A teacher demonstrates the reaction between colourless hydrogen gas and purple iodine vapour, which produces colourless hydrogen iodide gas: H2(g) + I2(g) → 2HI(g) The gases are mixed inside a sealed gas syringe, and the fading of the purple colour is used to judge how far the r…

Answer: ((a)) Reducing the volume at constant temperature increases the number of gas particles present in each unit of volume. This means the H2 and I2 particles are, on average, closer together, so they collide with each other more frequently. Since the reaction can only occur when particle… ((b)) Increasing temperature has two separate effects on the particles: (1) the particles move faster (have greater kinetic energy on average), so they collide with each other more frequently; (2) a greater proportion of the particles now possess kinetic energy greater than or equal t… ((c)) The platinum catalyst provides an alternative reaction pathway with a lower activation energy (70 kJ/mol instead of 170 kJ/mol). Since particles now need less energy to react successfully, a much greater proportion of collisions between H2 and I2 particles have enough energy to…. Part (a): Effect of increasing pressure Pushing in the plunger reduces the volume that the same number of gas particles occupies. This increases the number of particles per unit volume of the mixture. With more H2 and I2 particles packed i…

IGCSE Chemistry 0620: Collision Theory. Pressure, Temperature and Catalysts

A teacher demonstrates the reaction between colourless hydrogen gas and purple iodine vapour, which produces colourless hydrogen iodide gas: $\text{H}_2(g) + \text{I}_2(g) \rightarrow 2\text{HI}(g)$

The gases are mixed inside a sealed gas syringe, and the fading of the purple colour is used to judge how far the reaction has progressed.

(a) The teacher pushes in the plunger of the syringe, reducing the volume of the gas mixture and increasing its pressure, while keeping the temperature the same. Explain, using collision theory, why this increases the rate of reaction. [3]

(b) In a separate trial at the original pressure, the teacher instead increases the temperature of the gas mixture. Explain, using collision theory, why increasing the temperature increases the rate of reaction. Your answer should refer to two separate effects of a higher temperature on the reacting particles. [4]

(c) A finely divided platinum catalyst is then added to the gas mixture, at the original pressure and temperature. The activation energy of the uncatalysed reaction is $170\text{ kJ/mol}$ ; with the platinum catalyst present, the reaction instead proceeds by a pathway with an activation energy of only $70\text{ kJ/mol}$ . Explain why this increases the rate of reaction. [2]

Part (a): Effect of increasing pressure

Pushing in the plunger reduces the volume that the same number of gas particles occupies. This increases the number of particles per unit volume of the mixture.

With more $\text{H}_2$ and $\text{I}_2$ particles packed into the same space, the particles are closer together on average, so they collide with each other more frequently.

Since a reaction between $\text{H}_2$ and $\text{I}_2$ can only happen when particles of each collide, a higher frequency of collisions per second means more collisions succeed per second, so the rate of reaction increases.

Part (b): Effect of increasing temperature

Raising the temperature affects the particles in two distinct ways:

Collision frequency increases. At a higher temperature, particles have more kinetic energy on average, so they move faster and collide with each other more often per second.
A greater proportion of collisions have enough energy. Collision theory states that a collision is only successful if the colliding particles have combined energy greater than or equal to the activation energy, $E_a$ . At a higher temperature, a much larger fraction of particles have kinetic energy $\geq E_a$ , so a much larger fraction of collisions result in a reaction.

Because both effects act at the same time (more collisions overall, and a greater proportion of them succeeding) the rate of reaction increases very sharply as temperature rises, more sharply than collision frequency alone would predict.

Part (c): Effect of the platinum catalyst

Without the catalyst, particles need at least $170\text{ kJ/mol}$ of energy to react. The finely divided platinum catalyst provides an alternative reaction pathway with a much lower activation energy of only $70\text{ kJ/mol}$ :

$E_a(\text{catalysed}) = 70\text{ kJ/mol} \; < \; E_a(\text{uncatalysed}) = 170\text{ kJ/mol}$

Since particles now only need to reach this lower energy threshold to react successfully, a much greater proportion of the $\text{H}_2$ – $\text{I}_2$ collisions that occur now have sufficient energy to be successful. More successful collisions per second means the rate of reaction increases. Even though the temperature and pressure have not changed.

Final answers

(a) Higher pressure $\Rightarrow$ more particles per unit volume $\Rightarrow$ more frequent collisions $\Rightarrow$ faster rate.
(b) Higher temperature $\Rightarrow$ (1) more frequent collisions, and (2) a greater proportion of particles with energy $\geq E_a$ $\Rightarrow$ faster rate.
(c) Catalyst lowers $E_a$ from $170\text{ kJ/mol}$ to $70\text{ kJ/mol}$ $\Rightarrow$ a greater proportion of collisions succeed $\Rightarrow$ faster rate.