Chem 121 Winter 2015: Section 03, Sample Problems
Thermochemistry
Problems, some adapted, from Chemistry - The Central Science, 3rd edition.
14.2 The contents of the closed box in each of the following illustrations represents a system and
the arrows show the changes to the system during some process. The lengths of the arrows indicate
the relative magnitude of q and w. (a) Which of these processes, if any, is endothermic? (b) For
which if these processes, if any, is U<0? (c) For which process, if any, is there a net gain in internal
energy?
14.16 (a) State the first law of thermodynamics. (b) How is this law represented as an equation?
(c) What is meant by the internal energy of a system? (d) How is internal energy stored within
molecules? (e) By what means can the internal energy of a system increase?
14.18 Calculate U and determine whether the process is exothermic or endothermic for the
following cases: (a) A system absorbs 85 kJ of heat from its surroundings while doing 29 kJ of work
on the surroundings; (b) q = 1.50 kJ and w = - 657 J; (c) the system releases 57.5 kJ of heat while
doing 13.5 kJ of work on the surroundings.
14.20 (a) What is meant by the term state function? (b) Give one example of a quantity that is a
state function and one example that it is not. (c) Is work a state function? Why or why not?
14.25 Which has the higher enthalpy in each case? (a) 1 mole of CO2 (g) or 1 mol CO2 (l);
(b) 3 mol CO2 or 5 mol CO2 ; (c) I mole of H2 and ½ mol O2 at 25oC or 1 mol water at 25oC.
14.26 Consider the following reaction:
CH3OH (g)  CO(g) + 2H2(g) H = +90.7 kJ
(a) Is heat evolved or absorbed during the reaction? (b) Is the reaction exothermic or endothermic?
(c) Calculate the amount of heat released or absorbed when 45g of methanol decomposes at
constant pressure. (d) For a given sample of methanol the enthalpy change on reaction is 18.5 kJ;
how many grams of hydrogen are produced? (e) What is H for the reverse reaction?
14.42 The enthalpy of the reaction H2(g) + F2(g)  2HF (g) is H = - 537 kJ. The standard enthalpies
of formation of CF4 and C2H4 (ethene) are -680 kJ and +52.3 kJ respectively. Calculate the enthalpy
of the reaction between ethene and fluorine to give CF4 (g) and HF.
14.44 What is meant by the following terms: (a) standard conditions; (b) enthalpy of formation;
(c) standard enthalpy of formation?