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Dr.elhelece chemistry kinetics 330chem

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Rate of chemical reactions معدل التفاعل الكيميائي
factors affecting the rate of reaction العوامل المؤثرة على سرعة التفاعل الكيميائي
order of reaction and half life time رتبة التفاعل وفترة عمر النصف
determination of rate تعين سرعة التفاعل
order and rate constant of chemical reaction رتبة وثابت التفاعل الكيميائي
Arrhenius equation معادلة ارهينوس
determination of activation energy تعين طاقة التنشيط
collision theory transition state نظرية التصادم والحالة البينية
chain reaction and reaction mechanism تفاعلات السلسلة وميكانيكية التفاعل.

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Dr.elhelece chemistry kinetics 330chem

  1. 1. Chemical Kinetics ‫الحركية‬ ‫الكيمياء‬ 330 Chem 1Dr. Wael A. El-Helece
  2. 2. Chemical Kinetics ‫الحركية‬ ‫الكيمياء‬ Rate of chemical reactions ‫الكيميائي‬ ‫التفاعل‬ ‫معدل‬ factors affecting the rate of reaction ‫الكيميائي‬ ‫التفاعل‬ ‫سرعة‬ ‫على‬ ‫المؤثرة‬ ‫العوامل‬ order of reaction and half life time ‫النصف‬ ‫عمر‬ ‫وفترة‬ ‫التفاعل‬ ‫رتبة‬ determination of rate ‫التفاعل‬ ‫سرعة‬ ‫تعين‬ order and rate constant of chemical reaction ‫الكيميائي‬ ‫التفاعل‬ ‫وثابت‬ ‫رتبة‬ Arrhenius equation ‫ارهينوس‬ ‫معادلة‬ determination of activation energy ‫التنشيط‬ ‫طاقة‬ ‫تعين‬ collision theory transition state ‫البينية‬ ‫والحالة‬ ‫التصادم‬ ‫نظرية‬ chain reaction and reaction mechanism ‫التفاعل‬ ‫وميكانيكية‬ ‫السلسلة‬ ‫تفاعل ت‬. 2Dr. Wael A. El-Helece
  3. 3. Chemical Reactions ‫الكيميائية‬ ‫التفاعل ت‬ Making new substances out of old substances ‫قديمة‬ ‫أخرى‬ ‫مواد‬ ‫من‬ ‫جديدة‬ ‫مواد‬ ‫إنتاج‬ How fast do reactions go? ‫ما؟‬ ‫تفاعل‬ ‫سرعة‬ ‫مدى‬ ‫ما‬ This is the subject of ‫موضوع‬ ‫هو‬ ‫هذا‬ Chemical Kinetics ‫الكيميائية‬ ‫الحركية‬ 3Dr. Wael A. El-Helece
  4. 4. examples. ‫أمثلة‬ 2 H2(g) + O2(g) → 2 H2O(l). C(diamond) → C(graphite). 2 NO(g) + O2(g) → 2 NO2(g). 4Dr. Wael A. El-Helece
  5. 5. Factors Influence Reaction Rates ‫الكيميائى‬ ‫التفاعل‬ ‫سرعة‬ ‫على‬ ‫المؤثرة‬ ‫العوامل‬ Nature of Reactants ‫المتفاعلة‬ ‫المواد‬ ‫طبيعة‬ Acid-base reactions, formation of salts, and exchange of ions are fast reactions. Reactions in which large molecules are formed or break apart are usually slow. Reactions breaking strong covalent bonds are also slow. Temperature ‫الحرارة‬ ‫درجة‬ Usually, the higher the temperature, the faster the reaction. The temperature effect is discussed in terms of activation energy. Concentration Effect ‫التركيز‬ ‫تأثير‬ The dependences of reaction rates on concentrations are called rate laws. Rate laws are expressions of rates in terms of concentrations of reactants. •rate laws: differential and integrated rate laws. •Integrated rate laws: First Order Reactions & Second Order Reactions Rate laws apply to homogeneous reactions in which all reactants and products are in one phase (solution). •Heterogeneous reactions: reactants are present in more than one phase For heterogeneous reactions, the rates are affected by surface areas. Catalysts: substances used to facilitate reactions By the nature of the term, catalysts play important roles in chemical reactions. 5Dr. Wael A. El-Helece
  6. 6. Rate Equation ‫التفاعل‬ ‫معادلة‬ [A ] and CA mean concentration of A in mol/L. Reaction Velocity υ ‫التفاعل‬ ‫سرعة‬ aA + bB → cC + dD 6Dr. Wael A. El-Helece
  7. 7. Rate Laws ‫السرعة‬ ‫قوانين‬ The reaction velocity is related to a time- derivative of one of the concentrations. .‫للزمن‬ ‫بالنسبة‬ ‫المتفاعل ت‬ ‫أحد‬ ‫تركيز‬ ‫في‬ ‫دالة‬ ‫التفاعل‬ ‫سرعة‬ 7Dr. Wael A. El-Helece
  8. 8. where x, y, z,… are small whole numbers or simple fractions.  k is the rate constant. ‫التفاعل‬ ‫ثابت‬ k The sum of x + y + z + . . . is called the "order" of the reaction. ‫التفاعل‬ ‫برتبة‬ ‫يعرف‬ ‫النسب‬ ‫مجموع‬ 8Dr. Wael A. El-Helece
  9. 9. Common types of rate laws ‫السرعة‬ ‫قوانين‬ ‫من‬ ‫المشهورة‬ ‫النواع‬ First Order Reactions ‫اللولى‬ ‫الرتبة‬ ‫من‬ ‫تفاعل‬ Second Order Reactions ‫الثانية‬ ‫الرتبة‬ ‫من‬ ‫تفاعل‬ Third Order Reactions ‫الثالثة‬ ‫الرتبة‬ ‫من‬ ‫تفاعل‬ 9Dr. Wael A. El-Helece
  10. 10. First Order Reactions ‫اللولى‬ ‫الرتبة‬ ‫تفاعل ت‬ B + other reactants → products υ = υ = rate = k[B] 10Dr. Wael A. El-Helece
  11. 11. Background on Rates & Mechanisms ‫والميكانيكيات‬ ‫السرعات‬ ‫على‬ ‫مراجعة‬ Chemists study reactions. Some of what we study: : ,‫ندرسه‬ ‫مما‬ ‫بعضا‬ ‫التفاعلت‬ ‫يدرسون‬ ‫الكيميائيين‬ - Major & Minor Products ‫الساسية‬ ‫ججج‬‫ج‬‫ت‬‫النوا‬ ‫والفرعية‬ - Reactants ‫المتفاعلت‬ - % Completion ‫الكتمال‬ ‫نسبة‬ - Effects of Catalysts ‫المساعدة‬ ‫العوامل‬ ‫تأثير‬ - Separation and Purification of Products ‫فصل‬ ‫النواتج‬ ‫وتنقية‬ - Effects of Variables on Rxn Speed (rate) & on Products ‫على‬ ‫وكذا‬ ‫عل‬‫التفا‬ ‫سرعة‬ ‫لى‬‫ع‬ ‫المتغيرات‬ ‫ير‬‫تأث‬ ‫النواتج‬ 11Dr. Wael A. El-Helece
