 It's called the activation energy and has the symbol of a large E with a small subscript A. In all reactions, there's a certain amount of energy that needs to be added, regardless of whether it's exothermic or endothermic, before the reaction can get started. The reason for this is that bonds in the reactant molecules usually need to be broken before the atoms can be rearranged to form the products. Breaking bonds requires energy, so a certain amount of energy has to be put in to make that happen. This sketch here shows a good analogy. Once the rock is at the top of the hill, it will then move down the hill of its own accord under the force of gravity. But it requires some extra applied energy to get it to the top of the hill in the first place. In chemical terms, think about a gas flame. You know that natural gas, which is mostly methane, burns in oxygen and that this is an exothermic reaction. But if you release some natural gas into the air at room temperature, it doesn't just burst into flame all by itself. A spark or an extra flame is needed to get it going. The heat from that spark or flame provides just enough energy to get the reaction started, to start breaking bonds, and after that it will proceed of its own accord, which would be the gas burning. Some reactions have really big activation energies and need a lot of energy to be put in before the reaction will start, while others have small activation energies and they're really easy to start. Think back to those endothermic examples in the last video. The coal pack reaction had a very small activation energy, so simply mixing the chemicals was enough to get it going. Whereas decomposing copper carbonate requires a lot of bonds to be broken, so it has a large activation energy, and that's why it needed to be heated in order for the reaction to occur.