How would Figure 2 be different if no catalyst was used?

A catalyst increases the rate of a chemical reaction without being used up in the process. It does this by providing an alternative reaction pathway that has a lower activation energy. The activation energy is represented by the peak of the curve on a reaction profile. The diagram shows the catalysed reaction, which has a relatively low peak. If no catalyst were used, the reaction would still happen, but it would follow a pathway with a higher activation energy. This means the peak of the curve would be higher. Importantly, a catalyst does not change the energy of the reactants or the products, so the starting and ending energy levels of the profile would be exactly the same.