Enthalpy Calculations Review
1. What is energy?
2. What is heat?
3. What is the SI unit for heat?
4. What is an exothermic reaction?
5. What is an endothermic reaction?
6. What is enthalpy?
7. What is the sign of ΔH for an exothermic reaction?
8. What is the sign of ΔH for an endothermic reaction?
9. What are the 3 main assumptions made in calorimetry experiments?
10. Consider the following reaction, which occurs at room temperature and pressure:
2Cl (g)  Cl2 (g) ΔH = -243.4 kJ
a) Which has higher enthalpy under these conditions, 2Cl (g) or Cl2(g)
b) Show this in a potential energy graph
11. Consider the following reaction:
2Mg(s) + O2(g)  2MgO(s) ΔH = -1204 kJ
a) Is this reaction exothermic or endothermic?
b) Calculate the amount of heat transferred when 2.4 g of Mg react
c) Will the surroundings get warmer or colder when the reaction proceeds?
12. If 500 J of heat are added to 100.g sample of each of the substances listed below, which
will have the largest temperature increase?
a)
b)
c)
d)
Gold specific heat = 0.129 J/ g⁰C
Silver specific heat = 0.237 J /g⁰C
Copper specific heat = 0.385 J/ g⁰C
Water specific heat = 4.18 J/ g⁰C
13. If 100.0 J of heat are added to a 150.0 g sample of water at 25.0 ⁰C, what is the final
temperature of the water?
14. Will exothermic reactions feel hot or cold?
15. Will endothermic reactions feel hot or cold?
16. The heat released from the combustion of 0.0500 g of white phosphorus increases the
temperature of 150.0 g of water from 25.0 ⁰C to 31.5 ⁰C. Calculate the value of the
enthalpy change in kJ mol-1 of the combustion of phosphorus.
17. When a 6.50 g sample of solid sodium hydroxide dissolves in 100.0 g of water in a
Styrofoam cup, the temperature rises from 21.6 ⁰C to 37.8 ⁰C. Calculate ΔH (in kJ/mol of
NaOH) for the solution process.
18. Consider the combustion of liquid methanol:
CH3OH (l) + 3/2 O2(g)  CO2 (g) + 2H2O (l) ΔH = -726.5 kJ
a) What is the enthalpy change for the reverse reaction?
b) Balance the forward reaction with whole-number coefficients. What is ΔH for the
reaction represented by this equation?
c) If the reaction were written to produce H2O (g) instead of H2O (l), would you expect
the magnitude of ΔH to increase, decrease, or stay the same? Explain.
19. Given the following enthalpies of reaction:
a) P4(s) + 3O2(g)  P4O6 (s) ΔH = -1640.1 kJ
b) P4(s) + 5O2(g) P4O10(s) ΔH = -2940.1 kJ
Calculate the enthalpy change for P4O6(s) + 2O2(g)  P4O10(s)
20. Given the following enthalpies of reaction:
a) H2(g) + F2(g)  2HF(g) ΔH = -537kJ
b) C(s) + 2F2(g)  CF4(g) ΔH= -680 kJ
c) 2C(s) + 2H2(g)  C2H4(g) ΔH=52.3 kJ
Calculate the enthalpy change for C2H4 (g) + 6F2(g)  2CF4(g) + 4HF(g)
21. What is meant by standard conditions with reference to enthalpy changes?
22. Which of the following does not have a standard heat of formation value of zero at 25 ⁰C
and 1.00 atm?
a. Cl2(g)
b. I2 (s)
c. Br2(g)
d. Na(s)
23. Calculate the enthalpy change for the reactions
1. Fe3O4 (s) + 2C(s)  3Fe(s) + 2CO2 (g)
2. 2NO2(g)  N2O4 (g)
3. 4FeO(s) + O2 (g)  2Fe2O3(s)
Given the following ΔHf
Fe3O4 (s) -118 kJ/ mol
CO2 (g) -394 kJ /mol
NO2(g) 33.2 kJ/ mol
N2O4 (g) 9.2 kJ /mol
FeO(s) -271.9 kJ /mol
Fe2O3(s) -822.16 kJ/mol
24. Is breaking bonds endothermic or exothermic?
25. Is forming bonds endothermic or exothermic?
26. Using bond energies from the textbook, find the enthalpy change for the following
reactions.
a) C2H4 + H2  C2H6
b) 2H2 + O2  2H2O
27. How many grams of water can be heated from 20.0 oC to 75oC using 12500.0 Joules?
28. In a coffee-cup calorimeter, 100.0 mL of 1.0 M NaOH and 100.0 mL of 1.0 M HCl are
mixed. Both solutions were originally at 24.6 oC. After the reaction, the temperature is
31.3 oC. What is the enthalpy change for the neutralization of HCl by NaOH?