  12. 12.  These chapters deal with rates & mechanisms of reactions. ‫حدوث‬ ‫وطريقة‬ ‫التفاعلت‬ ‫سرعة‬ ‫مع‬ ‫تتعامل‬ ‫الجزاء‬ ‫هذه‬ ‫التفاعل‬  Mechanism: Step-by-step progress of the chemical reaction. ‫الميكانيكية‬. :‫الكيميائى‬ ‫التفاعل‬ ‫تقدم‬ ‫بخطوة‬ ‫خطوة‬  Rate: How fast the reaction proceeds (usually Δ M / Time) ‫المعدل‬) :‫الزمنى‬ ‫المعدل‬ ‫عادةا‬ ‫التفاعل‬ ‫حدوث‬ ‫سرعة‬ ‫مدى‬ 12Dr. Wael A. El-Helece
  13. 13. Background on Rates & Mechanisms ‫والميكانيكيات‬ ‫المعدلت‬ ‫على‬ ‫مراجعة‬  Study of rates is useful since the results will: :‫فى‬ ‫نتائجه‬ ‫نافع‬ ‫علم‬ ‫المعدلت‬ ‫دراسة‬ 1) Indicate how to manipulate factors to control the reaction 1(‫فى‬ ‫والتحكم‬ ‫المختلفة‬ ‫العوامل‬ ‫من‬ ‫الستفادة‬ ‫يمكن‬ ‫كيف‬ ‫تبين‬ .‫التفاعل‬ 2) Lead to the mechanism of the reaction 2. (‫التفاعل‬ ‫حدوث‬ ‫طريقة‬ ‫تقود‬ 3) Indicate time needed to get a given amount of product 3. (‫النواتج‬ ‫من‬ ‫محددة‬ ‫كمية‬ ‫على‬ ‫للحصول‬ ‫اللزم‬ ‫الزمن‬ ‫تحدد‬ 4) Indicate amount of product in a given amount of time 13Dr. Wael A. El-Helece
  14. 14. ‫السرعات‬ ‫على‬ ‫مراجعة‬ ‫والميكانيكيات‬ Study of mechanisms important. With the mechanism we can: ,‫طريق‬ ‫عن‬ ‫في‬ ‫مة‬ ‫مه‬ ‫عل‬ ‫التفا‬ ‫حدوث‬ ‫قة‬ ‫طري‬ ‫سة‬ ‫درا‬ ‫نستطي‬ ‫الميكانيكية‬‫ع‬ 1) Predict products of similar reactions ‫التفاعلت‬ ‫نواتج‬ ‫تحدد‬ ‫المماثلة‬ 2) Better understand the reaction ‫للتفاعل‬ ‫جيد‬ ‫فهم‬ 3) Accurately manipulate the reaction for a desired result ‫محددة‬ ‫نواتج‬ ‫على‬ ‫للحصول‬ ‫التفاعل‬ ‫توجيه‬ ‫تحديدا‬ 4) Organize and simplify the study of organic chemistry ‫العضوية‬ ‫الكيمياء‬ ‫دراسة‬ ‫وتبسيط‬ ‫تنظيم‬ Example: ‫مثال‬ OH I 14Dr. Wael A. El-Helece
  15. 15. Background on Rates & Mechanisms ‫والميكانيكيات‬ ‫السرعات‬ ‫على‬ ‫مراجعة‬  Main Factors which influence reaction rate: :‫التفاعل‬ ‫معدل‬ ‫في‬ ‫تؤثر‬ ‫التي‬ ‫الساسية‬ ‫العوامل‬  Concentrations of Reactants ‫المتفاعلة‬ ‫المواد‬ ‫تراكيز‬ Rates usually increase as reactant concentrations increase. .‫المتفاعلة‬ ‫المواد‬ ‫تركيزات‬ ‫بزيادة‬ ‫تزداد‬ ‫عادةا‬ ‫المعدلت‬  Reaction Temperature ‫التفاعل‬ ‫حرارة‬ ‫درجة‬ An increase in temperature increases the rate of a reaction. .‫التفاعل‬ ‫معدل‬ ‫تزيد‬ ‫الحرارة‬ ‫درجة‬ ‫في‬ ‫الزيادة‬  Presence of a Catalyst ‫حفاز‬ ‫عامل‬ ‫وجود‬ (not all rxns have catalysts) ‫مواد‬ ‫توجد‬ ‫التفاعلت‬ ‫كل‬ ‫ليس‬ ‫مساعدة‬ 15Dr. Wael A. El-Helece
  16. 16. Background on Rates & Mechanisms ‫والميكانيكيات‬ ‫السرعات‬ ‫على‬ ‫مراجعة‬ A catalyst is a substance which increases the rate of a reaction without being consumed in the overall reaction. .‫تتغير‬ ‫أن‬ ‫دون‬ ‫التفاعل‬ ‫معدل‬ ‫تزيد‬ ‫مادة‬ ‫هو‬ ‫المساعد‬ ‫العامل‬ The concentration of the catalyst or its surface area (if insoluble) are variables which influence the rate. .‫التفاعل‬ ‫تحث‬ ‫متغيرات‬ ‫سطحه‬ ‫مساحة‬ ‫أو‬ ‫الحفاز‬ ‫العامل‬ ‫تركيز‬ Some catalysts are incredibly complex - like enzymes; and others are quite simple: ,‫وبعضها‬ ‫النزيمات‬ ‫مثل‬ ‫كبير‬ ‫بشكل‬ ‫معقدة‬ ‫تكون‬ ‫الحفازة‬ ‫العوامل‬ ‫بعض‬ .‫بسيط‬ ‫يكون‬ H+ + H2O + CH2 = CH2 ------) CH3-CH2-OH + H+ Type of Reactants ‫المتفاعلت‬ ‫أنواع‬ 16Dr. Wael A. El-Helece
  17. 17. Rates ‫التفاعل‬ ‫معدلت‬  Reaction Rate = either the increase in M of product per unit time or the decrease in M of reactant per unit time; ΔM / ΔT Note: [X] = moles X / Liter , :‫فى‬ ‫النقصان‬ ‫أو‬ ‫الزمن‬ ‫لوحدة‬ ‫النواتج‬ ‫تركيز‬ ‫فى‬ ‫الزيادة‬ ‫اما‬ ‫التفاعل‬ ‫معدل‬ .‫للزمن‬ ‫بالنسبة‬ ‫المتفاعلت‬ ‫تركيز‬  Example: H+ Catalyst Sucrose + H2O ‫جججججججججججججججججججججج‬ Glucose + Fructose ‫فركتوز‬ + ‫جلكوز‬ ‫جججججججججججججججججججججججججججج‬ ‫+الماء‬ ‫السكروز‬ Rate = rate of formation of either product. Rate = Δ M of glucose / Δsec = + Δ[glucose]/ Δsec or Rate = rate of disappearance of either reactant. Rate = - Δ[sucrose]/ Δsec  In order to obtain rate, we need a way to measure ΔM of any reactant or product with respect to time. ‫لقياس‬ ‫طريقة‬ ‫الى‬ ‫نحتاج‬ ‫التفاعل‬ ‫سرعة‬ ‫تعين‬ ‫نستطيع‬ ‫لكى‬ 17Dr. Wael A. El-Helece
  18. 18. Rates ‫التفاعل‬ ‫معدلت‬  Example: 2 N2O5 -----) 4 NO2 + 1 O2 If we want to equalize the rates then: Rate = Δ[O2] = 1/4 Δ[NO2] = - 1/2 Δ[N2O5] Δt Δt Δt *divide by balancing coefficients when we equalize rates. .‫السرعا ت‬ ‫معادلة‬ ‫نستطيع‬ ‫الوزن‬ ‫معامل ت‬ ‫على‬ ‫القسمة‬  Various Rates can be determined: ‫تعينها‬ ‫يمكن‬ ‫المختلفة‬ ‫المعدل ت‬ 1) instantaneous rate at a given time; ‫محدد‬ ‫زمن‬ ‫عند‬ ‫الوقتية‬ ‫السرعة‬ 2) average rate over a long period of time; ‫كبيرة‬ ‫زمنية‬ ‫فترا ت‬ ‫خل ل‬ ‫فى‬ ‫المتوسطة‬ ‫السرعة‬ 3) the initial rate – rate at the beginning of the rxn (this is used the most). .(‫استخداما‬ ‫)أكثر‬ ‫التفاعل‬ ‫بدء‬ ‫عند‬ ‫السرعة‬ : ‫اللولى‬ ‫السرعة‬ 18Dr. Wael A. El-Helece
  19. 19. Dr. Wael A. El-Helece 19 Rates ‫التفاعل‬ ‫معدلت‬ On next slide the Δ[O2] versus time is plotted for a reaction. :‫الزمن‬ ‫مع‬ ‫سجين‬‫الك‬ ‫غاز‬ ‫يز‬‫ترك‬ ‫فى‬ ‫ير‬‫التغ‬ ‫ية‬‫التال‬ ‫حة‬‫الشري‬ ‫فى‬ ‫للتفاعل‬ ‫رسم‬ 2N2O5 4NO2 + O2 Note: ‫لحظ‬ 1) how the rate changes with time. ‫الزمن‬ ‫مع‬ ‫المعدل‬ ‫يتغير‬ ‫كيف‬. 2) that rate is the tangent at a given point on the curve.
