4b Flashcards
What is a reversible reaction?
A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants. Reversible reactions can be represented like this:
A+B ⇌ C+D
The double arrow means that the reaction can go in either direction.
Dynamic Equilibrium
If a reversible reaction takes place in a closed system then a state of dynamic equilibrium will always be reached. Dynamic equilibrium is when the amounts of reactants and products reach a balance - their concentrations stop changing.
As the reactants react in a reversible reaction, their concentrations fall - So the forward reaction will slow down. But as more and more products are made and their concentrations rise, the backward reaction will speed up. After a while the forward reaction will be going at exactly the same rate as the backward one the system has reached dynamic equilibrium. Both reactions are still happening, but the overall effect is nil because the forward and reverse reactions cancel each other out.
What is a closed system?
A ‘closed system’ just means that none of the reactants or products can escape and nothing else can get in.
equilibrium
When a reaction is at equilibrium it doesn’t mean the amounts of reactants and products are equal. The exact position of equilibrium depends on the conditions, as well as the reaction itself:
If the equilibrium lies to the right, the concentration of products (C+D) is greater than that of the reactants (A+B). A+B ⇌ C+D
If the equilibrium lies to the left, the concentration of reactants (A+B) is greater than that of the products (C+D). A+B ⇌ C+D
Changes in the position of equilibrium
Three things can change the position of equilibrium (which changes the amounts of products and reactants present at equilibrium). These are temperature, pressure (when some or all of the products or reactants are gases), and the concentrations of reactants or products.
What is Le Chatelier’s principle?
Le Chatelier’s principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract the change. This means the effect of any changes to a system can be predicted. So by altering the temperature, pressure or concentration of the reactants, you can alter the yield of the reaction making sure that you end up with more of the product you want (and less of the reactants).
Temperature
All reversible reactions are exothermic in one direction and endothermic in the other.
If you raise the temperature of A+B ⇌ C+D where -> = exothermic, the equilibrium will shift to oppose the change. The system wats to absorb heat energy to cool it down. Therefore, the equilibrium will shift to the left (the endothermic direction).
If you reduce the temperature of A+B ⇌ C+D where -> = exothermic, the equilibrium will shift to oppose the change. The system wats to produce heat energy to heat it up. Therefore, the equilibrium will shift to the right (the exothermic direction).
Pressure
Changing the pressure affects reactions where the reactants and products are gases. Many of these reactions have a greater volume on one side (either of products or reactants), Greater volume means there are more gas molecules on that side of the equation and less volume means there are fewer gas molecules.
Example - The reaction below is used to make hydrogen gas. It has two gas molecules on the left and four on the right.
CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g)
If you increase the pressure, the equilibrium shifts to oppose the change and moves to decrease the pressure. The position of equilibrium will shift to the left which generates less gas particles – CH4 and H₂O.
If you decrease the pressure, the equilibrium shifts to oppose the change and moves to increase the pressure. The position of equilibrium will shift to the right which generates more gas particles - CO and H₂.
Concentration
If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium. So, the system will respond to bring itself back to equilibrium again. A+B ⇌ C+D
If you add/increase the concentration of A or B, the position of equilibrium will shift to oppose the change and so the equilibrium will shift to the right forming more C & D and decreasing the amount of A & B.
If you add/increase the concentration of C or D, the position of equilibrium will shift to oppose the change and so the equilibrium will shift to the left forming more A & B and decreasing the amount of C & D.
