Q: As the temperature decreases, the rate constant ($k$) decreases. Given the following data at 298 K for the reaction $2HBr(aq) + Ca(OH)_2(aq) \rightarrow CaBr_2(aq) + 2H_2O(l)$ (second order in HBr, zero order in Ca(OH)$_2$), and the activation energy ($E_a$) is 79.3 kJ/mol, what is the rate constant at 500 K? Exp 1: [HBr] = 0.0025 M, [Ca(OH)$_2$] = 0.0050 M, Rate = 0.0221 M/s Exp 2: [HBr] = 0.0075 M, [Ca(OH)$_2$] = 0.0050 M, Rate = 0.0884 M/s Exp 3: [HBr] = 0.0035 M, [Ca(OH)$_2$] = 0.0100 M, Rate = 0.0221 M/s

$7.45 \times 10^{-1}$ M$^{-1}$s$^{-1}$

Question 1

Which of the following statements best describes the energy changes during the evaporation of water at 100°C, given that $H_2O(l) \rightarrow H_2O(g)$ requires 42.1 kJ/mol and the vapor does 2.39 kJ/mol of work expanding into the surroundings?

Accepted Answer

The overall change in energy ($\Delta E$) for water is positive.

Question 2

Using the following enthalpy values, what is the $\Delta H_{	ext{rxn}}$ for the reaction: $\mathrm{Ca(s) + 2H_2O(l) ightarrow Ca^{2+}(aq) + 2OH^-(aq) + H_2(g)}$?

$\Delta H_f^\circ$ (kJ/mol):
Ca$^{2+}$(aq): $-542.8$
OH$^-$ (aq): $-230.0$
H$_2$O (l): $-285.83$
H$_2$ (g): $0$

Accepted Answer

$52.5$ kJ

Question 3

Given the following reactions and enthalpy changes:
1: $C_6H_{12}O_6(s) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l)$, $\Delta H = -2805$ kJ
2: $6LiOH(s) + 6CO_2(g) \rightarrow 2Li_2CO_3(s) + 4LiHCO_3(s) + 3H_2O(l)$, $\Delta H = -1385$ kJ
3: $C_6H_{12}O_6(s) + 12LiOH(s) \rightarrow 6Li_2CO_3(s) + 6LiHCO_3(s) + 6H_2O(l)$, $\Delta H = ?$
What is $\Delta H$ for reaction 3?

Accepted Answer

$-1974$ kJ

Question 4

How much enthalpy ($\Delta H$) is released if 7.0 mol of $H_2$ are combusted according to $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$, $\Delta H = -483.6$ kJ? Is this reaction endothermic or exothermic?

Accepted Answer

$-1,690$ kJ, exothermic

Question 5

The melting point of gallium metal (Ga) is 29.76°C. When a cube of gallium is placed in your hand (37°C), it melts. What is the sign of $q_{	ext{sys}}$ for the gallium?

Accepted Answer

$q_{	ext{sys}}$ is positive

Question 6

When 4.25 g of NH$_4$NO$_3$ is dissolved in 60.0 g of water at 22.0°C, this endothermic process removes 1.37 kJ of energy from the solution. What is the final temperature of the solution? (Assume $c_{	ext{water}} = 4.184$ J/g°C, $MM = 80.0$ g/mol, and a coffee cup calorimeter.)

Accepted Answer

$18.7$ °C

Question 7

In general, as activation energy ($E_a$) increases, the rate constant ($k$) ____.

Accepted Answer

decreases

Question 8

For the reaction $2NO + O_2 \rightarrow 2NO_2$, the following initial rates were measured:
Exp 1: [NO] = 0.090 M, [O$_2$] = 0.050 M, Rate = 0.440 M/s
Exp 2: [NO] = 0.090 M, [O$_2$] = 0.100 M, Rate = 0.440 M/s
Exp 3: [NO] = 0.270 M, [O$_2$] = 0.050 M, Rate = 1.32 M/s
What is the correct rate law for this reaction?

Accepted Answer

Rate = k[NO]$^2$[O$_2$]

Question 9

Which of the following would increase the rate of a reaction?

Accepted Answer

Increasing the concentration of reactants.

Question 10

The half-life ($t_{1/2}$) of N$_2$O$_5$ is 54.6 s and its initial concentration is 0.85 M. Which equation should be used to determine the rate constant ($k$) for the reaction $2N_2O_5 \rightarrow 4NO_2 + O_2$?

Accepted Answer

$k = \frac{0.693}{t_{1/2}}$

Question 11

Which of the following statements about reaction rates is correct?

Accepted Answer

The rate increases when the concentration of reactants is increased.

Question 12

For a reaction that is second order in $A$ and zero order in $B$, which rate law is correct?

Accepted Answer

Rate = k[A]$^2$

Question 13

Given the following data for the reaction $2HBr(aq) + Ca(OH)_2(aq) \rightarrow CaBr_2(aq) + 2H_2O(l)$ at 298 K:
Exp 1: [HBr] = 0.0025 M, [Ca(OH)$_2$] = 0.0050 M, Rate = 0.0221 M/s
Exp 2: [HBr] = 0.0075 M, [Ca(OH)$_2$] = 0.0050 M, Rate = 0.0884 M/s
Exp 3: [HBr] = 0.0035 M, [Ca(OH)$_2$] = 0.0100 M, Rate = 0.0221 M/s
If the reaction is second order in HBr and zero order in Ca(OH)$_2$, what is the rate constant $k$?

Accepted Answer

$0.0221$ M$^{-1}$s$^{-1}$

Question 14

Which of the following graphs best represents the relationship between concentration and time for a reaction that follows the rate law: Rate = k[A]?

Accepted Answer

A straight line with a negative slope for $\ln[A]$ vs. time

Question 15

Consider the following two-step reaction mechanism:
Step 1: Cl$_2$ + O$_3$ $ightarrow$ ClO + O$_2$, $k_1 = 5.2 	imes 10^6$
Step 2: ClO + O $ightarrow$ Cl + O$_2$, $k_2 = 1.37 	imes 10^4$
Which statement is true?

Accepted Answer

Step 2 is the slow step, and Step 1 is the fast step.

Question 16

As the temperature decreases, the rate constant ($k$) decreases. Given the following data at 298 K for the reaction $2HBr(aq) + Ca(OH)_2(aq) \rightarrow CaBr_2(aq) + 2H_2O(l)$ (second order in HBr, zero order in Ca(OH)$_2$), and the activation energy ($E_a$) is 79.3 kJ/mol, what is the rate constant at 500 K?
Exp 1: [HBr] = 0.0025 M, [Ca(OH)$_2$] = 0.0050 M, Rate = 0.0221 M/s
Exp 2: [HBr] = 0.0075 M, [Ca(OH)$_2$] = 0.0050 M, Rate = 0.0884 M/s
Exp 3: [HBr] = 0.0035 M, [Ca(OH)$_2$] = 0.0100 M, Rate = 0.0221 M/s

Accepted Answer

$7.45 	imes 10^{-1}$ M$^{-1}$s$^{-1}$

Question 17

Which of the following best describes the effect of a catalyst on a chemical reaction?

Accepted Answer

It decreases the activation energy and increases the rate of reaction.

Question 18

Which of the following is the correct formula for calculating the change in internal energy ($\Delta E$) of a system?

Accepted Answer

$\Delta E = q + w$

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