  20. 20. Rates ‫التفاعل‬ ‫معدلت‬ 20Dr. Wael A. El-Helece
  21. 21. Rates ‫التفاعل‬ ‫معدلت‬ 21Dr. Wael A. El-Helece
  22. 22. Rates ‫التفاعل‬ ‫معدلت‬  Calculate the Average Rate for: ‫ل‬ ‫المتوسط‬ ‫المعدل‬ ‫احسب‬ I- + ClO- -----) Cl- + IO- Given the [I- ] and time in seconds, then what is the average rate? ‫المعدل‬ ‫ماهو‬ ‫ثم‬ ‫ومن‬ ‫بالثوانى‬ ‫والزمن‬ ‫اليوديد‬ ‫تركيز‬ ‫علما‬ ‫المتوسط؟‬ Time (s) M I- 2.00 0.00169 8.00 0.00101 - Rate = - ΔM / ΔT - Rate = - (0.00101 - 0.00169) M / (8.00 - 2.00) s = 6.8x10-4 M / 6.00s22Dr. Wael A. El-Helece
  23. 23. Rates ‫التفاعل‬ ‫معدلت‬  Need to obtain the change in M of a given reagent per change in time; could follow any parameter related to concentration. ).‫للزمن‬ ‫بالنسبة‬ ‫التركيز‬ ‫فى‬ ‫التغير‬ ‫على‬ ‫الحصول‬ ‫الى‬ ‫نحتاج‬‫يمكن‬ .(‫التركيز‬ ‫على‬ ‫يعتمد‬ ‫عامل‬ ‫اى‬ ‫تتبع‬  Examples of what one might follow to obtain rates: .‫المعدل‬ ‫على‬ ‫للحصول‬ ‫نتتبعه‬ ‫ما‬ ‫على‬ ‫أمثلة‬ - A change in pressure (if gas produced or consumed in the rxn) -.( )‫غاز‬ ‫على‬ ‫التفاعل‬ ‫احتوى‬ ‫اذا‬ ‫الضغط‬ ‫فى‬ ‫التغير‬ - A change in pH (if acidity changes in the rxn) -)‫فى‬ ‫ةةةةة‬‫الحموض‬ ‫ةرة‬‫ي‬‫تغ‬ ‫اذا‬ ‫ةة‬‫ة‬‫ةط‬‫الوس‬ ‫ةةةةة‬‫حموض‬ ‫ةىةة‬‫ف‬ ‫ةرة‬‫ي‬‫التتغ‬ .(‫التفاعل‬ - A change in absorbance of electromagnetic radiation (EMR) -.‫كهرومغناطيسى‬ ‫لشعاع‬ ‫امتصاص‬ ‫فى‬ ‫التغير‬ 23Dr. Wael A. El-Helece
  24. 24. Usually measure absorbance of Visible or UV EMR at a given λ - caused by a change in reactant or product concentration. ‫موجى‬ ‫طول‬ ‫ند‬ ‫ع‬ ‫سجية‬ ‫البنف‬ ‫فوق‬ ‫عة‬ ‫الش‬ ‫ئى‬ ‫المر‬ ‫الضوء‬ ‫صاص‬ ‫امت‬ ‫ةا‬ ‫عاد‬ .‫النواتج‬ ‫او‬ ‫المتفاعلت‬ ‫احد‬ ‫تركيز‬ ‫فى‬ ‫لتغير‬ ‫نتيجة‬ ‫يكون‬ ‫محدد‬ A = Εbc at given λ (wavelength) This is Beer’s Law. ‫قانون‬ ‫محدد‬ ‫موجى‬ ‫طول‬ ‫ةدة‬‫ن‬‫ع‬ )‫بيير‬)‫لمبرت‬ Rates ‫التفاعل‬ ‫معدل ت‬ 24Dr. Wael A. El-Helece
  25. 25. Rates ‫التفاعل‬ ‫معدلت‬ A = absorbance; use spectrophotometer to measure; has no units. A) :‫وحدة‬ ‫لها‬ ‫يس‬ ‫ل‬ ‫صاص‬ ‫المت‬ ‫يف‬ ‫ط‬ ‫قياس‬ ‫جهاز‬ ‫ستخدم‬ ‫ن‬ ‫صاصية‬ ‫المت‬ (.‫قياس‬ Ε = molar absorptivity = a constant @ given λ; has units of M-1 cm-1 Ε) :‫وحدة‬ ‫المحدد‬ ‫الموجى‬ ‫الطول‬ ‫ند‬ ‫ع‬ ‫طى‬ ‫يع‬ ‫بت‬ ‫ثا‬ ‫ية‬ ‫النوع‬ ‫ةةة‬‫ي‬‫النفاذ‬ ‫مولر‬ ‫القياس‬-1 ‫سم‬-1 ). b = pathlength of EMR through sample; usually 1.00 cm cuvette used. b) (.‫جهاز‬ ‫لكل‬ ‫ثابت‬ ‫المسار‬ ‫طول‬ c = concentration in M c.‫بالمولر‬ ‫التركيز‬ A plot of A versus M @ given λ will yield a straight line and the equation: ‫مستقيم‬ ‫خط‬ ‫طى‬‫يع‬ ‫محدد‬ ‫جى‬‫مو‬ ‫طول‬ ‫ند‬‫ع‬ ‫يز‬‫الترك‬ ‫مع‬ ‫صاصية‬‫المت‬ ‫سم‬‫ر‬ :‫تكون‬ ‫والمعادلة‬ 25Dr. Wael A. El-Helece
  26. 26. III. Rate Law ‫السرعة‬ ‫قانون‬ rate = k x [A]m x [B]n for A + B Products  Rate Law relates the rate to temperature & concentration. . :‫والتركيز‬ ‫الحرارة‬ ‫بدرجة‬ ‫المعدل‬ ‫يربط‬ ‫السرعة‬ ‫قانون‬  Rate law is given in terms of REACTANTS only (convention). .( )‫اتفاق‬ ‫فقط‬ ‫المتفاعلت‬ ‫بدللة‬ ‫السرعة‬ ‫قانون‬  k = rate constant & handles the temperature variable. .‫الحرارة‬ ‫متغير‬ ‫ممسك‬ ‫التفاعل‬ ‫ثابت‬  The exponents are the order & handle the concentration variables. .‫التركيز‬ ‫متغيرات‬ ‫فى‬ ‫وتتحكم‬ ‫الرتبة‬ ‫هى‬ ‫الدلة‬  General form of the rate law for: ‫التفاعل‬ ‫لمعادلة‬ ‫العام‬ ‫الشكل‬ a A + b B c C + d D rate = k x [A]m x [B]n 26Dr. Wael A. El-Helece
  27. 27. Dr. Wael A. El-Helece 27 Order for A is m ‫هى‬ ‫رتبة‬ order for B is n; ‫هى‬ ‫رتبة‬ Overall order is: m + n ‫الكلية‬ ‫الرتبة‬‫هى‬ - m & n are determined experimentally. - k, also determined experimentally & units depend upon overall order. III. Rate Law ‫السرعة‬ ‫قانون‬ rate = k x [A]m x [B]n for A + B Products
  28. 28. III. Rate Law ‫السرعة‬ ‫قانون‬ Rate Constant & Units ‫والوحدات‬ ‫السرعة‬ ‫ثابت‬ Note: Assume time is in seconds (s). Rate = k [A]x Solve for k & plug in units; k = Rate / [A]x Overall Rxn Order, x Units for k zero Ms-1 first s-1 second M-1 s-1 third M-2 s-1 28Dr. Wael A. El-Helece
  29. 29. III. Rate Law ‫السرعة‬ ‫قانون‬  Example: 2 NO2 + 1 F2 ----) 2 NO2F Rate= k [NO2]n x [F2]m  In the laboratory, the overall rate was found to be second order. n + m = 2 Possibilities: n=2 & m=0; n=0 & m=2; n=1 & m=1  Experiments demonstrated that n=1 and m=1; How? By running the reaction at least three times: #1 – getting rate at certain initial concentrations of NO2 & F2 ; #2 – getting rate when keeping [NO2] the same & doubling [F2]; #3 – getting rate when keeping [F2] the same & doubling [NO2].  They found that doubling [NO2] doubled the rate & doubling [F2] doubled the rate; so, both coefficients had to be 1.  Rate = k [NO2]1 [F2]1 29Dr. Wael A. El-Helece
