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Energy changes
1.
Chapter 15
Energy Changes
2.
3.
Expt I
: Addition of NaOH to water Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Sodium hydroxide pellets are added to water. The mixture is carefully stirred to dissolve the pellets. The temperature of the water is recorded before and after adding sodium hydroxide.
4.
Expt II :
Addition of NH 4 C l to water Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Ammonium chloride crystals are added to water. The mixture is carefully stirred to dissolve the pellets. The temperature of the water is recorded before and after adding ammonium chloride.
5.
Results of Both
Experiments Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Experiment Solid Initial temperature (°C) Final temperature (°C) I NaOH 28 34 II NH 4 C l 28 22
6.
What conclusions about
energy changes can be made from these 2 experiments? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. In experiment I, heat energy was given out when the solid dissolved in water. Thus, the temperature of the solution rose . We say this change is exothermic .
7.
What conclusions about
energy changes can be made from these 2 experiments? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. In experiment II, heat energy was taken in when the solid dissolved in water. Thus, the temperature of the solution dropped . We say this change is endothermic .
8.
9.
Initially, the temperature
of the reaction mixture rises until the highest temperature is reached. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. How does temperature change in an exothermic reaction? When the reaction is completed, the temperature of the reaction mixture falls until it reaches room temperature.
10.
Energy Level Diagram
– Exothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. This means that the total energy of the products is less than that of the reactants. Hence, the energy level of the products is lower. Consider an exothermic reaction. Heat is given out to the surroundings. reactants heat of reaction (Δ H = negative) products Energy level
11.
12.
How does temperature
change in an endothermic reaction? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Initially, the temperature of the reaction mixture falls until the lowest temperature is reached. When the reaction is completed, the temperature of the reaction mixture rises until it reaches room temperature.
13.
Energy Level Diagram
– Endothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. This means that the total energy of the products is more than that of the reactants. Consider an endothermic reaction. Heat is taken in from the surroundings. reactants heat of reaction (Δ H = positive) products Energy level
14.
15.
All reactions need
energy in order to get started. For example, a mixture of hydrogen and oxygen will not explode unless it is ignited. Many reactions that do not occur at room temperature will proceed rapidly when the reactants are heated. Heat provides the energy to change less energetic particles into more energetic particles and thus to start a reaction. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Energy changes in actual reactions
16.
Copyright © 2006-2011
Marshall Cavendish International (Singapore) Pte. Ltd. The minimum energy that reacting particles must possess in order for a chemical reaction to occur is called the activation energy, E a . What is activation energy?
17.
Energy Profile Diagram
for an Exothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. reactants activation energy ( E a ) enthalpy change (Δ H is negative) products Energy Progress of reaction
18.
Energy Profile Diagram
for an Endothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. reactants activation energy ( E a ) enthalpy change (Δ H is positive) products Progress of reaction Energy
19.
What causes energy
changes in chemical reactions? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. The breaking of bonds or the making of new bonds during a reaction leads to energy changes.
20.
When bonds
between atoms are broken , heat energy is absorbed . Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. When bonds are formed between atoms, heat energy is given out . • Bond breaking is an endothermic process. • Bond formation is an exothermic process. Most reactions involve both bond breaking and bond making. What causes energy changes in chemical reactions?
21.
How can you
tell if a reaction is exothermic or endothermic? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Exothermic reaction: Δ H bond breaking < Δ H bond making Endothermic reaction: Δ H bond breaking > Δ H bond making
22.
How do we
determine whether a reaction is exothermic or endothermic? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Is the reaction 2H 2 (g) + O 2 (g) 2H 2 O(g) exothermic or endothermic? To find out, we calculate the energy involved in bond breaking and bond forming in the reaction.
23.
Calculating the
Δ H bond breaking Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Energy required to break 1 mol of H–H bonds = +436 kJ (given) 2H 2 (g) + O 2 (g) 2H 2 O(g) Energy required to break 2 mol of H–H bonds = 2 × (+436) kJ = +872 kJ Δ H bond breaking = (+872) + (+496) = +1368 kJ Energy required to break 1 mol of O=O bonds = +496 kJ (given) “ +” because bond breaking has positive enthalpy.
24.
Calculating the
Δ H bond making Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Each molecule of H 2 O is made up of 2 O–H bonds 2H 2 (g) + O 2 (g) 2H 2 O(g) Energy required to make 4 mol of O–H bond = 4 × ( −926) kJ = −3704 kJ Δ H bond making = −3704 kJ Energy liberated to make 1 mol of O–H bonds = − 926 kJ (given) *** Overall, Δ H reaction = (+1368) + (−3704) kJ = −2336 kJ Hence, the reaction is exothermic . “ – ” because bond breaking has negative enthalpy.
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