# Reversible Chemical Reactions

## Not all reactions go to completion...Equilibrium 1

Equilibrium: Crash Course Chemistry #28

## The graphical way of looking at equilibrium

• As a system approaches equilibrium, both the forward and reverse reactions are occurring.

• At equilibrium, the forward and reverse reactions are proceeding at the
same rate.

• Once equilibrium is achieved, the amount of each reactant and product remains constant.

## The weird double arrow!

Since, in a system at equilibrium, both the forward and reverse reactions are being carried out, we write its equation with a double arrow.

## The equilibrium constant, K

First of all, notice that the equilibrium constant is an upper-case K while the rate constant, k, is lower-case. The two k's are related however! If the rates of the forward and reverse reactions are the same, then rate of forward = rate of reverse and K is kf/kr.

From our study of kinetics, we know that rate and the rate constant are not the same (unless it is a zero order reaction).

## The magnitude of K tells us if one direction is favored over the other.

Since K represents a ratio of products/reactants, the magnitude of that ratio is very telling as to which side of the reaction the equilibrium "lies".

## Practice

1. Consider the reaction A + B ↔ C + D. assume that both the forward reaction and the reverse reaction are elementary processes and that the value of the equilibrium constant is very large. (a) Which species predominate at equilibrium, reactants or products? (b) Which reaction has the larger rate constant, the forward or the reverse? Explain.

## What is Q?

Q is referred to as the reaction quotient. You can see from the picture above that K and Q are calculated exactly in the same manner. HOWEVER, you only put in equilibrium concentrations (square brackets = molarity) for K and "questionable" concentrations for Q (when you do not know whether a system is at equilibrium). If Q = K, then you are at equilibrium.

Pure solids and liquids do not have a "concentration" so these are left out of equilibrium expressions. Reactions with mixed phases are referred to as heterogeneous and reactions with uniform phases across reactants and products are homogeneous.

## Since gas concentrations can be expressed using pressure, there is a K for gas reactions.

In all of the other types of equilibrium expressions, Keq, Kc, Ka, Kb, Kw, etc, we use molarity in the equilibrium expression. We can use pressure for gases and get a Kp. The two types (K's based on concentration and Kp which is based on pressure) are related as indicated below. You can derive this expression from the ideal gas law...if you want to.

## If you put a heavier person on that seesaw or change the rate of water flow into or out of the container above...

Many things can effect equilibrium BUT the position of the equilibrium will not change (i.e., K will not change) unless the TEMPERATURE is changed. For all other "stresses", the system will adjust but come back to the same position as the figure to the left indicates.

## Changes in concentration: think seesaw again!

Le Chatelier's Principle

## Solving Equilibrium Problems

Equilibrium Equations: Crash Course Chemistry #29