  30. 30. III. Rate Law ‫السرعة‬ ‫قانون‬ Rate Constant ‫التفاعل‬ ‫ثابت‬  Determination of the rate constant, k. You are given: 1) aA + bB  cC + dD 2) Rate = k[A]0 [B]1 (0 & 1 were determined experimentally) 3) M of A & B = 2.00 moles/L & Rate = 2.50 x 10-2 M/s Determine the value for k & give complete rate expression. Rate = k[A]0 [B]1 k = Rate [A]0 [B]1 k = Rate = 2.50 x 10-2 M/s = 2.50 x 10-2 M/s [A]0 [B]1 [2.00 M]0 x [2.00 M]1 [1] x [2.00 M]1 k = 1.25 x 10-2 s-1 Rate = (1.25 x 10-2 s-1 )[A]0 [B]1 (completed rate expression) 30Dr. Wael A. El-Helece
  31. 31. III. Rate Law ‫السرعة‬ ‫قانون‬  Rate Law for a reaction is found experimentally except for a single step in a mechanism (elementary reaction). Assume rxn is NOT elementary unless told that it is a one step in mechanism.  Given: 1NO2 + 1CO -----) 1NO + 1CO2 By experiment, the rate law was found to be: rate = k[NO2]2 [CO]0 or rate = k[NO2]2 ( Note: [CO]0 = 1 ) The order WRT each reactant & the overall order are: 2nd order WRT NO2 0th order WRT CO 2nd order overall 31Dr. Wael A. El-Helece
  32. 32. IV. Order  The concentration variables are handled by the exponents - the order.  The orders are determined experimentally except for one case: An elementary reaction.  Elementary reactions are one step reactions which are the individual steps in a mechanism. (For an elementary reaction only: the balancing coefficients determine the order.) - Important Example 1 for a multistep reaction: 1 CH2Br2 + 2 KI ---) 1 CH2I2 + 2 KBr If experimentation found that m & n were both first order; then: rate = k [CH2Br2 ]1 [KI]1 Example 2 for an elementary reaction: 2 O3 ---) 3 O2 (told it is elementary) No need for experimentation; order comes from balancing coefficients: rate = k[O3]2 32Dr. Wael A. El-Helece
  33. 33. IV. Determination of Order ‫التفاعل‬ ‫رتبة‬ ‫تعين‬  Order - from units of k: If you are given the units of the rate constant for a reaction, then you will know the overall order. :‫التفاعل‬ ‫ثابت‬ ‫وحدات‬ ‫علمت‬ ‫اذا‬ ‫الثابت‬ ‫وحدات‬ ‫من‬ ‫الرتبة‬ .‫الرتبة‬ ‫معرفة‬ ‫يمكن‬ ‫اذا‬  Order by Method 1 - from altering M: Measure initial rates keeping one reactant constant and change the concentration of another; observe the rates. ‫ة‬‫ة‬‫ةة‬‫بالطريق‬ ‫ةةة‬‫ب‬‫الرت‬1:‫على‬ ‫حفاظا‬ ‫الولية‬ ‫المعدلت‬ ‫نقيس‬ ‫ونلحظ‬ ‫ةرةة‬‫الخ‬ ‫ةرة‬‫ي‬‫وتغ‬ ‫ةتة‬‫ب‬‫ثا‬ ‫المتفاعلت‬ ‫ةدةة‬‫اح‬ ‫ةزة‬‫ي‬‫ترك‬ .‫المعدل‬  Order by Method 2 - from integrated rate expression: The rate between the limits of time = 0 & time = t. By plotting out the variables of these integrated rate expressions you can determine the order. ‫بالطريقة‬ ‫الرتبة‬3:‫الزمن‬ ‫بين‬ ‫المعمل‬ ‫فى‬ ‫المعدل‬0‫والزمن‬t. 33Dr. Wael A. El-Helece
  34. 34. IV. Determination of Order by varying M ‫التركيز‬ ‫تغيير‬ ‫طريق‬ ‫عن‬ ‫الرتبة‬ ‫تعين‬  Example #1: Determine the order for & rate expression for: 2N2O5 ---) 4NO2 + 1O2 rate = k [N2O5]m Exp #1: Rate = 4.8x10-6 Ms-1 at 1.0x10-2 M N2O5 Exp #2: Rate = 9.6x10-6 Ms-1 at 2.0x10-2 M N2O5 Order: Note that when [N2O5] doubles, the rate doubles. Since rate α [N2O5]m & rate doubles when [N2O5] doubles, the value of must be 1; the order is 1. - rate α [N2O5]m & rate doubles when [N2O5] doubles, then: go from [1]m = 1 to [2]m = 2 m has to be 1 34Dr. Wael A. El-Helece
  35. 35. IV. Determination of Order by varying M ‫التركيز‬ ‫تغيير‬ ‫طريق‬ ‫عن‬ ‫الرتبة‬ ‫تعين‬ Summary ‫ملخص‬ EFFECTS of doubling reagent M while keeping others constant: ‫تراكيز‬ ‫على‬ ‫الحفاظ‬ ‫بينما‬ ‫التركيز‬ ‫مضاعفة‬ ‫تأثيرات‬ :‫الخرى‬ ‫المتفاعلت‬ Rate remains the same ‫نفسه‬ ‫المعدل‬ ‫يظل‬0 th order:[M]0 Rate doubles ‫المعدل‬ ‫يتضاعف‬1 st order:[M]1 Rate quadruples ‫اضعاف‬ ‫اربعة‬ ‫المعدل‬2 nd order:[M]2 Rate increases eightfold ‫اضعاف‬ ‫ثمانية‬ ‫المعدل‬3 rd order:[M]3 35Dr. Wael A. El-Helece
