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Chemical Kinetics - Collision theory and rates of reaction

Grade 11IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Collision Theory: For a reaction to occur, reactant particles must collide with each other. These collisions are only 'effective' if they occur with the correct orientation and a minimum kinetic energy known as the Activation Energy (EaE_a).

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Rate of Reaction: This is defined as the change in concentration of a reactant or product per unit time. The standard units are mol dm−3 s−1\text{mol dm}^{-3}\text{ s}^{-1}.

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Activation Energy (EaE_a): The minimum energy that colliding particles must possess to overcome the energy barrier and break chemical bonds, leading to a reaction.

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Maxwell-Boltzmann Distribution: A graph showing the distribution of kinetic energies of particles in a gas or liquid. The area under the curve to the right of EaE_a represents the fraction of particles that have enough energy to react.

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Temperature Influence: Increasing the temperature increases the average kinetic energy of the particles. This results in a much larger fraction of particles having E≥EaE \ge E_a, significantly increasing the rate of reaction.

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Concentration and Pressure: Increasing the concentration of reactants (in solution) or the pressure (in gases) increases the frequency of collisions between particles, thereby increasing the reaction rate.

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Catalysts: A catalyst increases the reaction rate by providing an alternative reaction pathway with a lower activation energy (EaE_a). It is not consumed in the overall reaction.

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Surface Area: For heterogeneous reactions (e.g., a solid reacting with a liquid), increasing the surface area of the solid increases the number of exposed particles available for collision.

📐Formulae

Rate=−Δ[Reactant]ΔtRate = -\frac{\Delta [Reactant]}{\Delta t}

Rate=Δ[Product]ΔtRate = \frac{\Delta [Product]}{\Delta t}

k=Ae−EaRTk = A e^{-\frac{E_a}{RT}}

💡Examples

Problem 1:

During the reaction Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g), the concentration of HClHCl decreases from 2.0 mol dm−32.0\text{ mol dm}^{-3} to 1.2 mol dm−31.2\text{ mol dm}^{-3} over a period of 4040 seconds. Calculate the average rate of reaction with respect to HClHCl.

Solution:

Rate=−[HCl]final−[HCl]initialΔt=−1.2−2.040=0.02 mol dm−3 s−1Rate = -\frac{[HCl]_{final} - [HCl]_{initial}}{\Delta t} = -\frac{1.2 - 2.0}{40} = 0.02\text{ mol dm}^{-3}\text{ s}^{-1}

Explanation:

The rate is determined by the change in concentration divided by time. Since HClHCl is a reactant, we use a negative sign to ensure the rate value is positive.

Problem 2:

Explain, using the Maxwell-Boltzmann distribution, why a small increase in temperature leads to a large increase in the rate of reaction.

Solution:

At a higher temperature T2T_2 (where T2>T1T_2 > T_1), the distribution curve flattens and shifts to the right. While the total area under the curve remains constant, the area to the right of the activation energy EaE_a increases significantly.

Explanation:

Even a small shift in the average kinetic energy means a much larger proportion of molecules now possess energy E≥EaE \ge E_a, leading to a higher frequency of successful collisions.

Collision theory and rates of reaction Revision - Grade 11 Chemistry IB