First slide

Hydrogen

Question

H_2(g) + Cl_2(g) → 2HCl_(g) ...... rate = r₁
D_2(g) + Cl_2(g) → 2DCl_(g) ..... rate = r₂.
The value of r₁ > r₂. Then the correct statement among the following is

Moderate

A

H - H and D - D bond lengths are same
B

H - H has greater bond energy than D - D
C

The activation energy for the first reaction is less than the of the second reaction
D

The activation energy for the first reaction is greater than the second reaction

Solution

The order of bond dissociation energy (BDE):
$\begin{array}{{20}{c}} {D - D\; > \;H - H\;} \\ {\left[ {\;\;{D_{2\;}}\;\;\; > \;\;{H_2}\;\;\;\;} \right]} \end{array}$
The order of bond lengths (B.L):
$\begin{array}{{20}{c}} {D - D\; \approx \;H - H\;} \\ {\left[ {\;\;{D_{2\;}}\;\;\; \approx \;\;{H_2}\;\;\;\;} \right]} \end{array}$
→ The bond dissociation energy of heavier isotopes is greater than that of lighter isotopes.
→ As the bond dissociation energy increases the time required to break the bond in the reaction increases, therefore rate of reaction (r) decreases. i.e,. The reaction which involes heavier isotopes proceeds with slower rate (r₁) than the reaction which involes lighter isotopes(r₁)

$\large {H_{{2_{(g)}}}}\; + \;C{l_{{2_{(g)}}}}\; \to \;2HC{l_{(g)}}\;\;\;{r_1}$

$\large {D_{{2_{(g)}}}}\; + \;C{l_{{2_{(g)}}}}\; \to \;2HC{l_{(g)}}\;\;\;{r_2}$
(1) B.L : H-H ≈ D-D
→ Correct but it is not correct explaination for the rates of above reaction
(2) B.L (or) B.D.E: D-D > H-H
(3)

⇒ [Ea]₂> [Ea]₁ and

Activation Energy:(Ea)
The energy difference between the Activated complex and reactants is called Activation energy.
The reaction with high activation energy proceeds with slow rate

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