ThermoBounce is an interactive online visualization designed to help students understand how enthalpy, entropy, and temperature contribute to the reaction rate and equilibrium of a unimolecular chemical change between reactant and product states, for example, molecular isomerizations or phase transitions. At the start of the animation, reactant molecules are represented by spheres bouncing around in a reactant room, with an average kinetic energy proportional to the temperature. This room is connected to an adjacent product room by a window. When the user triggers the animation, spheres may convert to the product state by passing through the window into the product room, changing color as they do so. The difference between the floor heights of the reactant and product rooms represents the enthalpy change of the reaction, and the floor areas of the two rooms represent the entropies of the reactant and product states. The height of the bottom of the window above the floor of the reactant room represents the enthalpy of activation, and the width of the window represents the entropy of activation. Users can change the geometry of the rooms and window to adjust all these quantities and can modify the temperature before or during the reaction. They can then observe the resulting changes in the reaction rate and in the accumulation of product molecules, as shown on a reaction progress graph. The visualization and instructions for interacting with it are freely available on the Internet.