Thermochemistry

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?

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.

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”

Enthalpy

• Heat absorbed or released by a reaction at constant pressure

H = qq = H = m x C x T

Note: We cannot calculate the actual value of enthalpy, only the change in enthalpy

Thermochemical equation

• Includes the ΔH as either a product or a reactant

CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ

Heat of Reaction

H = Hproducts – Hreactants

Type of Reaction Sign of H Exothermic Negative

(Hproducts < Hreactants)Endothermic Positive

(Hproducts > Hreactants)

Exothermic Reaction 2CO(g) + O2(g) 2CO2(g) H = -566.8 kJ

Or

2CO(g) + O2(g) 2CO2(g) + 566.8 kJ

Negative sign means energy is released

10

Ene

rgy

Reactants Products®

Change is down

ΔH is <0= Exothermic (heat is given off)

11

2CO(g) + O2(g) → 2CO2(g) + 566.8 kJE

nerg

y

Reactants Products®

2CO + O2

2CO2

566.8kJ given off

Endothermic Reaction

2CO2(g) 2CO(g) + O2(g) H = +566.8 kJ

Or

2CO2(g) + 566.8 kJ 2CO(g) + O2(g)

Positive sign means energy is absorbed

13

Ene

rgy

Reactants Products®

Change is upΔH is > 0

= Endothermic (heat is absorbed)

14

CaCO3 → CaO + CO2E

nerg

y

Reactants Products®

2CO2(g)

2CO(g) + O2(g)

566.8 kJ absorbed

2CO2(g) + 566.8 kJ → 2CO(g) + O2(g)

15

Chemistry Happens in

MOLES• An equation that includes energy is called a

thermochemical equation• CH4 + 2O2 ® CO2 + 2H2O + 802.2 kJ

–1 mole of CH4 releases 802.2 kJ of energy.

–When you make 802.2 kJ you also make 2 moles of water

• 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

CH4 + 2O2 ® CO2 + 2H2O; ΔH = -802.2 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 10. 3 grams of CH4 are burned completely, how much heat will be produced?

10. 3 g CH4

16.05 g CH4

1 mol CH4

1 mol CH4

802.2 kJ

= 514 kJ

ΔH = -514 kJ, which means the heat is released for the reaction of 10.3 grams CH4

Ratio from balanced equation

Start with known valueConvert to moles Convert moles to desired unit

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.

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.

Heat of Reaction

CaO(s) + H2O(l) Ca(OH)2(s) ΔH = -65.2 kJEn

thal

py (H

) CaO + H2O

Ca(OH)2

ΔH = -65.2 kJ