Lesson: Chapter - 9

Enthalpy:Practice Questions


1. Statement I Statement II
The reaction shown above is exothermic. BECAUSE   The total enthalpy of the products in this reaction is less than that of the reactants.
2. Use the bond energies given below to estimate the enthalpy, DH, for the following reaction:

(A) DH = -800 kJ
(B) DH = -680 kJ
(C) DH = -150 kJ
(D) DH = +150 kJ
(E) DH = +200 kJ
3. Each of two solutions are mixed separately, and both solutions are found to be the same temperature. The two solutions are mixed, and a thermometer shows that the mixture’s temperature has decreased in temperature. Which of the following statements is true?
(A) The chemical reaction is exothermic.
(B) The chemical reaction is absorbing energy.
(C) The chemical reaction is releasing energy.
(D) The energy released could be found by multiplying the temperatures together.
(E) The energy absorbed by the solution is equal to the difference in temperature of the solutions.
4. Consider the reaction below. When a 45.00 gram sample of ethanol is burned with excess oxygen, about how much energy is released as heat?
C2H5OH(l) + 3O2(g) ? 2CO2(g) + 3H2O(l) ?H = -1.40 × 103 kJ
(A) 0.995 kJ
(B) 5.1 × 102 kJ
(C) 1.40 × 103 kJ
(D) 2.80 × 103 kJ
(E) 5000 kJ
5. In neutralizing 500 mL of 1.0 M HCl with 500 mL of 1.0 M NaOH, the temperature of the solution rises 5.0ºC. Given that the density of the solution is 1.0 g/mL and the specific heat of the solution is 4.184 J/g ºC, calculate the approximate energy released from this experiment.
(A) 20 J
(B) 1000 J
(C) 4200 J
(D) 2.1 × 104 J
(E) 1.0 × 104 kJ
6. Based on the relationship of entropy to the degree of disorder of a system, which response includes all the occurrences listed that represent a decrease in entropy?
  I. The freezing of water  II. The vaporization of water III. Sublimation (vaporization) of dry ice, solid CO2 IV. The extraction of Mg and pure water from seawater
(A) I and II
(B) II and IV
(C) I and IV
(D) III
(E) II and III
7. Spontaneous reactions are driven by
(A) Low enthalpy values and high entropy values
(B) Low enthalpy values and low entropy values
(C) High enthalpy values and low entropy values
(D) High enthalpy values and high entropy values
(E) High temperatures and low pressures
8. If an exothermic process is spontaneous, which of the following statements must be true?
(A) DG must be positive.
(B) DS must be positive.
(C) DS must be negative.
(D) The temperature must be over 500K.
(E) DG must be negative.
9. Consider the following hypothetical reaction (at 375K). The standard free energies in kJ/mol are given below each substance in parentheses.What is the value of the Gibb’s free energy for the reaction at this temperature? Is the reaction spontaneous?
(A) -220; yes
(B) -180; no
(C) -160; yes
(D) +180; no
(E) -160; no
10. Which of the following events is least likely to occur with an increase in temperature for the reaction given? (WithH= -45.9 kJ/mol.)
N2+3H2 ?3NH3
(A) The gas particles will move more quickly.
(B) The reaction will produce more ammonia in a shorter time.
(C) The reaction will reverse and ammonia will decompose.
(D) The entropy of the system will increase.
(E) The equilibrium constant will become smaller.
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