SQA National 5 Chemistry Practice Exam 2025 – Complete Study Resource

Decreasing the temperature of a system reduces the kinetic energy of the particles involved in a chemical reaction. As the temperature drops, the frequency and energy of collisions between the reactant molecules also decrease. This means that the particles are less likely to collide with sufficient energy to overcome the activation energy barrier required for a reaction to occur. Consequently, a lower temperature generally leads to a slower reaction rate because the particles are moving more slowly and have fewer successful collisions.

In contrast, decreasing particle size increases the surface area of reactants, which typically accelerates the reaction by allowing more collisions to occur. Increasing concentration means more reactant particles are present in the same volume, leading to a higher frequency of collisions and a faster reaction rate. Adding a catalyst lowers the activation energy necessary for a reaction to proceed, also increasing the reaction rate. Hence, among the options provided, decreasing temperature is the factor that negatively affects the rate of a chemical reaction.