Chemical reactions - Rate of reaction (3)
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1.
Describe and explain the effect on the rate of reaction of adding a catalyst, including enzymes, using collision theory.
(4 marks)
Adding a catalyst increases the rate of reaction. A catalyst provides an alternative reaction pathway with a lower activation energy. Collision theory explains this by stating that a catalyst provides a surface or intermediate that allows reactant particles to collide more effectively. Instead of reactants colliding directly, they collide with the catalyst, forming an intermediate complex. This complex then breaks down to form the products, releasing the catalyst to participate in further reactions. Because the activation energy is lower, a larger proportion of reactant particles will have sufficient energy to react, resulting in a faster reaction rate. Enzymes are biological catalysts that work in a similar way, providing a specific binding site for reactants, thus lowering the activation energy for the reaction.
Without Catalyst: Activation Energy (Ea) is high. Few collisions lead to product formation. |
With Catalyst: Activation Energy (Ea) is lowered. More collisions lead to product formation. |
2.
The rate of a chemical reaction is affected by various factors. State the role of a catalyst in a chemical reaction, specifically relating it to the energy required for the reaction to take place.
A catalyst increases the rate of a chemical reaction by lowering the activation energy (Ea).
The activation energy is the energy threshold that reactant molecules must overcome for a reaction to occur. A catalyst provides an alternative reaction pathway with a lower Ea.
By lowering the activation energy, a greater proportion of reactant molecules have enough energy to react, resulting in a faster reaction rate.
Crucially, a catalyst is not consumed in the reaction; it participates in the reaction mechanism but is regenerated at the end. Therefore, it does not change the overall enthalpy change of the reaction.
3.
Describe the effect on the rate of reaction of changing the concentration of solutions. Explain your answer using collision theory.
Effect of Changing Concentration: Increasing the concentration of reactants generally increases the rate of reaction. This is because a higher concentration means there are more reactant particles present in a given volume.
Collision Theory Explanation: Collision theory states that for a reaction to occur, reactant particles must collide with sufficient energy (activation energy) and in the correct orientation. Increasing the concentration increases the frequency of collisions between reactant particles. More collisions lead to a higher probability of successful collisions (those with sufficient energy and correct orientation), and therefore a faster rate of reaction. The rate of reaction is directly proportional to the concentration of the reactants (assuming the reaction order is independent of concentration). This can be represented mathematically as rate ∝ concentration.