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Imagine a game of tug of war. You have two teams pulling on a rope. Let's say one is more muscular. But, the other team has more people. They match up perfectly even! We've reached equilibrium. The forces from each side are precisely balanced.
We can also have balance in chemical reactions. In a tug of war, the forces balance. But, in chemical reactions, energy balances. What happens when enthalpy and entropy change counteract? Consider Gibb's free energy. The free energy change would be zero. So, the forward and reverse reactions are both spontaneous. We've reached chemical equilibrium!
What is Equilibrium?
Equilibrium happens when we have a reversible reaction. So, the chemical reaction is happening in both directions. The reactants are becoming the products. But, at the same time, the products are becoming reactants. We've reached chemical equilibrium when the rates of both reactions are equal.
Chemical equilibrium is a dynamic process. That means the system's macro view is constant. At equilibrium, the reactant and product molar concentrations stay constant. But, microscopically, the atoms and molecules are still reacting. Because the system is still active, it's dynamic.
How do we Reach Equilibrium?
Imagine putting two reactants in a beaker. At first, they don't react. But, within nanoseconds, some of the molecules collide. We know that they need the correct orientation. Only a few of the collisions will have it. But, they also need enough energy. That's the reaction's activation energy. Very few collisions have everything correct. But some do. So, they react to form the products.
At first, the products' concentrations are minuscule. To react, they need to collide. But, since there are so few, it's improbable. It's even less likely that they collide with proper orientation and energy. But meanwhile, the forward reaction continues. We begin to build up the product concentrations. Soon, they are also reacting. But, they reform the reactants. Eventually, both reactions happen at the same rate. We've reached equilibrium. Once we've reached equilibrium, none of the concentrations change.
How do we Write Equilibrium Reactions?
Writing equilibrium reactions is easy! You start by writing a balanced chemical equation. But, instead of using a single arrow, you put two half-arrows. One points right, the other left. The two arrows show that there is a forward and reverse reaction.
Here's an example. It's the reaction of carbonic acid with water. Carbonic acid is a weak polyprotic acid. It plays a significant role in many branches of chemistry. In environmental chemistry, carbonic acid causes ocean acidification. In biochemistry, carbonic acid helps to maintain blood's pH at 7.4.
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More about Reaction Rate
Do you remember how to write a reaction's rate law? At equilibrium, our reactions are elementary steps. So, the rate law coefficients equal the mole ratios. Let's look at a generic reaction, .
For the forward reaction, .
For the reverse reaction, .
At equilibrium the rates are equal. So, . We define this quotient as the equilibrium constant, . In future lessons, we'll explore how to interpret and calculate the equilibrium constant.
