Measuring and Expressing Heat Changes

17.2

2

Enthalpy

The heat content of a substance that has at given temperature and pressureChange in heat content is a change in enthalpyKeeping pressure constant heat and enthalpy are interchangeableH= m x C x Change in enthalpy is H

Enthalpy

If heat is released the heat content of the products is lowerH is negative (exothermic)If heat is absorbed the heat content of the products is higherH is positive (endothermic)

4

Ene

rgy

Reactants Products

Change is down

H is <0

5

Ene

rgy

Reactants Products

Change is upH is > 0

6

Heat of Reaction

The heat that is released or absorbed in a chemical reactionEquivalent to HC + O2(g) CO2(g) +393.5 kJ

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

H2(g) + 1/2O2 (g) + 241.8 kJ H2O(g) H = 241.8KJ

In thermochemical equation it is important to say what state

Example Problem

Calculate the Kilojoules of heat required to decompose 2.24 mol NaHCO3.

2NaHCO3 + 129 KJ Na2CO3 + H2O + CO2

2.24 mol NaHCO3 ( 129 KJ )2 mol NaHCO3

= 144 KJ

You Try

When Carbon dioxide is formed from its elements, heat is absorbed. Calculate the amount of heat (KJ) absorbed when 5.66 g of carbon disuflide is formed.C + 2S CS2 ΔH=89.3 KJHow many KJ are produced when 3.4 mol Fe2O3 reacts with an excess of CO?Fe2O3 + 3CO 2 Fe + 3CO2 + 26.3 KJ

9

Heat of Combustion

The heat from the reaction that completely burns 1 mole of a substanceRemember: a combustion reaction produces CO2 + H2O