29. Calculate the enthalpy change for the formation of lead(IV) chloride by the reaction of
lead (II) chloride with chlorine.
3 PbCl2(s) + 3 Cl2(g)
3 PbCl4(l)
Use the following thermochemical equations :
Pb(s) + 2Cl2(g)
PbCl4(l)
Pb(s) + Cl2
∆H = -329.2 kJ
∆H = +359.4 kJ
PbCl2(s)
Give your answer with the energy term in the equation.
30. Determine the heat of reaction for this equation :
4 NO(g) + 2O2(g)
4 NO2(g)
Use the following reaction data :
1/2N2(g) + 1/2O2(g)
1/2N2(g) + O2(g)
NO(g)
∆H = -90.4 kJ/mol
∆H = +33.6 kJ/mol
NO2(g)
Give the answer with the energy term beside the equation.
31. Calculate the heat of formation for ethane:
C2H4(g) + H2(g)
C2H6(g)
Based on the following information :
2H2(g) + O2(g)
2H2O(l)
C2H4(g) + 3O2(g)
2C2H6(g) + 7O2(g)
2H2O(l) + 2CO2(g)
6H2O(l) + 4CO2(g)
Give the answer with a potential energy diagram.
∆H = -572 kJ
∆H = -1401 kJ
∆H = +3100 kJ
32. Use the standard heat of formation table to calculate the change in enthalpy for each
reaction.
a. C2H4(g) + 3O2(g)
b. CaCO3(s)
2CO2(g) +
2H2O(l)
CaO(s) + CO2(g)
c. C6H6(l) + 9/2O2(g)
d. 4NH3(g) + 5O2(g)
6CO(g) + 3H2O(l)
4NO(g) + 6H2O(g)
33. When 1.37 g of barium reacts with oxygen, there is a release of 5.57 x 103 J of energy.
How much heat would be produced during the formation of 1.00 mol of barium oxide?
34. The heat produced by burning 1.00 g of hydrazine (N2H4) is collected in 3.95 kg of water
in a calorimeter. If the temperature rise is 1.2 ⁰C, calculate the molar heat of combustion
of hydrazine.
35. Octane, (C8H18), is one component of gasoline. The heat produced by burning 1.00 g of
octane is collected in 2.00 kg of water in a calorimeter. The temperature rise was 24.4 ⁰C.
Calculate the molar heat of combustion of octane. Give your answer with the energy
term in the equation.
36. A particular calorimeter contains 3.54 kg of water. When 8.36 g of calcium are made to
react with excess chlorine gas in this calorimeter, the observed temperature rise is 11.2
⁰C. Calculate the molar heat of formation of calcium chloride.
37. A 0.50g sample of NH4NO3 is added to 35.0g of water in a coffee cup calorimeter and
stirred until it dissolves. The temperature of the solution drops from 22.7 ⁰C to 21.6 ⁰C.
What is the heat of solution expressed in kJ/mol of NH4NO3? Give the answer using a
potential energy diagram.
38. A coffee cup calorimeter having a heat capacity of 4.18J/g ⁰C is used to measure the heat
evolved when the following aqueous solutions , both initially at 22.6C are mixed, 100g of
a solution containing 6.62g of lead (II) nitrate, and 100g of solution containing 6.00g of
sodium iodide. The final temperature is 24.2 ⁰C. Assume that the specific heat of the
mixture is the same as that of water.
a) Write a balanced equation for the reaction that occurs
b) Calculate the heat evolved in the reaction.
c) Calculate the ΔH for the reaction per mole of lead (II) nitrate consumed.
d) Give the answer with the energy term outside the reaction equation.
39. Draw Lewis structures for reactants and products. Estimate the enthalpy change (∆Hrxn)
for the reactions using bond energies.
a) H2 (g) + CO2 (g)  H2O (g) + CO (g)
e) H2 (g) + C2H4 (g)  C2H6 (g)
b) 2H2O2 (g)  2H2O (g) + O2 (g)
f) 2C2H6 (g) + 7O2 (g)  4CO2 (g) + 6H2O (g)
c) CO (g) + 2H2 (g)  CH3OH (g)
g) CH4 (g) + 3Cl2 (g)  CHCl3 (g) + 3HCl (g)
d) N2 (g) + 3H2 (g)  2NH3 (g)
h) HCN (g) + 2H2 (g)  CH3NH2 (g)