1. Stoichiometry Calculations involving Enthalpy
H2(g) + Cl2(g) → 2HCl(g) ΔH = −184.6 kJ How much energy is given off when 8.22 mol of H2 react?

2. Practice
When carbon disulfide is formed from its elements, heat is absorbed. Calculate the amount of heat (in kJ) absorbed when 5.66 g of carbon disulfide is formed.
C (s) + 2S (s)  CS2 (l) ∆H=89.3 kJ

3. Practice
The production of iron and carbon dioxide from iron (III) oxide and carbon monoxide is an exothermic reaction. How many kilojoules of heat are produced when 3.40 moles Fe2O3 reacts with an excess of CO?
Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) kJ

4. Hess’s Law
An indirect way of calculating the enthalpy for a chemical reaction.
The change in enthalpy for any chemical reaction is constant, whether the reaction occurs in one step, or a series of steps.
The overall enthalpy change in a reaction is equal to the sum of the enthalpy changes for the individual steps in the process.

5. Hess’s Law Characteristics of ΔH:
1. If the reaction is reversed, the sign of ΔH is also reversed.
2. If the coefficients in the balanced equation are multiplied (or divided), the value of ΔH is multiplied (or divided).

6. Hess’s Law
1. calculate the enthalpy change for the reaction N2(g) + 2 O2(g)  2 NO2(g) ΔH = _________
using Hess's law and the following information: N2(g) + O2(g)  2 NO(g) ΔH = 180 kJ/mol NO(g) + O2(g)  2 NO2(g) ΔH = -112 kJ/mol

7. Hess’s Law
2. calculate the enthalpy change for the reaction C(s) + O2(g)  2 CO(g) ΔH = _________
using Hess's law and the following information: C(s) + O2(g)  CO2(g) ΔH = kJ/mol CO(g) + O2(g)  2 CO2(g) ΔH = kJ/mol

8. Hess’s Law
3. calculate the enthalpy change for the reaction NH3(g) + 3 N2O(g)  4 N2(g) + 3 H2O(l) ΔH = ________
using Hess's law and the following information: 4 NH3(g) + 3 O2(g)  2 N2(g) + 6 H2O(l) ΔH=-1531 kJ/mol N2O(g) + H2(g)  N2(g) + H2O(l) ΔH = kJ/mol H2(g) + ½ O2(g)  H2O(l) ΔH = kJ/mol