Learn the definition of chemical equilibrium and how it is dynamic. Discover what the equilibrium constant is and how it shows whether the reaction favors the reactants or products. Learn how chemists designate equilibrium in an equation and how they show the difference in reaction rate.
Chemical equilibrium is when the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants remain unchanged. Equilibrium is a dynamic state, meaning that things are always moving. Products are being broken down into reactants, and reactants are being combined into products. Things are moving, but the concentrations stay the same. When the reaction is written, it is written with a double arrow instead of an equal sign to show that the reaction is reversible. A + B go to C + D.
In some reactions, the forward reaction is almost completed before the reverse reaction starts. In this case, there's a higher concentration of products than reactants, but the reaction can still be in equilibrium because the concentrations of both the reactants and products stay the same. The reaction equilibrium lies to the right because there are more products than reactants. In this case, the reaction is written with two different-length arrows, with the longer arrow pointing to the right, showing that more product is made than reactant. A + B goes to C + D.
The opposite is also true. The forward reaction of making products has barely started, and the reverse reaction is already going like gangbusters. In this case, the equilibrium of the reaction is said to lie to the left and the longer arrow points left. A + B goes to C + D.
The rate of reactions are often shown in a graph like this one. This graph compares the rate of the forward reaction to the rate of the reverse reaction. To start, the forward reaction has the maximum rate possible, and the reverse reaction has no rate because it hasn't started yet. As the reaction is under way, the forward reaction decreases as the reactants are used up, and the reverse reaction increases as there is more product to turn back into reactants. Eventually, equilibrium is reached, and the graph turns into one straight horizontal line.
Once equilibrium is reached, the concentrations of the reactants and products don't change. When this happens, an equilibrium constant K can be written for the reaction. Only the substances whose concentrations change are included in the equilibrium constant equation. If the reaction equation is nA + mB goes to xC + yD, then the equilibrium constant equation can be written as:
The brackets indicate concentration. The superscripts are the same as the coefficients of each substance in the balanced equation. Let me show you how this looks in a real balanced equation and it may make it clearer for you.
The K for a reaction at a given temperature shows how many reactants are converted into product. If the K is small, the forward reaction barely starts before the reverse reaction gets going and equilibrium is established. If K is large, then most of the reactants were made into products before the reaction reaches equilibrium.
You can solve for any of the unknowns in an equilibrium constant equation if you know the other information.
Example: Determine the K for the reaction N2 + O2 goes to 2NO.
- [N2] = 0.0064 mol/L
- [O2] = 0.0017 mol/L
- [NO] = 1.1 * 10-5 mol/L
Here's another example: Determine the [I2] in the following equation: H2 + I2 goes to 2HI.
- K = 54
- [HI] = 0.017
- [H2] = 0.002
Chemical equilibrium is when the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants remain unchanged. Equilibrium is a dynamic state, meaning that things are always moving.
Once equilibrium is reached, the concentrations of the reactants and products don't change, and an equilibrium constant equation can be written for the reaction. If the reaction equation is nA + mB goes to xC + yD, then the equilibrium constant equation can be written as:
After viewing this video, you should be able to paraphrase what chemical equilibrium is and how it is always in motion but has equal reactions within it.