Name _________________________________________ Date _____________________ Period _____
Enthalpies of Reaction
1. (a) Define
.
(b) what is another term for enthalpy of reaction?
(c) What is the symbol used for enthalpy of reaction?
(d) What are the most common units for enthalpy of reaction?
(e) Enthalpy of reaction is an
- explain what this means in terms of chemical
reactions and equations.
(f) If the amount of reactants are doubled what will happen to the enthalpy of reaction?
(g) How can the enthalpy change for the reverse reaction be determined?
(h) Why is it important to specify the states of the reactants and products in thermochemical equations?
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2. What is an
?
3. When methane is combusted completely to carbon dioxide gas and liquid water the 890 kJ of energy are
released. When methane is combusted completely to carbon dioxide gas and water vapor 802 kJ of energy
are released.

Draw an enthalpy diagram for the combustion of methane.

Put enthalpy on the y-axis and clearly indicate the direction in which enthalpy is increasing or decreasing.

Use chemical formulas (with phases of matter) to indicate the energy of all reactants and products.

Be sure to include magnitude and sign of all enthalpy changes.

Determine, record, and label the difference in enthalpy changes between the two reactions.
4. (a) What is a thermochemical equation?
(b) The decomposition of solid calcium carbonate (limestone) into solid calcium oxide (lime) CaO( s )
and carbon dioxide gas at constant pressure requires the addition of 179 kJ of heat per mole of
solid calcium carbonate. i) Write a balanced thermochemical equation for the reaction.
(c) Draw an enthalpy diagram for the reaction.
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5. Without referring to tables, predict which of the following has the higher enthalpy in each case. Explain
your answer in terms of endothermic or exothermic processes and the sign of ΔH
(a) 1 mol CO
2
(s) or 1 mole CO
2
(g) at the same temperature
(b) 2 mol of hydrogen atoms or 1 mole of H
2
gas
(c) 1 mol of H
2
(g) and 0.5 mol of O
(d) 1 mole nitrogen gas at 100 o
2
(g) at 25
C or 1 mole N
2 o
C or 1 mol H
2
O (g) at 25
(g) at 300 o
C o
C
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6. Consider the following reaction:
CH
3
OH(g) → CO(g) + 2H
2
(g) ΔH = + 90.7 kJ
(a) Is the reaction exothermic or endothermic?
(b) What is the value of ΔH for the reverse process?
CO(g) + 2H
2
(g) → CH
3
OH(g)
(c) Calculate the enthalpy change when 45.0 g of CH
3
OH(g) are decomposed by this reaction at
constantpressure. Follow ALL math work rules!
(d) If the enthalpy change is 16.5 kJ, how many grams of hydrogen gas are produced? Follow ALL
math work rules!
(e) How many kilojoules of heat are released when 10.0 g of CO( g ) reacts completely with H
2
( g ) to
form CH
3
OH( g ) at constant pressure? Follow ALL math work rules!
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7. At one time, a common means of forming small quantities of oxygen gas in the laboratory was to heat solid
potassium chlorate.
2KClO
3
(s) → 2KCl(s) + 3O
2
(g) ΔH = ─ 89.4 kJ
(a) Is the reaction exothermic or endothermic?
For this reaction, calculate ΔH for the formation of each of the following. Follow ALL math work rules!
(b) 1.24 mol of O
2
Follow ALL math work rules!
(c) 4.89 g of KCl Follow ALL math work rules!
9. A gas is confined to a cylinder under constant atmospheric pressure, as illustrated in Figure 5.3 on page168
of the textbook. When 450 J of heat is added to the gas, it expands and does 95 J of work on the
surroundings. a) What is the values of ΔH for this process? Follow ALL math work rules!
b) What is the value of ΔE for this process? Follow ALL math work rules!
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10. Consider the decomposition of liquid benzene, C
6
H
6
(l), to gaseous acetylene, C
2
H
2
(g):
C
6
H
6
(l) → 3 C
2
H
2
(g) ΔH = + 630 kJ
(a) What is the enthalpy change for the reverse reaction?
(b) What is ΔH for the decomposition of 1 mol of benzene to acetylene?
(c) If C
6
H
6
(g) were consumed instead of C
6
H
6
(l), would you expect the magnitude of ΔH to increase,
decrease, or stay the same? Explain.
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