Chapter 11 Notes, part Chapter 11 Notes, part IVIV

Heat and ReactionsHeat and Reactions

Why are some reactions endothermic and others

exothermic?• All compounds have some All compounds have some potential energy stored in their potential energy stored in their bonds.bonds.

• What would happen if there was What would happen if there was more potential energy in the more potential energy in the reactants than in the products? reactants than in the products? Where would that energy go?Where would that energy go?

Into the surroundings!!!

What about…

• If there was more potential energy in the products than in the reactants? Where would that energy come from?

From the surroundings!!!

Heat in ReactionsHeat in Reactions

• Enthalpy (H)—the measure of Enthalpy (H)—the measure of internal heat content of a internal heat content of a substance (stored energy). substance (stored energy).

• The change in enthalpy (The change in enthalpy (H) is H) is the same as heat lost/gained.the same as heat lost/gained.

Heat of ReactionHeat of Reaction

• In a reaction, if In a reaction, if H is negative H is negative the reaction is exothermic.the reaction is exothermic.

• If If H is positive, the reaction is H is positive, the reaction is endothermic.endothermic.

• Why?Why?

Heat of ReactionHeat of Reaction• A A thermochemical equationthermochemical equation is an is an

equation that includes the heat change equation that includes the heat change ((H) that occurs during that reaction.H) that occurs during that reaction.

• The The heat of reactionheat of reaction is the heat change is the heat change for the equation as written.for the equation as written.

NOTE: There is absolutely no reason for this NOTE: There is absolutely no reason for this animation.animation.

Using Heat of Reaction

• Heat of reaction gets used the Heat of reaction gets used the same way as calculating same way as calculating stoichiometry.stoichiometry.

2NaOH + H2SO4 Na2SO4 + 2H2O H = -145kJ

• How much heat would be How much heat would be produced by the reaction of produced by the reaction of 12.2g H12.2g H22SOSO44 with excess with excess NaOH?NaOH?

CaCO3 CaO + CO2 H = +110kJ

• How much heat is required to How much heat is required to decompose 12.5g of CaCOdecompose 12.5g of CaCO33??

• If the temperature is 200If the temperature is 200ooC, C, what volume of gas would be what volume of gas would be generated at 1.00atm with generated at 1.00atm with 1200kJ heat applied?1200kJ heat applied?

Heat of Solution

• Heat that is produced or Heat that is produced or absorbed by the dissolving of absorbed by the dissolving of a substance into water.a substance into water.

• Essentially, it is the same as Essentially, it is the same as the heat of reaction, but since the heat of reaction, but since it is a physical process, it has it is a physical process, it has a different name.a different name.

Practice Problem 3

• How many kilojoules of heat is released when 2500 mol NaOH(s) is dissolved in water?

NaOH(s) + H2O(l) Na+(aq) + OH(aq)

ΔHsoln = –445.1 kJ/mole

Finding H

• H can be found for reactions experimentally, but some reactions are too slow or too difficult to measure.

Rusting is exothermic, but takes too long to easily measure!

Hess’s Law• Hess’s Law says that heat of reaction is

a state function.• In a nutshell, this means that if you add

2 or more thermochemical equations, you can determine the heat of reaction indirectly!

Hess’s Law• Find the heat of reaction when:C(s, diamond) C(s, graphite)

Given that:C(s, diamond) + O2(g) CO2(g) H = -395.4kJ

C(s, graphite) + O2(g) CO2(g) H = -393.5kJ

Practice Problem• Find the heat of reaction for:PbCl2(s) + Cl2(g) PbCl4(l)

• Given that:Pb(s) + 2Cl2(g) PbCl4(l) H = -329.2kJ

Pb(s) + Cl2(g) PbCl2(s) H = -359.4kJ

Finding Ho with Hfo

• We can find the heat of reaction by using heat of formation!

• What do you think heat of formation is?

Ho = Hfo(products) –

Hfo(reactants)

• The heat of reaction is equal to the heat of formation of products minus reactants.

• Remember, heat of formation for elements is usually zero.

Practice Problem• What is the heat of reaction for sulfur

dioxide gas and oxygen gas producing sulfur trioxide gas at 25oC and 101.3kPa?

Practice Problem• What is the heat of reaction for the

combustion of methane (a gas) with oxygen gas to form carbon dioxide gas and water vapor?