a work in progress slideshow on chem topics: heat, reactions, equilibrium

1. EnergyEnergy
► Changes form butChanges form but
does not change indoes not change in
amount oramount or
disappear.disappear.

2. EnergyEnergy
Chemical potential energy isChemical potential energy is
stored in chemical bonds of astored in chemical bonds of a
substancesubstance
Cl Cl
Cl2 Bond

3. Heat: Energy is released or absorbedHeat: Energy is released or absorbed
as heat during chemical processesas heat during chemical processes
and reactions.and reactions.

4. Let’s Review What We Know…Let’s Review What We Know…
► Specific Heat formula is:Specific Heat formula is:
 q = mCq = mCΔΔTT
 When do we use this? What for??When do we use this? What for??
► Practice:Practice: Calculate the heat required to raise theCalculate the heat required to raise the
temperature of 1,250 g of ice -25 C to 0 C. Thetemperature of 1,250 g of ice -25 C to 0 C. The
specific heat capacity of ice is 1.84 J/gC.specific heat capacity of ice is 1.84 J/gC.
q = (1,250)(1.84)(25)
q =

5. Now we will see what else weNow we will see what else we
can do with heat…can do with heat…
►During a phase change, we don’t haveDuring a phase change, we don’t have
a change in temperature.a change in temperature.
►Plug in “0” for yourPlug in “0” for your ΔΔT in your specificT in your specific
heat formula. What happens?heat formula. What happens?
►We need a new formula!!!We need a new formula!!!

6. Heat (enthalpy) ofHeat (enthalpy) of
VaporizationVaporization
►Hvap: the heat required to change aHvap: the heat required to change a
liquid to a gas orliquid to a gas or L to G.L to G.
 Formula: q = mFormula: q = mΔΔHHvapvap

7. Heat (enthalpy) of FusionHeat (enthalpy) of Fusion
►Hfus: the heat required to change aHfus: the heat required to change a
liquid to a solid (liquid to a solid (L to a S) or back.L to a S) or back.
 Formula:Formula: q = mq = mΔΔHHfusfus

8. Let’s Practice!Let’s Practice!
►Calculate the amount of energy required toCalculate the amount of energy required to
melt 30.0 grams of ice.melt 30.0 grams of ice.
►Calculate the amount of energy required toCalculate the amount of energy required to
freeze 250 grams of water.freeze 250 grams of water.
►Calculate the amount of energy required toCalculate the amount of energy required to
vaporize 750 grams of liquid water.vaporize 750 grams of liquid water.

10. Heat in ReactionsHeat in Reactions
► Sometimes we want to know how muchSometimes we want to know how much
heat is in a whole reaction.heat is in a whole reaction.
►Two words we will need to know:Two words we will need to know:
 Activation Energy: the energy needed for aActivation Energy: the energy needed for a
rxn to take placerxn to take place
 Catalyst:Catalyst: substance we can add to lower thesubstance we can add to lower the
activation energyactivation energy

11. Two Types of Rxns:Two Types of Rxns:
►If a rxn LOSES HEAT it is EXOTHERMICIf a rxn LOSES HEAT it is EXOTHERMIC
►If a rxn GAINS HEAT it is ENDOTHERMICIf a rxn GAINS HEAT it is ENDOTHERMIC

12. ►Endothermic Rxn VideoEndothermic Rxn Video

14. Exothermic RxnExothermic Rxn
►Reactants higher inReactants higher in
energy than productsenergy than products
►Energy is RELEASED inEnergy is RELEASED in
the form of heatthe form of heat
►EXOTHERMIC reactionEXOTHERMIC reaction
►ΔHrxn = -ΔHΔHrxn = -ΔH
Hproducts – HreactantsHproducts – Hreactants
= negative= negative