  36. 36. IV. Determination of Order by varying M ‫التركيز‬ ‫تغيير‬ ‫طريق‬ ‫عن‬ ‫الرتبة‬ ‫تعين‬  Example #2: 2 NO + Cl2 -----) 2 NOCl - Calculate order of Rxn Exp Initial [NO] Initial [Cl2] Initial Rate, Ms-1 1 0.0125 0.0255 2.27x10-5 2 0.0125 0.0510 4.57x10-5 3 0.0250 0.0255 9.08x10-5 Rate = k[NO]m [Cl2]n a) calculate n: From 1 & 2 - double [Cl2] & keep [NO] constant & rate increases by factor of 2.01; n = 1 b) calculate m: From 1 & 3 - double [NO] & keep [Cl2] constant & rate increases by factor of 4.00; m = 2 Rate = k[NO]2 [Cl2]1 2nd order wrt [NO]; 1st order wrt [Cl2]; 3rd order overall 36Dr. Wael A. El-Helece
  37. 37. Order by Method #2 ‫بطريقة‬ ‫الرتبة‬2 from integrated rate expression: ‫التكاملى‬ ‫المعدل‬ ‫من‬  Integrate the rate expression between the limits of time = 0 and time = t. By plotting out the variables of these integrated rate expressions you can determine the order. You will be doing this in the kinetics lab. ‫والزمن‬ ‫صفر‬ ‫الزمن‬ ‫الحدود‬ ‫بين‬ ‫السرعة‬ ‫معادلة‬ ‫كامل‬t‫العلقة‬ ‫برسم‬ .‫المعمل‬ ‫فى‬ ‫يمكن‬ ‫وهذا‬ ‫المعدل‬ ‫ايجاد‬ ‫نستطيع‬ ‫النتغيرات‬ ‫بين‬ 37Dr. Wael A. El-Helece
  38. 38. IV. Determination of Order by Integrated Rate Expression  Summary on use of logarithms  Log: involves #’s to the base 10; Log 10x = x  Ln: Natural log uses #’s to the base e; Ln ex = x  Ln [A/B] = Ln A - Ln B (or Log)  Ln [A x B] = Ln A + Ln B (or Log)  Ln Ab = b Ln A (or Log)  To obtain either log or ln use the appropriate calculator function.  Log 2.1x10-4 = - 3.68 (note significant figure change - see below) (-4.0000000…. + 0.32 = -3.68 ; cut off at first doubtful digit)  To remove Ln & Log use the inverse; ex & 10x functions on cal.  Inverse [log 3.00] or 103.00 = 1.0 x 103  Inverse [ln 3.00] or e3.00 = 20. 38Dr. Wael A. El-Helece
  39. 39. IV. Order Integrated Rate Law - First Order Rxns ‫الولى‬ ‫الرتبة‬ ‫من‬ ‫تفاعل‬ ‫المتكامل‬ ‫التفاعل‬ ‫معدل‬ ‫الرتبة‬ [A] Time 39Dr. Wael A. El-Helece 1) 1st Order Reactions: aA → Products If 1st order, then -Δ[A]/Δt = k[A]1 (rate expression) - This plot for first order data only gives minimal information -‫الرتبة‬ ‫من‬ ‫للتفاعل‬ ‫الرسم‬ ‫هذا‬1‫الدنى‬ ‫الحد‬ ‫تعطي‬ ‫البيانات‬ .‫فقط‬ ‫المعلومات‬ ‫من‬
  40. 40. IV. Order Integrated Rate Law - First Order Rxns ‫الولى‬ ‫الرتبة‬ ‫من‬ ‫تفاعل‬ ‫المتكامل‬ ‫التفاعل‬ ‫معدل‬ ‫الرتبة‬ 1) 1st Order Reactions: aA -----) bB -Δ[A]/Δt = k[A]1 - if we integrate from time t to 0, we get the following: Y = mX + b ln[A]t = - kt + ln[A]o or ln{[A]t/[A]o} = -kt where [A]t = M of A at time = t & [A]o = M of A at t = 0 - A plot of ln[A]t versus t gives a straight line (Y = mX +b): b Slope (m) = - k Note: Only linear for 1st order ln[A]t Time, t 40Dr. Wael A. El-Helece
  41. 41. Must be 1st order since plot of ln[N2O5] vs t is linear. Can get k from the slope. Example of an integrated rate plot for a 1st order reaction Slope = -k = rise/run 41Dr. Wael A. El-Helece
  42. 42. IV. Order Example using 1st order integrated equation  Example: 2N2O5 ---) 4NO2 + O2 rate = k [N2O5]1 (1st order) Given: k = 4.80x10-4 s-1 & [N2O5]t=o = 1.65x10-2 M; what is [N2O5] at 825 s? ln [A]t = - k x t + ln [Ao] ln [N2O5] = - 4.80x10-4 s-1 x 825 s + ln [1.65x10-2 ] ln [N2O5] = - 0.396 + - 4.104 ln [N2O5] = - 4.500 Take inverse ln or anti ln of both sides & get: inverse {ln [-4.500]} or e -4.500 = 0.0111 [N2O5] = 0.0111 M 42Dr. Wael A. El-Helece
  43. 43. IV. Order Integrated Rate Law - First Order Rxns  Half-Life (t1/2) of 1st Order Reaction: t1/2= time it takes for [A]o to decrease to 1/2 initial M = ½[A]o ln [A]t/ [A]o = -kt ln 1/2[A]o/ [A]o = -kt1/2 ln 1/2 = -kt1/2 -0.693 = -kt1/2 t1/2 = 0.693 / k Note: 1) Time for 1/2 to disappear is independent of [A] for 1st order reaction. 2) This is an easy way to calculate 1st order rate constant, k. Example: If t1/2 = 189 sec for 1st order decomposition of 1.0 mole of H2O2, then how much H2O2 will be left after 378 sec? Note: 378/189 = 2 Goes through two half lives 1.0 mol → 0.50 mol → 0.25 mol 43Dr. Wael A. El-Helece
  44. 44. IV. Order Integrated Rate Law - First Order Rxns  Example: Given a) k = 3.66x10-3 s-1 for decomposition of H2O2 and b) [H2O2]o = 0.882 M. Calculate: 1) t1/2 2) How much will be left after one half-life? (Note: Reaction must be 1st order – examine units for k) 1) t1/2 = 0.693/k t1/2 = 0.693 / 3.66x10-3 s-1 = 189 s 2) M of [H2O2] cut in half in one half-life (t1/2); will go from 0.882 to 0.441 M in 189 s. 44Dr. Wael A. El-Helece
