 Let's use a couple of graphs to look at how this would play out during a reaction. I will start with a vessel containing only A. I'm going to draw two graphs, one that tracks the concentration of A and B, and another that tracks the rate of the forward and reverse reactions. The rate will be measured in moles per litre per second, or Big M over S. Think about that unit. It means how fast the concentration is changing. I'll draw A and the rate of the forward reaction in blue, and I'll draw the concentration of B and the rate of the reverse reaction in red. Okay, so only A is present initially. So the concentration of A is high and the concentration of B is zero. As the reaction proceeds, A gets used up, so its concentration drops, and B is produced, so its concentration climbs. Because the concentration of A starts high, the forward rate is also high, and because the concentration of B is initially zero, the reverse rate also starts at zero. But as the reaction proceeds, the concentration of A falls, so the rate of the forward reaction also decreases. Remember, concentration affects rate. Simultaneously, the concentration of B is climbing, so the rate of the reverse reaction climbs. This continues until the rate of the forward reaction equals the rate of the reverse reaction. When we reach this point, the concentrations of A and B are no longer changing. Since the rates are equal, neither species can win. But although the concentrations aren't changing anymore of A and B, they also don't have to be equal. Remember that for this hypothetical reaction, we needed a three times higher concentration of B to get the rates to be equal. So at equilibrium, the concentration of B will be three times the concentration of A. So the key points are these. If a chemical reaction is at equilibrium, the rates of the forward and reverse reactions are equal. This is the primary definition of equilibrium. When this is the case, the concentrations of the reactants in the products are not changing. But they don't necessarily have to be equal. And finally, it's the ratio of the rate constants of the forward and reverse reactions that determines the equilibrium concentrations. In equilibrium chemistry, we simplify the ratio of those rate constants into a single constant, KEQ, the equilibrium constant. And that's different for each reaction. I'll show you how to use this in coming videos.