(a) The reaction $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(a q)+\( \)\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(a q)$ is first order with in \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}(a q)\) and zero-order in \(\mathrm{H}_{2} \mathrm{O}\). At \(300 \mathrm{~K}\) the rate constant equals \(3.30 \times 10^{-2} \mathrm{~min}^{-1} .\) Calculate the half- life at this temperature. \((\mathbf{b})\) If the activation energy for this reaction is \(80.0 \mathrm{~kJ} / \mathrm{mol}\), at what temperature would the reaction rate be doubled?

Short Answer

Expert verified
The half-life of the reaction at 300 K is approximately 21.0 minutes. The reaction rate would double at a temperature of about 307.2 K.

Step by step solution

01

Part (a): Calculating the half-life at 300 K

In a first-order reaction, the integrated rate equation is given by the formula: \[ ln\frac{[A]_{0}}{[A]_{t}} = kt. \] To find the half-life (t₁/₂), we set \([A]_{t}\) as \(\frac{[A]_{0}}{2}\), since half the reactant remains at the half-life time. Plugging this into the formula, we get: \[ ln\frac{[A]_{0}}{\frac{[A]_{0}}{2}} = kt_{1/2}. \] Now, we are left with: \[ ln2 = kt_{1/2}. \] Solving for t₁/₂, we have: \[ t_{1/2} = \frac{ln2}{k}. \] The rate constant at 300 K is 3.30 x 10⁻² min⁻¹. Therefore, we can calculate the half-life as: \[ t_{1/2} = \frac{ln2}{3.30 × 10^{-2} \,\text{min}^{-1}}. \] Calculating, we get the half-life: \[ t_{1/2} \approx 21.0\, \text{min}. \]
02

Part (b): Calculating temperature for doubled reaction rate

To find the temperature at which the reaction rate doubles, we will use the Arrhenius equation, which is given by: \[ k = Ae^{-\frac{E_{a}}{RT}}, \] where k is the rate constant, A is the pre-exponential factor, Eₐ is the activation energy, R is the gas constant, and T is the temperature in Kelvin. Since the rate doubles, we can write another equation for the new rate constant, k': \[ k' = Ae^{-\frac{E_{a}}{R(T')}}, \] where k' is twice the original rate constant and T' is the new temperature. Now, we will divide the second equation by the first one: \[ \frac{k'}{k} = \frac{Ae^{-\frac{E_{a}}{R(T')}}}{Ae^{-\frac{E_{a}}{RT}}} = \frac{e^{-\frac{E_{a}}{R(T')}}}{e^{-\frac{E_{a}}{RT}}}. \] Since k' = 2k, we can rewrite the equation as: \[ 2 = \frac{e^{-\frac{E_{a}}{R(T')}}}{e^{-\frac{E_{a}}{RT}}}. \] Applying ln on both sides, we get: \[ ln2 = \frac{E_{a}}{RT} - \frac{E_{a}}{R(T')}. \] We are given Eₐ as 80.0 kJ/mol = 80000 J/mol. Also, R = 8.314 J/mol·K. We can now solve for T': \[ T' = \frac{E_{a}RT}{R(E_{a} - ln2RT)} = \frac{80000 × 8.314 × 300}{8.314(80000 - ln2 × 8.314 × 300)}. \] Calculating T', we find the new temperature: \[ T' \approx 307.2\, K. \] In conclusion, the half-life of the reaction at 300 K is approximately 21.0 minutes, and the reaction rate would double at a temperature of about 307.2 K.

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Most popular questions from this chapter

A certain enzyme catalyzes a biochemical reaction. In water, without the enzyme, the reaction proceeds with a rate constant of $6.50 \times 10^{-4} \mathrm{~min}^{-1}\( at \)37^{\circ} \mathrm{C} .$ In the presence of the enzyme in water, the reaction proceeds with a rate constant of $1.67 \times 10^{4} \mathrm{~min}^{-1}\( at \)37^{\circ} \mathrm{C}$. Assuming the collision factor is the same for both situations, calculate the difference in activation energies for the uncatalyzed versus enzyme-catalyzed reaction.

You have studied the gas-phase oxidation of \(\mathrm{HBr}\) by \(\mathrm{O}_{2}\) : $$ 4 \mathrm{HBr}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g)+2 \mathrm{Br}_{2}(g) $$ You find the reaction to be first order with respect to \(\mathrm{HBr}\) and first order with respect to \(\mathrm{O}_{2}\). You propose the following mechanism: $$ \begin{aligned} \operatorname{HBr}(g)+\mathrm{O}_{2}(g) & \longrightarrow \mathrm{HOOBr}(g) \\\ \mathrm{HOOBr}(g)+\mathrm{HBr}(g) & \longrightarrow 2 \mathrm{HOBr}(g) \\ \mathrm{HOBr}(g)+\mathrm{HBr}(g) & \longrightarrow \mathrm{H}_{2} \mathrm{O}(g)+\mathrm{Br}_{2}(g) \end{aligned} $$ (a) Confirm that the elementary reactions add to give the overall reaction. (b) Based on the experimentally determined rate law, which step is rate determining? (c) What are the intermediates in this mechanism? (d) If you are unable to detect HOBr or HOOBr among the products, does this disprove your mechanism?

Indicate whether each statement is true or false. (a) If you measure the rate constant for a reaction al different temperatures, you can calculate the overall enthalpy change for the reaction. (b) Exothermic reactions are faster than endothermic reactions. (c) If you double the temperature for a reaction, you cut the activation energy in half.

The addition of NO accelerates the decomposition of $\mathrm{N}_{2} \mathrm{O}$, possibly by the following mechanism: $$ \begin{aligned} \mathrm{NO}(g)+\mathrm{N}_{2} \mathrm{O}(g) & \longrightarrow \mathrm{N}_{2}(g)+\mathrm{NO}_{2}(g) \\ 2 \mathrm{NO}_{2}(g) & \longrightarrow 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \end{aligned} $$ (a) What is the chemical equation for the overall reaction? Show how the two steps can be added to give the overall equation. (b) Is NO serving as a catalyst or an intermediate in this reaction? (c) If experiments show that during the decomposition of \(\mathrm{N}_{2} \mathrm{O}, \mathrm{NO}_{2}\) does not accumulate in measurable quantities, does this rule out the proposed mechanism?

Urea \(\left(\mathrm{NH}_{2} \mathrm{CONH}_{2}\right)\) is the end product in protein metabolism in animals. The decomposition of urea in $0.1 \mathrm{M} \mathrm{HCl}$ occurs according to the reaction $$ \mathrm{NH}_{2} \mathrm{CONH}_{2}(a q)+\mathrm{H}^{+}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NH}_{4}^{+}(a q)+\mathrm{HCO}_{3}^{-}(a q) $$ The reaction is first order in urea and first order overall. When \(\left[\mathrm{NH}_{2} \mathrm{CONH}_{2}\right]=0.200 \mathrm{M},\) the rate at \(61.05^{\circ} \mathrm{C}\) $$ \text { is } 8.56 \times 10^{-5} \mathrm{M} / \mathrm{s} $$ (a) What is the rate constant, \(k\) ? (b) What is the concentration of urea in this solution after $4.00 \times 10^{3} \mathrm{~s}\( if the starting concentration is \)0.500 \mathrm{M}$ ? (c) What is the half-life for this reaction at \(61.05^{\circ} \mathrm{C}\) ?

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