  45. 45. IV. Order Integrated Rate Law - Second Order Rxns 2) Second Order Reactions: - Assume that aA -----) Products is 2nd order Rate = - Δ[A] / Δ t = k [A]2 Integrate rate expression from time t to 0 & get following: 1/[A]t = k t + 1/[A]o So, a plot of 1/[A]t vs t should give a straight line with slope = k and y intercept = 1/[A]o t1/2 = 1 Note: Now t1/2 depends on initial M k x [A]0 Note: can tell if reaction is 2nd order from 1/[A] vs t plot. 45Dr. Wael A. El-Helece
  46. 46. IV. Order Integrated Rate Law - Second Order Rxns Example Plot of ln[NO2] vs t is not linear – not 1st order. 46Dr. Wael A. El-Helece
  47. 47. IV. Order Integrated Rate Law - 0th Order 3) 0th Order Reactions Assume A → B is 0th order: Rate = -k[A]0 Rate = -k - “Integrated” Rate Equation for a 0th order reaction: [A]t = -k x t + [A]0 - a plot of [A]t versus t will give a straight line - Again, if you let [A]t = 1/2 [A]o then t = t1/2 - t1/2 = [A]0 / 2k 47Dr. Wael A. El-Helece
  48. 48. IV. Order Integrated Rate Law - Summary Δ Rate when double [M] None Double Quadruple 48Dr. Wael A. El-Helece
  49. 49. IV. Order Integrated Rate Law - Summary [A]t Time, t ln[A]t Time, t 1/[A]t Time, t 0th Order n=0 [A]t = - kt + [A]o 1st Order n=1 ln[A]t = - kt + ln[A]o 2th Order n=2 1/[A]t = kt + 1/[A]o A B Δ[A]/Δt = k[A]n Note: slope = -k or k in each case 49Dr. Wael A. El-Helece
  50. 50.  A collision needs to occur before a reaction can take place, & the rate constant (& rate) of the reaction depends upon the: ‫وكذا‬ ‫السرعة‬ ‫وثابت‬ ‫التفاعل‬ ‫يحدث‬ ‫كى‬ ‫تصادم‬ ‫يحدث‬ ‫ان‬ ‫يجب‬ :‫على‬ ‫يعتمد‬ ‫والتفاعل‬ ‫السرعة‬ 1) collision frequency ‫التصادم‬ ‫تردد‬ (temperature) ‫درجة‬ ‫الحرارة‬ 2) number of collisions having enough energy for rxn (Ea) .‫التفاعل‬ ‫لحدوث‬ ‫كافية‬ ‫طاقة‬ ‫تملك‬ ‫التى‬ ‫التصادمات‬ ‫عدد‬ 3) orientation of particles upon collision .‫التصادم‬ ‫اثناء‬ ‫الجزيئات‬ ‫اتجاهات‬  Ea = energy of activation = minimum energy of collision in order for the reaction to take place. :‫لحدوث‬ ‫التصادم‬ ‫اثناء‬ ‫الطاقة‬ ‫من‬ ‫قدر‬ ‫اقل‬ ‫التنشيط‬ ‫طاقة‬ .‫التفاعل‬  Ea & ΔH can be represented by Potential Energy Diagram; can draw for one step or for several steps in a mechanism. 50Dr. Wael A. El-Helece V. Temperature & Reaction Rate ‫التفاعل‬ ‫وسرعة‬ ‫الحرارة‬ ‫درجة‬
  51. 51. V. Temperature & Reaction Rate ‫التفاعل‬ ‫وسرعة‬ ‫الحرارة‬ ‫درجة‬ ΔH (Exothermic) ‫طارد‬ ‫للحرارة‬ What is Ea for reverse reaction? ‫للتفاعل‬ ‫التنشيط‬ ‫طاقة‬ ‫مقدار‬ ‫ما‬ ‫العكلسي؟‬ 51Dr. Wael A. El-Helece A) Potential Energy Diagram for an Elementary Reaction ‫واحدة‬ ‫خطوة‬ ‫من‬ ‫لتفاعل‬ ‫الوضع‬ ‫طاقة‬ ‫مخطط‬
  52. 52. V. Temperature & Reaction Rate ‫التفاعل‬ ‫ومعدل‬ ‫الحرارة‬ ‫درجة‬ Arrhenius Equation ‫اررهينوس‬ ‫معادلة‬  Arrhenius Equation relates: rate constant (k), temperature (T), energy of activation (Ea in J/mole), & orientation factor. ‫وكذا‬ ‫التنشيط‬ ‫وطاقة‬ ‫الحرارة‬ ‫ودرجة‬ ‫المعدل‬ ‫ثابت‬ ‫تربط‬ ‫اررهينوس‬ ‫معادلة‬ .‫التوجيه‬ ‫عامل‬ k = A e-Ea/RT R = gas constant; use R = 8.31 J/(Kmole) Take ln of both sides: ln k = -(Ea/R) 1/T + ln A Y = m X + b ‫مستقيم‬ ‫خط‬ ‫معادلة‬  Measure k at several temperatures and make plot of ln k versus 1/T. Slope of the curve = - Ea/R (will give Ea). ‫قيم‬ ‫نقيس‬k‫بين‬ ‫العلقة‬ ‫ونرسم‬ ‫مختلفة‬ ‫حرارة‬ ‫درجات‬ ‫عند‬ln k‫و‬1/T‫الميل‬ ‫يعطي‬-Ea/R‫قيمة‬ ‫ثم‬ ‫ومن‬Ea.  Note: A is a constant & includes orientation factor. :‫لحظ‬A.‫التوجيه‬ ‫عامل‬ ‫يعطي‬ ‫ثابت‬ 52Dr. Wael A. El-Helece
  53. 53. V. Temperature & Reaction Rate Arrhenius Equation Data below from 4 experiments - detn of rate constant, k, at 4 temperatures for a rxn ln k 1/T (in K-1 ) ln k = -(Ea/R) 1/T + ln A From: k = Ae-Ea/RT Y = m X + b Use: R = 8.31J/(K. mol) Slope = -Ea/R Can now determine Ea ln A o o o o 53Dr. Wael A. El-Helece
  54. 54. Integrated Rate Equation Half-Life t1/2 [A]t = - kt + [A]o [Ao]/2k 0th order ‫صفر‬ ‫الرتبة‬ ‫تفاعل‬ ln[A]t = - kt + ln[A]o 0.693/k 1st order ‫الولى‬ ‫الرتبة‬ ‫تفاعل‬ 1/[A]t = kt + 1/[A]o 1/k[Ao] 2nd order ‫الثانية‬ ‫الرتبة‬ ‫تفاعل‬ R = 8.31 J/K. mol. ‫للغازات‬ ‫العام‬ ‫الثابت‬ k = A e-Ea/RT Water ‫الماء‬ ‫ثوابت‬ ∆Hvap = 40.7 kJ/m ∆Hfus = 6.01 kJ/m o o o 54Dr. Wael A. El-Helece
  55. 55. Collision Theory ‫التصادمات‬ ‫نظرية‬
