1. Ch. 17
THERMOCHEMISTRY 1

2. Thermochemistry
Thermochemistry: study of energy changes that occur during a chemical rxn and changes in state.
Enthalpy (H): heat content of a system (constant pressure)
Heat always flows from warmer to cooler objects. 2

3. System v. Surrounding System: part of universe you are studying
Could be a test tube, entire lab, or planet.
Surrounding: everything else. 3

4. Bond energy
Breaking bonds requires(absorbs) energy while forming bonds releases energy
Energy stored in chemical bonds is chemical potential energy. 4

5. H Delta H = enthalpy or heat of reaction ΔH = H products – H reactants
If products have more stored energy, answer is positive (energy was put into system)
If reactants have more stored energy, answer is negative (energy was released from system)
Units are calories (cal) or Kilojoules (kJ) 5

6. Exothermic Rxns Exothermic System releases energy
Release means loses energy to surroundings
Means it gets warmer to us (we are surroundings)
Number that represents enthalpy is negative 6

7. Exothermic systems Ways to indicate an exothermic system
H = - # in kJ or cal (negative number)
CaO + H2O → Ca(OH) kJ 7

8. Exothermic Rxn 8

9. Endothermic Reactions
Endothermic System absorbs energy from surroundings
Gets cooler to us (we are surroundings)
Or energy must be provided from a heat source (hotplate)
Number that represents enthalpy is positive 9

10. Endothermic Systems Ways to indicate an endothermic system
H = + # in kJ or cal (positive number)
2NaHCO kJ → Na2CO3 + H2O + CO2 10

11. Endothermic Rxn 11

12. Phase change Calculations
When a substance goes through a phase change, a certain amount of energy is released or absorbed. This can be calculated:
Qphase change=mHf or v
The H value is a constant and will be provided. For example: Hf of ice=334 J/g or Hv of water=2260 J/g
Temp is constant!

13. Hf of ice=334 J/g Hv of water=2260 J/g
Hf is used when going from solid to a liquid
Hv is used when going from a liquid to a gas
When going in reverse, the H value is negative

14. Hf of ice=334 J/g Hv of water=2260 J/g
Calculate the energy needed to have 54 grams of water go from liquid to vapor
Calculate the energy needed to have 32 grams of water go from solid to liquid

15. Calculating Heat 15

16. Heat Equation c is also writen as Cp units are : J/goC or cal/goC
1 Calorie=1 kilocalorie=1000 calories
1 Joule (J)= cal
4.184 J = 1 cal 16

17. Phase Change or Heat Graph
Moving left to right(a-e) → is ENDOTHERMIC
Must put energy in to melt/boil
Energy goes into breaking bonds if temp is same
Moving Right to left (e-a) ← is EXOTHERMIC
Energy is release in condensation/freezing
Energy is released as bonds form keeping temp same 17

18. Phase changes Endothermic: melting, boiling
Exothermic: condensation, freezing
(B-C) Enthalpy of fusion: Q= m fusion
(D-E)Enthalpy of vaporization:Q=m vap 18

19. Phase change Equation
For phase changes, use a different equation because there is no temp. change
Q = m fusion for water see back of P.T.
Q = m vap 19

20. Summary of Equations
Depending on what is happening to temp dictates which equation to use. 20

21. 1. What amount of heat is required to raise the temperature of 85
1. What amount of heat is required to raise the temperature of 85.9 g of water by 7°C? The specific heat of water is J/g°C.
2. What amount of heat is required to increase the temperature of 75.0 g of water from 22.3°C to 36.1°C?

22. When 2 different substances of different temperatures are added to one system, the final Q values are the same. When solving the problems:
m1C1T1=m2C2T2
A copper cylinder has a mass of 76.8 g and a specific heat of cal/g·C. It is heated to 86.5° C and then put in 68.7 g of turpentine whose temperature is 19.5° C. The final temperature of the mixture is 31.9° C. What is the specific heat of the turpentine?