Q: Which of the following is the correct formula for calculating the heat ($q$) absorbed or released by a substance?

$q = m \times c \times \Delta T$

Q: Given the following data for the reaction $2\ \mathrm{HBr} + \mathrm{Ca(OH)_2} \rightarrow \mathrm{CaBr_2} + 2\ \mathrm{H_2O}$ at 298 K: Exp 1: [HBr]=0.0025 M, [Ca(OH)2]=0.0050 M, Initial Rate=0.0221 M/s; Exp 2: [HBr]=0.0070 M, [Ca(OH)2]=0.0050 M, Initial Rate=0.0884 M/s; Exp 3: [HBr]=0.0035 M, [Ca(OH)2]=0.0100 M, Initial Rate=0.0221 M/s. What is the rate constant $k$ if the reaction is second order with respect to HBr and zeroth order with respect to Ca(OH)2?

$1.8 \times 10^3\ \mathrm{M}^{-1}\mathrm{s}^{-1}$

Question 1

Which of the following statements best describes the energy changes during the evaporation of water at 100°C?

Accepted Answer

Heat energy is transferred from the surroundings to the water.

Question 2

Given the reaction $\mathrm{Ca(s) + 2H_2O(l) \rightarrow Ca^{2+}(aq) + 2OH^-(aq) + H_2(g)}$ and the following enthalpy values: $\Delta H_f^{\circ}(Ca^{2+}) = -226.7\ \mathrm{kJ/mol}$, $\Delta H_f^{\circ}(OH^-) = -230.0\ \mathrm{kJ/mol}$, $\Delta H_f^{\circ}(H_2O) = -241.8\ \mathrm{kJ/mol}$, $\Delta H_f^{\circ}(H_2) = 0\ \mathrm{kJ/mol}$, what is $\Delta H^{\circ}$ for the reaction?

Accepted Answer

$-233.5\ \mathrm{kJ}$

Question 3

Which of the following is the correct formula for calculating the heat ($q$) absorbed or released by a substance?

Accepted Answer

$q = m 	imes c 	imes \Delta T$

Question 4

If $2\ \mathrm{H_2(g) + O_2(g) \rightarrow 2\ H_2O(l)}$ has $\Delta H = -483.6\ \mathrm{kJ}$, how much enthalpy is released when $1.77\ \mathrm{mol}$ of $\mathrm{H_2}$ is combusted?

Accepted Answer

$-340.0\ \mathrm{kJ}$

Question 5

A cube of gallium metal ($\mathrm{Ga}$) melts in your hand at $29.76^{\circ}\mathrm{C}$. What is the sign of $q_{\mathrm{Ga}}$?

Accepted Answer

$q_{\mathrm{Ga}} > 0$

Question 6

When $4.25\ \mathrm{g}$ of $\mathrm{NH_4NO_3}$ is dissolved in $60.0\ \mathrm{g}$ of water at $22.0^{\circ}\mathrm{C}$, the process removes $1.37\ \mathrm{kJ}$ of energy from the solution. What is the final temperature of the solution? ($c_{\mathrm{water}} = 4.184\ \mathrm{J/g\cdot K}$)

Accepted Answer

$15.0^{\circ}\mathrm{C}$

Question 7

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

Accepted Answer

decreases

Question 8

For the reaction $2\ \mathrm{NO} + \mathrm{O_2} \rightarrow 2\ \mathrm{NO_2}$, which rate law matches the experimental data: Exp 1: [NO]=0.090 M, [O2]=0.050 M, Initial Rate=0.440 M/s; Exp 2: [NO]=0.090 M, [O2]=0.100 M, Initial Rate=0.440 M/s; Exp 3: [NO]=0.270 M, [O2]=0.050 M, Initial Rate=1.32 M/s?

Accepted Answer

Rate = k[NO]^2

Question 9

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

Accepted Answer

Increasing the concentration of reactants (for a 1st or 2nd order process).

Question 10

The half-life ($t_{1/2}$) of $\mathrm{N_2O_5}$ is $54.6\ \mathrm{s}$ and its initial concentration is $0.85\ \mathrm{M}$. Which equation should be used to determine the rate constant ($k$) for a first-order reaction?

Accepted Answer

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

Question 11

Which of the following statements about reaction rates is correct?

Accepted Answer

The rate increases if the concentration of reactants is increased.

Question 12

For a reaction where $A$ and $B$ are reactants, which rate law matches a second-order reaction?

Accepted Answer

$\mathrm{rate} = k[A][B]$

Question 13

Given the following data for the reaction $2\ \mathrm{HBr} + \mathrm{Ca(OH)_2} \rightarrow \mathrm{CaBr_2} + 2\ \mathrm{H_2O}$ at 298 K: Exp 1: [HBr]=0.0025 M, [Ca(OH)2]=0.0050 M, Initial Rate=0.0221 M/s; Exp 2: [HBr]=0.0070 M, [Ca(OH)2]=0.0050 M, Initial Rate=0.0884 M/s; Exp 3: [HBr]=0.0035 M, [Ca(OH)2]=0.0100 M, Initial Rate=0.0221 M/s. What is the rate constant $k$ if the reaction is second order with respect to HBr and zeroth order with respect to Ca(OH)2?

Accepted Answer

$1.8 	imes 10^3\ \mathrm{M}^{-1}\mathrm{s}^{-1}$

Question 14

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

Accepted Answer

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

Question 15

Consider the following reaction mechanism for the decomposition of $\mathrm{Cl_2O}$: Step 1: $\mathrm{Cl_2O ightarrow Cl_2 + O}$, $k_1 = 5.2 	imes 10^2$; Step 2: $\mathrm{Cl_2O + O ightarrow Cl_2 + O_2}$, $k_2 = 1.37 	imes 10^4$. Which statement is true?

Accepted Answer

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

Question 16

As the temperature decreases, what happens to the rate constant ($k$) for a chemical reaction?

Accepted Answer

It decreases.

Question 17

Given the Arrhenius equation $k = A e^{-E_a/(RT)}$, which of the following changes will increase the rate constant $k$?

Accepted Answer

Decrease the activation energy $E_a$.

Question 18

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

Accepted Answer

$\Delta E = q + w$

Question 19

Which of the following statements about enthalpy ($\Delta H$) is correct?

Accepted Answer

A positive $\Delta H$ indicates an endothermic reaction.

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