1. Change in Enthalpy
Unit 11

2. Learning Objective
You should be able to define and calculate the enthalpy of a reaction.
You should be able to identify whether a reaction is endothermic or exothermic from a chemical reaction or graph.

3. Chemical Changes
Chemical reactions involve the breaking of existing bonds (reactants) and the formation of new bonds (products).
Breaking of bonds requires energy.
Formation of bonds releases energy.
CH4 + Cl2  CH3Cl + HCl
+ Cl-Cl H-Cl +

4. Enthalpy Measure of the heat content of the system
The change in enthalpy for a reaction lets us know if energy is being taken in or if energy is being released.

5. Endothermic Reactions
Reaction consumes energy
Energy is listed on the reactant (left) side of the reaction
ΔH is a positive value
Products have more enthalpy (energy) than the reactants

6. Endothermic Reactions
The endothermic reaction absorbs heat; energy is a reactant 6

7. Exothermic Reactions Reaction releases energy
Energy is listed on the product (right) side of the reaction
ΔH is a negative value
Reactants have more enthalpy (heat) than the products

8. Exothermic Reactions
The exothermic reaction releases heat; energy is a product 8

9. Changes in Enthalpy ΔH
To measure and study energy changes in reactions, chemists defined a property called enthalpy (H)
Enthalpy is a measure of heat energy of a system at constant pressure
It can’t be measured directly, we measure the change in enthalpy. ΔHrxn
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10. 2 NaHCO3(s) + 129 kJ  Na2CO3(s) + H2O(g) + CO2(g)
Find the energy required to react 1mole of NaHCO3

11. 2 NaHCO3(s) + 129 kJ  Na2CO3(s) + H2O(g) + CO2(g)
1.Find the energy required to produce ½ mole of CO2(g) ? 2.Find the energy required to produce 1/3 mole of H2O(g) ?

12. Enthalpy Question
For the decomposition of hydrogen peroxide, it is known that: H2O2(l) → H2O(l) + 1/2 O2(g)
ΔH = kJ Using this information, determine ΔH for the reaction: 2 H2O(l) + O2(g) → 2 H2O2(l)

13. Find ΔH for each given the pair of equations
CaO(s) + H2O(l)  Ca(OH)2(s) kJ ½ Ca(OH)2(s)  ½ CaO(s) + ½ H2O(l)

14. Calculating Changes in Enthalpy
Change in enthalpy is called heat of reaction ΔHrxn
ΔHrxn = ΣHf(products) - ΣHf(reactants)
“Δ Hf0” is called standard heat of formation - the change in enthalpy from the formation of 1 mole of a compound from its elements
This information will come from a table
“Δ” is the letter delta – it means “change”
“Σ” is the letter sigma – it means “sum”
“ o ” refers to standard temperature and pressure (STP)
Units: kJ/mol
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15. Change in Enthalpy
All elements in their standard states have an enthalpy of zero
Because there is no change involved in their formation
Endothermic
When N2 and O2 combine to form NO2, the ΔH = kJ/mol
Exothermic
When S and O2 combine to form SO3, the ΔH = -396 kJ/mol.

16. Calculate ΔH for the following reaction: 8 Al(s) + 3 Fe3O4(s) --> 4 Al2O3(s) + 9 Fe(s)

17. 8 Al(s) + 3 Fe3O4(s) --> 4 Al2O3(s) + 9 Fe(s)
ΔH = Σ ΔHf products - Σ ΔHf reactants
ΔH = sum of ΔHf products – sum of ΔHf reactants :
ΔH = (0) + 4 ΔHf Al2O3(s) - 3 ΔHf Fe3O4(s) + (0)

18. The values for ΔHf may be found in the Heats of Formation of Compounds table. Plugging in these numbers:
ΔH = 4( kJ) - 3( kJ)
ΔH = kJ

19. Enthalpy Change Problem
Use standard heat of formation to calculate ΔHrxn for the combustion of methane. Then, decide if the reaction is exothermic or endothermic.
Molecule
Standard Heat of Formation ΔHf0
CH4 (g)
-75 kJ/mole
C2H2 (g)
227 kJ/mole
C2H6 (g)
-85 kJ/mole
C6H12O6 (s)
-1268 kJ/mole
O2 (g)
0 kJ/mole
H2 (g)
CO2 (g)
-394 kJ/mole
H2O (l)
-286 kJ/mole
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)