16. EndothermicEndothermic
RxnRxn
GAINING heat – heat isGAINING heat – heat is
kept in rxn.kept in rxn.
•Products higher inProducts higher in
energy than reactantenergy than reactant
•Energy is ABSORBEDEnergy is ABSORBED
in the form of heatin the form of heat
•ENDOTHERMICENDOTHERMIC
reaction (ENDO=inside)reaction (ENDO=inside)
ΔHrxn =ΔHrxn = ++ΔΔHH
Hproducts – HreactantsHproducts – Hreactants
= positive= positive

17. 5 Factors that affect reaction rates:5 Factors that affect reaction rates:
1.1. ConcentrationConcentration
2.2. ReactivityReactivity
3.3. Surface AreaSurface Area
4.4. TemperatureTemperature
5.5. Catalyst (lower activation energy)Catalyst (lower activation energy)

18. 1. Concentration1. Concentration
► Think Molarity.Think Molarity.
► The higher theThe higher the
concentration of theconcentration of the
reactants, the largerreactants, the larger
the reaction.the reaction.
How could we make wateredHow could we make watered
down Kool-Aid taste better?down Kool-Aid taste better?

19. 2. Reactivity2. Reactivity
► How “reactive” are theHow “reactive” are the
elements?elements?
► The more reactive theyThe more reactive they
are, the bigger theare, the bigger the
reaction!reaction!

20. 3. Surface Area3. Surface Area
► The more surfaceThe more surface
area, the more placesarea, the more places
for a reaction to takefor a reaction to take
place. Moreplace. More
COLLISIONS.COLLISIONS.
► 3GB to 8GB3GB to 8GB

21. 4. Temperature4. Temperature
►Why do we heatWhy do we heat
things in Chemistrythings in Chemistry
Class?Class?
►Increase in tempIncrease in temp
increases numberincreases number
of collisions.of collisions.
►Faster particles =Faster particles =
more collisions.more collisions.

22. 5. Catalyst5. Catalyst
►Something we addSomething we add
to make somethingto make something
happen FASTER.happen FASTER.
►Gives rxn lowerGives rxn lower
activation energy –activation energy –
the rxn doesn’tthe rxn doesn’t
need as muchneed as much
energy to start theenergy to start the
rxn.rxn.

23. Quick Review…Quick Review…

25. ►Reversible reaction: can take place in theReversible reaction: can take place in the
forward and reverse directionsforward and reverse directions
 aA + bB cC + dDaA + bB cC + dD
Chapt 18 Key PointsChapt 18 Key Points

26. EquilibriumEquilibrium
►When the forward and reverse directionsWhen the forward and reverse directions
have equal rates and the concentrations ofhave equal rates and the concentrations of
the reactants/products don’t changethe reactants/products don’t change
anymore.anymore.
►Equilibrium Constant (KEquilibrium Constant (Keqeq))

27. Le Chatlier’s principleLe Chatlier’s principle
► How equilibrium can shift in response to a stressHow equilibrium can shift in response to a stress
or disturbance.or disturbance.
 If you increase any reactant the equilibrium will shiftIf you increase any reactant the equilibrium will shift
towards the products (right or in the forward direction).towards the products (right or in the forward direction).
 If you increase any product the equilibrium will shiftIf you increase any product the equilibrium will shift
towards the reactants (left or in the reverse direction).towards the reactants (left or in the reverse direction).
 An increase in pressure will cause the equilibrium toAn increase in pressure will cause the equilibrium to
shift towards the direction with the most # of moles of ashift towards the direction with the most # of moles of a
gas.gas.

28. ►Equilibrium Constant (KEquilibrium Constant (Keqeq))
►aA + bB cC + dDaA + bB cC + dD
►KKeqeq == [C][C]cc
[D][D]dd
[A][A]aa
[B][B]bb
►Ex)Ex)
 NN22(g) + 3H(g) + 3H22(g)(g) 2NH2NH33(g)(g)
 KKeqeq = [NH= [NH33]]22
[N[N22][H][H22]]33