5PROBLEMS

After you have read the information about Hess’s Law and viewed the Practice Problems, you are ready to try some problems on your own. SAMPLE:

Hess's Law, also known as Hess's Law of Constant Heat Summation, states that the total enthalpy of a chemical reaction is the sum of the enthalpy changes for the steps of the reaction. So, you can find enthalpy change by breaking a reaction into component steps that have known enthalpy values.

Solve the problems below showing ALL YOUR WORK!

1. Calculate the enthalpy for the following reaction:

N2(g)+2O2(g)2NO2 (g) H=???kJ

Using the following two equations:

N2(g)+O2(g)2NO(g) H = +180kJ

2NO2 (g)2NO(g)+O2(g) H = +112kJ

2. Calculate Ho for this reaction:

2N2 (g)+5O2 (g)2N2O5(g)

using the following three equations:

H2 (g)+(1/2) O2 (g)H2O(l) H = -285.8kJ

N2O5(g)+H2O(l)2HNO3(l) H = -76.6kJ

(1/2)N2 (g)+(3/2)O2 (g)+(1/2)H2(g)HNO3(l) H = - 174.1kJ

3. Calculate Hf for this reaction:

6C(s)+6H2(g)+3O2(g)C6H12O6(s)

using the following three equations:

C(s)+O2(g)CO2 (g) H= -393.51 kJ

H2(g)+(1/2)O2(g)H2O(l) H= -285.83 kJ

C6H12O6(s)+6O2 (g)6CO2 (g)+6H2O(l) H= -2803.02 kJ

4. Given the following reactions and their enthalpy changes, calculate the enthalpy change for

2NO2(g)N2O4(g)

N2(g)+2O2(g)2NO2 (g) +67.8kJ

N2(g)+2O2(g)N2O4(g) +9.67kJ

5. Use the thermochemical equations shown below to determine the enthalpy for the reaction:

C2H6O(l)+3O2(g)2CO2(g)+3H2O(l)

2C2H4O(l)+2H2O(l)2C2H6O(l)+O2(g) H= +203.5kJ

2CO2 (g)+2H2O(l)C2H4O(l)+5/2O2(g) H= +583.5kJ

Save and Submit to 6.03 Hess’s Law Problems