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Sia

青州's Question
Chemistry
Posted 7 months ago

C2.
A reversible reaction between hydrogen and iodine occurs as follows:
H2( g)+I2( g)2HI(g)Kc=49.0 at 700 K
\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~g}) \leftrightharpoons 2 \mathrm{HI}(\mathrm{g}) \quad \mathrm{K}_{\mathrm{c}}=49.0 \text { at } 700 \mathrm{~K}


Where Kc\mathrm{K}_{c} is the equilibrium constant.
(a) If a 10 L10 \mathrm{~L} reaction vessel is fed with 1 mole1 \mathrm{~mole} of H2\mathrm{H}_{2} and 1 mole21 \mathrm{~mole}_{2} at 700 K700 \mathrm{~K}, calculate the equilibrium concentrations of all components?
(b) If kr=1.20×103M1 s1\mathrm{k}_{\mathrm{r}}=1.20 \times 10^{-3} \mathrm{M}^{-1} \mathrm{~s}^{-1} at 700 K700 \mathrm{~K}, determine the value of kt\mathrm{k}_{\mathrm{t}} at the same temperature? Where kf\mathrm{k}_{\mathrm{f}} is the forward reaction rate constant and kr\mathrm{k}_{r} is the reverse rate constant.
(c) If the forward reaction is endothermic, explain with justification how an increase in temperature effects the values of Kc,kf\mathrm{K}_{\mathrm{c}}, \mathrm{k}_{\mathrm{f}} and kr\mathrm{k}_{\mathrm{r}} ?
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Answer from Sia
Posted 7 months ago
Solution
1
Set up the equilibrium expression: The equilibrium constant expression for the reaction H2(g)+I2(g)2HI(g)\mathrm{H}_{2}(g) + \mathrm{I}_{2}(g) \leftrightharpoons 2 \mathrm{HI}(g) is given by Kc=[HI]2[H2][I2]K_c = \frac{[\mathrm{HI}]^2}{[\mathrm{H}_2][\mathrm{I}_2]}
2
Calculate initial concentrations: The initial concentrations of H2\mathrm{H}_2 and I2\mathrm{I}_2 are both 1 mole10 L=0.1 M\frac{1 \text{ mole}}{10 \text{ L}} = 0.1 \text{ M}. The initial concentration of HI\mathrm{HI} is 0 M0 \text{ M} since it has not yet formed
3
Define change in concentrations: Let xx be the change in concentration of H2\mathrm{H}_2 and I2\mathrm{I}_2 at equilibrium. Then the change in concentration of HI\mathrm{HI} is 2x2x
4
Write the equilibrium concentrations: At equilibrium, [H2]=[I2]=0.1x[\mathrm{H}_2] = [\mathrm{I}_2] = 0.1 - x and [HI]=2x[\mathrm{HI}] = 2x
5
Apply the equilibrium constant: Substitute the equilibrium concentrations into the equilibrium expression to get 49.0=(2x)2(0.1x)(0.1x)49.0 = \frac{(2x)^2}{(0.1 - x)(0.1 - x)}
6
Solve for x: Solve the quadratic equation to find the value of xx
7
Find equilibrium concentrations: Use the value of xx to calculate the equilibrium concentrations of H2\mathrm{H}_2, I2\mathrm{I}_2, and HI\mathrm{HI}
1 Answer
[Insert equilibrium concentrations here]
Key Concept
Equilibrium concentrations are determined by applying the equilibrium constant to the changes in concentrations of reactants and products.
Explanation
The equilibrium constant expression relates the concentrations of reactants and products at equilibrium. By setting up a system of equations based on the stoichiometry of the reaction and the equilibrium constant, we can solve for the unknown concentrations.
Solution
1
Use the equilibrium constant expression: The equilibrium constant KcK_c is related to the rate constants of the forward (kfk_f) and reverse (krk_r) reactions by the equation Kc=kfkrK_c = \frac{k_f}{k_r}
2
Solve for the reverse rate constant: Rearrange the equation to solve for krk_r: kr=kfKck_r = \frac{k_f}{K_c}
3
Substitute known values: Substitute kf=1.20×103 M1s1k_f = 1.20 \times 10^{-3} \text{ M}^{-1}\text{s}^{-1} and Kc=49.0K_c = 49.0 into the equation to find krk_r
2 Answer
[Insert value of krk_r here]
Key Concept
The reverse rate constant can be calculated from the equilibrium constant and the forward rate constant.
Explanation
Knowing the equilibrium constant and the forward rate constant allows us to find the reverse rate constant by rearranging the relationship between these constants.
Solution
1
Understand Le Chatelier's Principle: An increase in temperature for an endothermic reaction shifts the equilibrium to favor the formation of products, increasing KcK_c
2
Predict the effect on rate constants: According to the Arrhenius equation, an increase in temperature increases the rate constants kfk_f and krk_r, but the increase in kfk_f will be more significant for an endothermic reaction
3 Answer
An increase in temperature will increase KcK_c, kfk_f, and krk_r, with a more significant increase in kfk_f for an endothermic reaction.
Key Concept
Temperature changes affect equilibrium constants and rate constants differently depending on the endothermic or exothermic nature of the reaction.
Explanation
For an endothermic reaction, increasing temperature shifts the equilibrium towards products, increasing KcK_c. The rate constants kfk_f and krk_r also increase with temperature, but the forward rate constant increases more due to the reaction absorbing heat.

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