Name ______________________________________ Date ____________ Class ___________________
Modern Chemistry • CHAPTER 17
HOMEWORK 17-2
Multiple choice. Select the one best answer.
________1. Elements are not found in tables of standard heats of formation because
A. they are too small too measure.
B. the heats of formation can not be measured with standard tools.
C. the heat of formation of any element in its stable form is 0.
D. that information is not useful in solving problems with thermochemical reactions.
________2. What conditions must be present in order to measure changes in enthalpy in a system?
A. 0C, 1 atm pressure, and specified physical states
B. 0F, 1 atm pressure, specified physical states, and 1 M concentration
C. 25C, 1 atm pressure, specified physical states, 1 M concentration
D. 1 atm pressure, specified physical states, any temperature
________3. Specific heat is usually measured
A. at constant temperature.
B. for a particular volume of a substance.
C. in degrees Celsius or kelvins.
D. at constant pressure.
________4. A 2.5 kg sample of a substance was heated from 113C to 289C. The substance absorbed 0.45 kJ
of heat. What is the specific heat of this substance?
A. 1.02 x 10–3 J/g·K
B. 1.98 x 105 J/g·K
C. 1.02 x 10–6 J/g·K
D. 6.4 x 10–3 J/g·K
________5. The heat of combustion is defined in terms of
A. one mole of reactant.
B. one mole of product.
C. the physical state of the reactant only.
D. the mass of CO2 produced.
________6. Which statement is true?
A. An element in its standard state has a heat of formation with a value less than zero.
B. A compound with a highly negative heat of formation is less stable than a compound with a slightly
negative heat of formation.
C. Most heats of formation are negative values.
D. Heats of formation are given for reactants and products at 0F.
_______7. For elements in their standard state, the value of ΔHfo is ___ .
Name ______________________________________ Date ____________ Class ___________________
Modern Chemistry • CHAPTER 17
HOMEWORK 17-2
8. The formation and decomposition of water can be represented by the following thermochemical equations:
H2(g) + 1/2O2(g) H2O(g) + 241.8 kJ/mol
H2O(l ) + 241.8 kJ/mol H2(g) + 1/2O2(g)
________________a. Is heat being taken in or released as liquid H2O decomposes?
________________b. What is the appropriate sign for the enthalpy change in this decomposition reaction?
PROBLEMS Write the answer on the line to the left. Show all your work in the space provided. Be
sure to show all your work and use correct significant figures and units.
______________9. If 200. g of water at 20°C absorbs 41 840 J of heat, what will its final temperature be?
______________10. Aluminum has a specific heat of 0.900 J/(g•°C). How much energy in kJ is needed to raise the
temperature of a 625 g block of aluminum from 30.7°C to 82.1°C?
______________11. The products in a reaction have a total heat content of 458 kJ/mol and the reactants have a
total heat content of 658 kJ/mol.
______________a. What is the value of ΔH for this reaction?
______________b. Which is the more stable part of this system, the reactants or the products? Explain your
answer.
______________12. The heat of combustion of acetylene gas is -1301.1 kJ/mol of C2H2.
a. Write the balanced thermochemical equation for the complete combustion of C2H2.
______________b. If 0.25 mol of C2H2 react according to the equation in part a, how much heat is released?
______________c. How many grams of C2H2 are needed to react according to the equation in part a to release 3900
kJ of heat?