Suggested problems Chapter 6
6.25
A small car is travelling at twice the speed of a larger car, which has twice the mass of the smaller car. Which car
has the greater kinetic energy? (Or do they both have the same kinetic energy?)
6.37
A car whose mass is 4.85 x 103 lb is travelling at a speed of 57 miles per hour. What is the kinetic energy of the
car in joules? In calories? (1 mile = 1.609 km; 1 lb = 0.4536 kg).
6.39
Chlorine dioxide, ClO2, is a reddish-yellow gas used in bleaching paper pulp. The average speed of a ClO2
molecule at 25°C is 306 m/s. What is the kinetic energy (in joules) of a ClO2 molecule moving at this speed?
6.41
The process of dissolving ammonium nitrate, NH4NO3, in water is an endothermic process. What is the sign of q?
If you were to add some ammonium nitrate to water in a flask, would you expect the flask to feel warm or cold?
6.43
Nitric acid, a source of many nitrogen compounds, is produced from nitrogen dioxide. An old process for making
nitrogen dioxide employed nitrogen and oxygen.
N2(g) + 2O2(g) → 2NO2(g)
The reaction absorbs 66.2 kJ of heat per 2 mol NO2 produced. Is this reaction endothermic or exothermic? What
is the value of q?
6.47
When white phosphorus burns in air, it produces phosphorus(V) oxide.
P4(s) + 5O2(g) → P4O10(s), ∆H = -3020 kJ
What is the ∆H for the following reaction?
P4O10(s) → P4(s) + 5O2(g)
6.49
Phosphoric acid, H3PO4, can be prepared by the reaction of phosphorus(V) oxide, P4O10, with water.
¼ P4O10(s) + 3/2 H2O(l) → H3PO4 (ac); ∆H = -96.2 kJ
What is the ∆H for the reaction involving 1 mol of P4O10?
P4O10(s) + 6H2O(l) → 4H3PO4 (ac)
6.51
Colorless nitric oxide, NO, combines with oxygen to form nitrogen dioxide, NO2, a brown gas.
2NO(g) + O2(g) → 2NO2(g); ∆H = -114 kJ
What is the enthalpy change per gram of nitric oxide?
6.53
Ammonia burns in the presence of a copper catalyst to form nitrogen gas.
4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(g); ∆H = -1267 kJ
What is the enthalpy change to burn 35.8 g of ammonia?
6.55
Propane, C3H8, is a common fuel gas. Use the following to calculate the grams of propane you would need to
provide 369 kJ of heat
C3H8(g) + 5O2(g) → 2CO2(g) + 3H2O(g); ∆H = -2043 kJ
6.57
You wish to heat water to make coffee. How much heat (in joules) must be used to raise the temperature of
0.180 kg of tap water (enough for one cup of coffee) from 19°C to 96°C (near the ideal brewing temperature)?
Assume the specific heat is that of pure water, 4.18 J/(g · °C).
6.59
When steam condenses to liquid water, 2.26 kJ of heat is released per gram. The heat from 168 g of steam is
used to heat a room containing 6.44 x 104 g of air (20 ft x 20 ft x 8ft). The specific heat of air at normal pressure
is 1.015 J/(g · °C). What is the change in air temperature, assuming the heat from the steam is all absorbed by
the air?
6.65
Hydrazine, N2H4, is a colorless liquid used as a rocket fuel. What is the enthalpy change for the process in which
hydrazine is formed from its elements?
N2(g) + 2H2(g) → N2H4(l)
Use the following reactions and enthalpy changes:
N2H4(l) + O2(g) → N2(g) + 2H2O(l); ∆H = -622.2 kJ
H2(g) + 1/2 O2(g) → H2O(l); ∆H = - 285.8 kJ
6.67
Ammonia will burn in the presence of a platinum catalyst to produce nitric oxide, NO.
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)
What is the heat of reaction at constant pressure? Use the following thermochemical equations:
N2(g) + O2(g) → 2NO(g); ∆H = 180.6 kJ
N2(g) + 3H2(g) → 2NH3(g); ∆H = -91.8 kJ
2H2(g) + O2(g) → 2H2O(g); ∆H = -483.7 kJ
6.69
Compounds with carbon-carbon double bonds, such as ethylene, C2H4, add hydrogen in a reaction called
hydrogenation.
C2H4(g) + H2(g) → C2H6(g)
Calculate the enthalpy change for this reaction, using the following combustion data:
C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l); ∆H = -1411 kJ
C2H6(g) + 7/2O2(g) → 2CO2(g) + 3H2O(l); ∆H = -1560 kJ
H2(g) + ½ O2(g) → 2H2O(g); ∆H = -286 kJ
6.71
The cooling effect of alcohol on the skin is due to its evaporation. Calculate the heat of vaporization of ethanol
(ethyl alcohol), C2H5OH.
C2H5OH(l) → C2H5OH(g); ∆H° = ?
The standard enthalpy of formation of C2H5OH is -277.7(l) kJ/mol and that of C2H5OH(g) is -235.4 kJ/mol.
6.73
Hydrogen sulfide has is a poisonous gas with the odor of rotten eggs. It occurs in natural gas and is produced
during the decay or organic matter, which contains sulfur. The gas burns in oxygen as follows:
2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
Calculate the standard enthalpy change for this reaction using standard enthalpies of formation.
6.75
Iron is obtained from iron ore by reduction with carbon monoxide. The overall reaction is
Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)
Calculate the standard enthalpy for this reaction. See Appendix C for data or use this link.**
6.77
Hydrogen chloride gas dissolves in water to form hydrochloric acid (an ionic solution).
HCl(g)
H+(aq) + Cl-(aq)
Find ∆H° for the above reaction. The data are given in Table 6.2 or you can use this link.**
6.79
The Group IIA carbonates decompose when heated. For example,
MgCO3(s) → MgO(s) + CO2(g)
Use enthalpies of formation (see Appendix C or this link**) and calculate the heat required to decompose 10.0 g
of magnesium carbonate.
** http://bilbo.chm.uri.edu/CHM112/tables/thermtable.htm