Rate of reaction model answers

Surface Area

•Reactants can only occur on the surface boundary where the 2 phases meet.

•Grinding or breaking a solid (or spraying a liquid) provides a greater total surface area, allowing more reactant particles to collide.

•This increased frequency of collisions results in a higher number of successful collisions per unit time and hence an increased reaction rate.

•Can affect solid and liquid reactants as well as catalysts.

Concentration

•The concentration of a substance is the number of particles per unit of volume.

•Increasing the concentration means more particles per unit volume

•If the concentration of a reactant is increased the frequency of collisions between reactant particles increases.

•As the total number of collisions between particles increases (as a certain percentage are successful) the number of successful collisions increases and the reaction rate increases.

Pressure

•Pressure is the force per unit area that gas particles exert when they collide with the walls of their container.

•A high gas pressure is the consequence of having an increased number of gas particles in a given volume at constant temperature.

•If the pressure is increased the frequency of collisions between reactant particles increases.

•As the total number of collisions between particles increases (as a certain percentage are successful) the number of successful collisions increases and the reaction rate increases.

Temperature

•An increase in the temperature of a substance corresponds with an increase in the average kinetic energy of the particles that make up the substance.

•This increase in average kinetic energy causes the particles to move, on average, with an increased speed, causing a greater frequency of collisions, both successful and unsuccessful.

•An increase in the temperature of the substance will also result in a greater percentage of particles with kinetic energy greater than activation energy.

•This will cause a higher percentage of the existing collisions to be successful per unit time and hence, a faster rate of reaction

•Overall a higher temperature increases the overall frequency of collisions and increases the proportion of successful collisions.

Catalyst

•A substance that is added to a reaction but is not used up in a reaction.

•A catalyst provides an alternative reaction pathway with lower activation energy.

•A greater proportion of reactant particles will have sufficient energy to overcome the, now reduced, activation energy.

•This increased probability of a successful collision results in a greater number of successful collisions per unit time and hence a faster reaction rate.