1. Thermochemistry

2. DO NOW
The specific heat of ethanol is 2.44 J/g°C. How many kilojoules of energy are required to heat 50.0 g of ethanol from -20.0°C to 68°C?

3. Objective
Describe how calorimeters are used to measure heat flow.
Construct thermochemical equations.
Solve for enthalpy changes in chemical reactions by using heats of reaction.

4. Calorimetry
Calorimetry - the measurement of the heat into or out of a system for chemical and physical processes.
Based on the fact that the heat released = the heat absorbed
The device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”

6. H = q q = H = m x C x T Enthalpy
Heat absorbed or released by a reaction at constant pressure
H = q q = H = m x C x T
Note: We cannot calculate the actual value of enthalpy, only the change in enthalpy

7. Thermochemical equation
Includes the ΔH as either a product or a reactant
CaO(s) + H2O(l)  Ca(OH)2(s) kJ

8. H = Hproducts – Hreactants
Heat of Reaction
H = Hproducts – Hreactants
Type of Reaction
Sign of H
Exothermic
Negative
(Hproducts < Hreactants)
Endothermic
Positive (Hproducts > Hreactants)

9. Exothermic Reaction 2CO(g) + O2(g)  2CO2(g) H = -566.8 kJ Or
2CO(g) + O2(g)  2CO2(g) kJ
Negative sign means energy is released

10. = Exothermic (heat is given off)
Energy
Change is down
ΔH is <0
= Exothermic (heat is given off)
Reactants
Products

11. 2CO(g) + O2(g) → 2CO2(g) + 566.8 kJ Energy Reactants Products ®
2CO + O2
566.8kJ given off
2CO2

12. Endothermic Reaction
2CO2(g)  2CO(g) + O2(g) H = kJ Or 2CO2(g) kJ  2CO(g) + O2(g) Positive sign means energy is absorbed

13. = Endothermic (heat is absorbed)
Energy
Change is up
ΔH is > 0
= Endothermic (heat is absorbed)
Reactants
Products

14. CaCO3 → CaO + CO2 2CO2(g) + 566.8 kJ → 2CO(g) + O2(g) Energy Reactants
Products
2CO(g) + O2(g)
566.8 kJ absorbed
2CO2(g)

15. Chemistry Happens in MOLES 1 mole of CH4 releases 802.2 kJ of energy.
An equation that includes energy is called a thermochemical equation
CH4 + 2O2 ® CO2 + 2H2O kJ
1 mole of CH4 releases kJ of energy.
When you make kJ you also make 2 moles of water

16. The magnitude of ΔH is directly proportional to the amount of reactants or products.
1A + 2 B ----> 1C ΔH = -100 kJ
1/2 A + 1B ----> 1/2 C ΔH = -50 kJ

17. CH4 + 2O2 ® CO2 + 2H2O; ΔH = kJ
Rewrite chemical equation as a thermochemical equation. Exothermic or endothermic reaction? If 3 moles of O2 react with excess CH4 how much heat will be produced?

18. 1CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(l) + 802.2 kJ
If grams of CH4 are burned completely, how much heat will be produced?
Convert moles to desired unit
Convert to moles
Start with known value
1 mol CH4
802.2 kJ
10. 3 g CH4
16.05 g CH4
1 mol CH4
= 514 kJ
Ratio from balanced equation
ΔH = -514 kJ, which means the heat is released for the reaction of 10.3 grams CH4

19. 4 NO(g) + 6 H2O(l)  4 NH3(g) + 5 O2(g) ΔH = +1170 kJ
Based upon the thermochemical equation given, calculate the heat associated with the decomposition of 1.15 g of NO.

20. 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(l) ΔH = -3120 kJ
Calculate the mass of ethane, C2H6, which must be burned to produce 100 kJ of heat.

21. Heat of Reaction CaO + H2O ΔH = -65.2 kJ Ca(OH)2
CaO(s) + H2O(l)  Ca(OH)2(s) ΔH = kJ
CaO + H2O
Enthalpy (H)
ΔH = kJ
Ca(OH)2