Equilibrium

Equilibrium Many reactions can proceed in both the forward direction (towards products- to the right) and the reverse reaction (towards reactants- to the left) Recall weak acids and bases!

Dynamic Equilibrium Molecules are constantly moving, so in chemistry we always have dynamic equilibriums (not static) Dynamic equilibriums occur when the rate of the forward reaction is equal to the rate of the reverse reaction

Which is at equilibrium? Salt in water?

4 Conditions for ANY Equilibrium 1. Equilibrium is achieved in a reversible process when the rates of opposing changes are equal (represented by the double arrow) 2. Macroscopic properties are constant. No change in properties that depend on the total quantity of matter (ex. colour, pressure, concentration, pH, etc.) Dynamic change at the molecular level, but no change at the macroscopic level 3. Equilibrium can only be reached in closed systems, including constant temperature (energetically closed too) 4. Equilibrium can be approached from either direction and proportions will still be the same.

Equilibrium Concentrations Although the rates are the same for either direction, it is important to remember that the concentrations are not!

Don’t be fooled by a steady state! Reactants are constantly added and products are removed as they form- appears at equilibrium

Le Chatelier’s Principle Henri Louis Le Chatelier was the first to describe how chemical systems at equilibrium adjust to changes: If a system at equilibrium is subjected to an external stress, the equilibrium will shift to minimize the effects of that stress If you think about it- we do that too!

Le Chatelier External stresses are factors that cause either the forward or reverse reaction to change, throwing the system out of balance. Le Chatelier allows us to predict how stress will affect the system 4 factors to consider: 1. Concentration 2. Pressure and volume 3. Temperature 4. Catalyst

Le Chatelier- Concentration Consider the following equilibrium: Fe3+(aq) + SCN-(aq) ↔ FeSCN2+(aq) (colorless) (red) If more Fe3+ is added to the reaction, what will happen? The system needs to react to minimize the stress Since Fe3+ is on the reactant side, the rate of the forward reaction will increase to “use up” the additional reactant This causes a shift to the right (to produce more FeSCN2+) What color will the solution become?

NOTE: When we added more Fe3+ we could say any of the following and they all mean the same thing: Equilibrium shifts to the right Equilibrium shifts to products The forward reaction is favoured Conversely, if we add more FeSCN2+ The equilibrium will shift to the left The equilibrium will shift to reactants Reactants will be favoured Fe3+(aq) + SCN-(aq) ↔ FeSCN2+(aq) (colorless) (red)

Removing a Substance Concentration of one of the substances can also be changed by removing that substance This is usually done by adding another substance to react with one of the compounds/ions that is already in the reaction Ex. Pb2+ reacts with SCN-. If we add Pb2+ ions it will remove SCN- ions and the equilibrium will shift to replace these ions (shift left) Fe3+(aq) + SCN-(aq) ↔ FeSCN2+(aq) (colorless) (red)

Recall: Pressure Pressure is caused by molecules colliding

Changing Pressure and Volume ONLY AFFECTS GASES! If you increase the pressure, you typically do this by decreasing the volume Reaction will favour the side with the least number of mols of a gas Conversely, if you decrease the pressure the reaction will shift to favour the greater number of moles of gas If both sides of the equilibrium have the same number of moles, the equilibrium will not shift. SOLIDS, LIQUIDS AND AQUEOUS SOLUTIONS ARE NOT AFFECTED BY CHANGING THE PRESSURE/VOLUME

For Example: N2O4 is colorless, NO2 is brown Decreasing the volume increases the pressure, equilibrium shifts left

Changing Temperature Easiest way to see the affect of temperature is to include heat in the reaction equation Exothermic reactions give off heat as a product Endothermic reactions require heat (include in reactants) For N2O4(g) ↔ 2NO2 (g) the ΔH = + 53kJ So we can write this as: N2O4(g) + heat ↔ 2NO2 (g) Now it is much easier to see how heat will affect the equilibrium -addition of heat will cause a shift to the right

Addition of a Catalyst Catalysts speed up both the forward and reverse reactions- the no not affect the position of an equilibrium!