Le Chatelier’s Principle

Reversible Reactions Thus far, we have considered only one-way reactions: A + B → C + D Some reactions are reversible: a chemical reaction in which the products can react to re-from the reactants They go forward (“to the right”) : A + B → C + D and backwards (“to the left”) : A + B ← C + D Written with a two-way arrow: A + B ↔ C + D

Examples of reversible reactions: Melting & Freezing Recharging a rechargeable battery Examples of irreversible reactions: Burning paper Dropping an egg

Chemical Equilibrium A + B ↔ C + D + + + + + + For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established. A + B ↔ C + D A B + A B + A B + C D + C D + C D +

Le Chatelier’s Principle When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress. The system shifts in the direction of the reaction that is favored by the stress. A stress is a change in: Concentration Temperature (ENERGY) Volume Pressure

Shifts If you INCREASE volume, concentration or temperature from the REACTANT side, there will be a shift RIGHT towards the products. If you DECREASE volume, concentration or temperature from the REACTANT side, there will be a shift LEFT towards the reactants. *** Opposite is true for compounds on the products side*** If you INCREASE pressure, it shifts to the side with the least amount of moles. If you DECREASE pressure, it shifts to the side with the most amount of moles. If a catalyst is present NO shift happens

Example CO (g) + 2 H2 (g)  CH3OH (g) + energy Methanol, CH3OH, can be manufactured using the following equilibrium reaction: CO (g) + 2 H2 (g)  CH3OH (g) + energy Predict the direction that the reaction will shift for each of the following stresses. Decrease in temperature Increase in pressure Add more concentration of H2 Add a catalyst Decrease volume CO