39 7.5 Rates of Reaction

Figures

Figure 7.5.3: Collusion visualisations

A diagram of the concept of collision theory in chemical reactions. At the top is the general reaction A + BX yields B + AX. Below, two sequences represent molecular interactions: The first sequence illustrates an ineffective collision where molecule A approaches molecule BX but does not result in a reaction, as indicated by the text ineffective collision text and an X mark. The second sequence shows an effective collision where molecule A approaches molecule BX from an orientation that allows a reaction to occur, leading to the formation of molecules B and AX, labeled with the green effective collision text and a check mark, signifying a successful reaction outcome.

Figure 7.5.4: Potential energy and activiation energy

Graphical illustration of the course of a chemical reaction. The vertical axis is labeled potential energy indicating the energy of the molecules, while the horizontal axis is labeled reaction progress, representing the sequence of the reaction from start to finish.

The reactants start on the left with a certain level of energy, then the profile rises to a peak representing the activation energy barrier, labeled E_a. This is the minimum energy required for the reaction to proceed. After the peak the curve descends, with the energy of the products lower than that of the reactants, suggesting the release of energy in the process.

Figure 7.5.3: Temperature and reaction rate

A graph illustration of the distribution of kinetic energy among molecules at two different temperatures.

The vertical axis is labeled fraction of molecules as the proportion of the total molecules at a given kinetic energy level. The horizontal axis is labeled kinetic energy showing the energy each molecule possesses due to motion.

Two bell-shaped curves are plotted: the taller bell curve peaks at a lower kinetic energy value signifying lower temperature; and the red curve, labeled higher temperature peaks at a higher kinetic energy value, showing that molecules at higher temperatures have a greater average kinetic energy.

The area under each curve to the right of the labeled activation energy, Ea, shaded with diagonal lines, represents the fraction of molecules with sufficient energy to overcome the activation energy barrier.

Practice questions

Short-answer question

1. Classify the following factors into the correct category. Category A: Factors to increase the reaction rate and Category B: Factors to decrease the reaction rate:
   • Lower reactant concentration
   • Higher reactant concentration
   • Lower temperature
   • Higher temperature
   • A powdered reactant
   • A solidified reactant
   • Presence of a catalyst
   • No catalyst present

True/false question

1. Increasing the temperature lowers the activation energy requirement.

Multiple choice question (There is/ are one or more correct answer(s) for each question)

1. What is required for a reaction to take place under collision theory?
   1. Higher energy
   2. Correct orientation
   3. Sufficient energy
   4. Presence of a catalyst

Solutions

Short-answer question

1. • Category A: Higher reactant concentration; Higher temperature; A powdered reactant; Presence of a catalyst
   • Category B: Lower reactant concentration; Lower temperature; A solidified reactant; No catalyst present

True/false question

1. False

Multiple choice question

1. b, c