What happens to particle motion when the heat is added? How do you suppose this would affect the rate at which a chemical change takes place?

When heat is added to an object it's internal energy increase. What is meant by internal energy is the kinetic energy and potential energy of the particles in an object. Potential energy of particles is according to the structure of particle arrangement within the object.

As kinetic energy increases the motion of the particles within the molecule increases therefore the speed of the particles inside the object increases, so the probability of atoms interacting with each other to produce new substance also increases. So the rate of chemical reaction increases.

Actually what we feel as heat is a measurement of the amount of kinetic energy the particles in the object have.