  56. 56. The Collision Theory ‫التصادم‬ ‫نظرية‬ Link to Simulation of Molecular Motion 1. Matter is moving particles. ‫حركة‬ ‫حالة‬ ‫فى‬ ‫جزيئات‬ ‫هى‬ ‫المادة‬ 2. Temperature increases- particles move faster ‫حركة‬ ‫تزداد‬ ‫تزداد‬ ‫الحرارة‬ ‫درجة‬ ‫الجزيئات‬ -more collisions ‫اكثر‬ ‫تصادمات‬ -more collision energy.‫اكبر‬ ‫تصادم‬ ‫طاقة‬ 3. Chemical reactions ‫الكيميائية‬ ‫التفاعلت‬ -bonds break ‫للروابط‬ ‫كسر‬ -new bonds form ‫جديدة‬ ‫روابط‬ ‫تكوين‬
  57. 57. Collision Theory ‫التصادم‬ ‫نظرية‬ Most collisions are not successful ‫ناجحة‬ ‫غير‬ ‫التصادمات‬ ‫معظم‬ Collisions provide the energy required to break bonds. ‫الروابط‬ ‫لكسر‬ ‫اللمزمة‬ ‫الطاقة‬ ‫يوفر‬ ‫التصادم‬ You need a collision to have a reaction. ‫تفاعل‬ ‫على‬ ‫للحصول‬ ‫تصادم‬ ‫الى‬ ‫نحتاج‬
  58. 58. Collision Theory ‫التصادمات‬ ‫مبدأ‬ products no products 1. Favourable Geometry ‫الفراغ‬ ‫فى‬ ‫معين‬ ‫ترتيب‬ A successful collision requires:‫يحتاج‬ ‫الناجح‬ ‫التصادم‬ Poor Geometry
  59. 59. 2. Sufficient Energy to break the chemical bonds ‫الكيميائية‬ ‫الروابط‬ ‫لكسر‬ ‫محددة‬ ‫طاقة‬ Activation energy is the minimum amount of energy required for a successful collision. ‫ناجح‬ ‫تصادم‬ ‫على‬ ‫للحصول‬ ‫الطاقة‬ ‫من‬ ‫قدر‬ ‫اقل‬ ‫هى‬ ‫التنشيط‬ ‫طاقة‬
  60. 60. The Collision Theory can be used to explain how the rate of a reaction can be changed. ‫التفاعل‬ ‫معدل‬ ‫يتأثر‬ ‫كيف‬ ‫تفسير‬ ‫التصادمات‬ ‫نظرية‬ ‫تسطيع‬ And that’s it! 3. Lower activation energy or Ea- low energy collisions are more effective. ‫اقل‬ ‫تصادم‬ ‫طاقة‬ ‫يعنى‬ ‫اقل‬ ‫تنشيط‬ ‫طاقة‬ 2. Harder collisions- greater collision energy‫اكبر‬ ‫تصادم‬ ‫طاقة‬ ‫بالرأس‬ ‫تصادم‬ 1. More collisions ‫اكثر‬ ‫تصادم‬ Reaction rates can increase due to :‫ل‬ ‫نتيجة‬ ‫تزداد‬ ‫التفاعل‬ ‫سرعة‬
  61. 61. The Collision Theory can be used to explain how the rate of a reaction can be changed. Harder collisions ‫بالراس‬ ‫تصادمات‬ More collisions ‫اكثر‬ ‫تصادمات‬ 1. Increasing the temperature increases the rate because there are: 2:‫بسبب‬ ‫المعدل‬ ‫يزداد‬ ‫الحرارة‬ ‫درجة‬ ‫بزيادة‬
  62. 62. 2. Increasing the reactant concentration increases the rate because there are: More frequent collisions
  63. 63. The Collision Theory can be used to explain how the rate of a reaction can be changed. The catalyst KI is added to H2O2, food colouring, and dishwashing detergent. The O2 produced makes foam. Lowers the activation energy or Ea- allowing low energy collisions to be successful 3. Adding a catalyst Movie
  64. 64. The Collision Theory can be used to explain how the rate of a reaction can be changed. Lower activation energy or Ea- allowing low energy collisions to be successful 4. Changing the nature of the reactant for a more reactive chemical increases the rate
  65. 65. The Collision Theory can be used to explain how the rate of a reaction can be changed. More frequent collisions 5. Increasing the surface area of a solid reactant increases the rate because:
  66. 66. Explain each Scenario Using the Collision Theory The spark provides the Ea and it explodes because it is exothermic A small spark ignites causes an explosion. Ea is too high for the room temperature collisions 1. A balloon full of H2 and O2 do not react at room temperature.
  67. 67. Explain each Scenario Using the Collision Theory It burns because it is exothermic The candle continues to burn The match provides the Ea A match causes the candle to burn Ea is too high for the room temperature collisions 2. A candle does not burn at room temperature
  68. 68. Explain each Scenario Using the Collision Theory 3. H2 O2 decomposes very slowly at room temperature. 2H2 O2(aq) → O2(g) + 2H2 O(l) KI increases the reaction rate dramatically. Lowers the Ea- allows low energy collisions to be successful KI is a catalyst as it is not a reactant and it speeds up the rate.
  69. 69. Describe and Graph the Relationship between the Following Ea and the rate Ea Rate Decreasing the Ea increases the rate- inverse.
  70. 70. Describe and Graph the Relationship between the Following Temperature and the rate Temp Rate Increasing the temperature increases the rate- direct.
  71. 71. Describe and Graph the Relationship between the Following Concentration and the rate Conc Rate Increasing the concentration increases the rate- direct.
  72. 72. Describe and Graph the Relationship between the Following Ea and the temperature Ea Temp No relationship! The only way to change the Ea is by adding a catalyst!
