1. Measuring and Expressing Heat Changes 17.2

2. 2 Enthalpy The heat content of a substance that has at given temperature and pressure Change in heat content is a change in enthalpy Keeping pressure constant heat and enthalpy are interchangeable  H= m x C x  Change in enthalpy is  H

3. Enthalpy If heat is released the heat content of the products is lower  H is negative (exothermic) If heat is absorbed the heat content of the products is higher  H is positive (endothermic)

4. 4 Energy ReactantsProducts  Change is down  H is <0

5. 5 Energy ReactantsProducts  Change is up  H is > 0

6. 6 Heat of Reaction The heat that is released or absorbed in a chemical reaction Equivalent to  H C + O 2 (g)  CO 2 (g) +393.5 kJ C + O 2 (g)  CO 2 (g)  H = -393.5 kJ H 2 (g) + 1/2O 2 (g) + 241.8 kJ  H 2 O(g) –  H = 241.8KJ In thermochemical equation it is important to say what state

7. Example Problem Calculate the Kilojoules of heat required to decompose 2.24 mol NaHCO 3. 2NaHCO 3 + 129 KJ  Na 2 CO 3 + H 2 O + CO 2 2.24 mol NaHCO3 ( 129 KJ ) 2 mol NaHCO3 = 144 KJ

8. 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  CS 2 ΔH=89.3 KJ How many KJ are produced when 3.4 mol Fe 2 O 3 reacts with an excess of CO? Fe 2 O 3 + 3CO  2 Fe + 3CO 2 + 26.3 KJ

9. 9 Heat of Combustion The heat from the reaction that completely burns 1 mole of a substance Remember: a combustion reaction produces CO 2 + H 2 O