If we're studying chemical reactions that are in equilibrium, those reactions must be reversible. Let's explore what a reversible reaction is. When you're learning about chemistry in junior science, you usually think of chemical change as irreversible. You bake a loaf of bread and the bread can't be turned back into its ingredients. You burn a candle and you can't turn the gases that are produced back into wax. However, all chemical reactions are, in fact, potentially reversible. Whether they're reversible in practice depends on several factors, including the conditions under which they occur and whether the products escape. But one of the most important factors that affects the reversibility of a reaction is its activation energy. Imagine a reaction that has an energy profile like this. The activation energy is quite small, but the enthalpy change is large and exothermic. Remember that the rate of a reaction is determined by the probability of successful collisions between the reactant molecules, and that in turn depends on the activation energy. So this reaction with a low activation energy goes quite fast, and the reactants are easily converted into products. If the reaction were reversible, it would mean that it could also proceed in the reverse direction. For this reaction, that would mean turning the reds back into the blues. But look at the energy profile in this direction. To get from red to blue, a much greater activation energy has to be met. This means the reaction would be very slow, possibly so slow as to be unmeasurable. A reaction with a very asymmetric energy profile like this is effectively irreversible, because it's much more likely that all the reactants will be converted into products than it is for any of the products to be converted into reactants. This is the case for most of the chemical reactions that you're familiar with, and we tend to class them as irreversible reactions. If instead we have a reaction where the activation energy in either direction is relatively small. Then the chances are that there will be some product molecules with sufficient energy to undergo the reverse reaction, and turn back into reactants. This means that two reactions are happening simultaneously, the forward reaction and the reverse reaction. If you were to start with only blue molecules, then after a sufficient time the sample would contain both reds and blues, since the forward and reverse reactions are both occurring. And this remains the case indefinitely. This type of reaction is called a reversible reaction, and it's these reactions that we study when we're looking at the phenomenon of equilibrium.