  73. 73. Which factors increase the percentage of successful collisions? I.Increasing temperature II.Increasing concentration III.Increasing surface area IV. Adding a catalyst
  74. 74. VI. Mechanisms ‫التفاعل‬ ‫حدوث‬ ‫طريقة‬ A) Introduction ‫مقدمة‬  Mechanism = step by step progress of chemical reaction. . :‫الكيميائي‬ ‫التفاعل‬ ‫تقدم‬ ‫بخطوة‬ ‫خطوة‬ ‫الميكانيكية‬  Most likely mechanism is determined experimentally from a study of rate data. ‫دراسة‬ ‫نننن‬‫م‬ ‫نان‬‫ي‬‫معمل‬ ‫نةن‬‫ن‬‫المعي‬ ‫نىنن‬‫ه‬ ‫ننة‬‫ن‬‫نح‬‫ص‬ ‫نرن‬‫ث‬‫الك‬ ‫نةن‬‫ي‬‫الميكانيك‬ .‫المعدل‬ ‫بيانات‬  The mechanism consists of one or more elementary reactions which add up to give you the overall reaction. ‫والذى‬ ‫اولي‬ ‫نلنن‬‫تفاع‬ ‫نرن‬‫ث‬‫اك‬ ‫او‬ ‫ندنن‬‫واح‬ ‫نننن‬‫م‬ ‫تتكون‬ ‫نةن‬‫ي‬‫الميكانيك‬ .‫الكلي‬ ‫التفاعل‬ ‫يعطى‬ ‫ويجمع‬ ‫يضاف‬  Species which is generated & then consumed in the mechanism is called an intermediate; Species which is added, consumed & then regenerated is a catalyst. ,‫بالمتوسطة‬ ‫تعرف‬ ‫الميكانيكية‬ ‫فى‬ ‫وتعتبر‬ ‫تخلق‬ ‫التى‬ ‫المواد‬ ‫تعاد‬ ‫ثم‬ ‫ن‬‫ن‬‫نن‬‫وم‬ ‫نلنن‬‫التفاع‬ ‫ن‬‫ن‬‫نى‬‫ف‬ ‫نبنر‬‫ت‬‫وتع‬ ‫نقن‬‫ل‬‫تخ‬ ‫نىن‬‫ت‬‫ال‬ ‫والمواد‬ .‫بالمساعدات‬ ‫تعرف‬  Step with largest Ea (slowest step) is called the rate determining step & governs overall reaction rate. 74Dr. Wael A. El-Helece
  75. 75. VI. Mechanisms ‫الميكانيكيات‬ Example 1 – Information ‫مثال‬1-‫المعلومات‬ Overall Rxn: O3 + 2NO2 -----) O2 + N2O5  Suggested two-step mechanism (from experimentation): .‫خطوتين‬ ‫التفاعل‬ ‫نفرض‬ ‫العملية‬ ‫النتائج‬ ‫من‬ Step 1) O3 + NO2 -----) NO3 + O2 (slow) Step 2) NO3 + NO2 -----) N2O5 rate = k [O3]1 [NO2]1 - from slow first step .‫البطأ‬ ‫الخطوة‬ ‫من‬ ‫يحدد‬ ‫المعدل‬  Notes: a) Two elementary reactions (NOTE: balancing coefficients = orders in an elementary rxn). ) := ‫المعادلة‬ ‫فى‬ ‫الوزن‬ ‫ارقام‬ ‫ابتدائيين‬ ‫تفاعلين‬ ‫ملحظات‬ (‫البتدائي‬ ‫التفاعلى‬ ‫فى‬ ‫الرتبة‬ 75Dr. Wael A. El-Helece
  76. 76. Dr. Wael A. El-Helece 76 b) Steps add up to give overall rxn. .‫الكلي‬ ‫التفاعل‬ ‫لتعطى‬ ‫الخطوات‬ ‫تجمع‬ c) NO3 is an intermediate (produced & used up). .( )‫ويستهلك‬ ‫يعطى‬ ‫بينية‬ ‫حالة‬ ‫يعتبر‬ ‫النترات‬ ‫ايون‬ d) There is no catalyst. .‫التفاعل‬ ‫هذا‬ ‫فى‬ ‫مساعد‬ ‫عامل‬ ‫ليوجد‬ e) Slowest step governs the overall rate. .‫الكلية‬ ‫للسرعة‬ ‫المحددة‬ ‫البطأهى‬ ‫الخطوة‬  mechanism is useful & will give us: :‫وتساعدنا‬ ‫مفيدة‬ ‫الميكانيكية‬ a) practical data, b) rate law, c) theoretical data, d) understanding of reaction.
  77. 77. VI. Mechanisms ‫الميكانيكيات‬ Example 2 - Calculate Rate Expression ‫مثال‬2-‫تعبيرات‬ ‫حساب‬ ‫المعدل‬  Determine a) general rate expression & b) complete rate law from the following mechanism : – :‫التالية‬ ‫الميكانيكية‬ ‫من‬ ‫الكلي‬ ‫المعدل‬ ‫قانون‬ ‫العام‬ ‫المعدل‬ ‫مصطلح‬ ‫عين‬ Note: can directly get the order for an elementary rxn from the balancing coefficients. ‫المعاملت‬ ‫من‬ ‫الرتبة‬ ‫على‬ ‫الحصول‬ ‫تستطيع‬ ‫البتدائي‬ ‫للتفاعل‬ ‫المعادلة‬ ‫فى‬ ‫.الوزنية‬ 1) 1I2 2Io (fast equilibrium) 2) 2Io + 1H2 2HI (slow) 77Dr. Wael A. El-Helece
  78. 78. Dr. Wael A. El-Helece 78 a) Overall Rxn from addn of steps: ‫الخطوات‬ ‫اضافة‬ ‫من‬ ‫الكلي‬ ‫المعدل‬ 1I2 + 1H2 2HI General rate law: rate = k[I2]x [H2]y ‫العام‬ ‫المعدل‬ ‫قانون‬ : b) Complete rate expression from mechanism: ‫الميكانيكية‬ ‫من‬ ‫الكامل‬ ‫المعدل‬ From step #2: (rxn rate = slow step rate) rate = k2 [Io]2 x [H2]1 ‫خطوة‬ ‫من‬2–‫الشكل‬ ‫على‬ ‫يكون‬ ‫المعدل‬ From step #1: Keq = [Io]2 /[I2]1 [Io]2 = Keq[I2]1 Substitute into above: ‫الخطوة‬ ‫من‬1-‫يكون‬ ‫المعدل‬ rate = k2 Keq [I2]1 [H2]1 rate = k [I2]1 [H2]1
  79. 79. IV. Mechanisms ‫الميكانيكيات‬ C) Catalysts ‫الحفازة‬ ‫العوامل‬  Catalyst = A chemical which speeds up a reaction without being consumed in the reaction. :‫تكون‬ ‫ان‬ ‫دون‬ ‫التفاعل‬ ‫تسرع‬ ‫كيميائية‬ ‫مادة‬ ‫المساعد‬ ‫العامل‬ .‫فيه‬ ‫طرفا‬ - They operate by lowering the Ea for the rate determining step. .‫للتفاعل‬ ‫المعينة‬ ‫الخطوة‬ ‫فى‬ ‫التنشيط‬ ‫طاقة‬ ‫بتقليل‬ ‫تعمل‬ - One example is Pt which speeds up the following rxn: :‫التالي‬ ‫التفاعل‬ ‫يسرع‬ ‫وهو‬ ‫البلتين‬ ‫ذلك‬ ‫على‬ ‫مثال‬ CO + 1/2 O2 -----) CO2 - Pt can be used in catalytic converter for your car exhaust. .‫السيارات‬ ‫عادم‬ ‫جهاز‬ ‫فى‬ ‫مساعد‬ ‫محول‬ ‫يستخدم‬ ‫البلتين‬ 79Dr. Wael A. El-Helece
  80. 80. Dr. Wael A. El-Helece 80 Most famous catalysts are proteins called enzymes. .( )‫النزيمات‬ ‫الحفازة‬ ‫العوامل‬ ‫انواع‬ ‫اشهر‬ - Enzymes = extremely specific biochemical catalysts that allow complex reactions to take place in living systems under mild conditions. :‫حدوث‬ ‫على‬ ‫تساعد‬ ‫والتى‬ ‫حيوية‬ ‫عضوية‬ ‫حفازة‬ ‫عوامل‬ ‫النزيمات‬ .‫الحية‬ ‫الكائنات‬ ‫فى‬ ‫التفاعلت‬ ‫معظم‬ - Enzymes are very complex, well designed, and usually have molecular weights in the tens of thousands. .‫باللف‬ ‫نوعي‬ ‫وزن‬ ‫لها‬ ‫تكون‬ ‫وعادة‬ ‫معقدة‬ ‫النزيمات‬ - Their mode of operation uncovered only ~ 60 years ago. ‫حوالي‬ ‫بسيطة‬ ‫مدة‬ ‫من‬ ‫ال‬ ‫محددة‬ ‫غير‬ ‫تاثيرها‬ ‫طريقة‬60.‫سنة‬
  81. 81. VI. Mechanisms C) Catalysts A catalyst speeds up the rxn by lowering the Ea – provides a different mechanism with a lower Ea New Ea 81Dr. Wael A. El-Helece
  82. 82. VI. Mechanisms C) Catalysts 82Dr. Wael A. El-Helece

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