Equilibrium

Reversible Reactions Able to proceed in both directions (forward and reverse) PE (kJ) Reaction coordinate Forward AE f Reverse AE r HH

Equilibrium The rate of the forward and reverse reactions are equal The rate of the forward and reverse reactions are equal The concentration of the products and reactants remains constant The concentration of the products and reactants remains constant Any change in conditions (temperature, pressure, concentration) may alter the equilibrium Any change in conditions (temperature, pressure, concentration) may alter the equilibrium

Phase Equilibrium Equilibrium between two different phases Example: Rate at which ice is melting is equal to the rate at which water is freezing H 2 O(s)  H 2 O(l)

Solution Equilibrium Saturated Solutions - equilibrium between the dissolved and crystallized solute –Rate of dissolving = Rate of crystallization Example: NaCl(s)  NaCl(aq)

Chemical Equilibrium Concentrations of the reactants and the products remain constant Concentrations of the reactants and the products remain constant

LE CHATELIER’S PRINCIPLE If a system at equilibrium is “disturbed” in some way, it will change to reduce the effects of the applied stress – “Re-Balance” The system will try to re-establish equilibrium, at a new equilibrium point Stresses that may be applied to a system are changes in: –Concentration –Pressure (for gases) –Temperature

EFFECT OF CONCENTRATION If the concentration of a substance increases, the reaction that reduces the amount of that substance will be favored (Re-Establish Balance) Move away from the substance that increased If a substance decreases, move towards it Example: 2NO(g) + O2(g)  2NO2(g) Add more NO, Add more NO2, Add more O2, Remove NO2,

Effect of Temperature Dependant on whether the reaction is endothermic or exothermic Dependant on whether the reaction is endothermic or exothermic An Increase in temperature will favor the more endothermic side, absorption of heat (use up the extra heat) An Increase in temperature will favor the more endothermic side, absorption of heat (use up the extra heat) Shift away from the heat Shift away from the heat A Decrease in temperature favors the more exothermic reaction (replace the lost heat) A Decrease in temperature favors the more exothermic reaction (replace the lost heat) Shift towards the heat Shift towards the heat

Effect of Temperature Example: Example: 2N 2 (g) + 3H 2 (g)  2NH 3 (g) KJ What happens when you raise the temperature? Shift to the left Shift to the left

Effect of Temperature A + B + heat  AB What happens when the temperature of the reaction is decreased?

Effect of Pressure Only effects gases – change in volume Only effects gases – change in volume An increase in pressure decreases the volume An increase in pressure decreases the volume A decrease in pressure increases the volume A decrease in pressure increases the volume Look at number of moles of products and reactants (coefficients) Look at number of moles of products and reactants (coefficients) If pressure increases, volume decreases, go to the side with less moles of gas If pressure increases, volume decreases, go to the side with less moles of gas If pressure decreases, volume increases, go to the side with more moles of gas If pressure decreases, volume increases, go to the side with more moles of gas

Effect of Pressure Example Example 2CO(g) + O 2 (g) ↔ 2CO 2 (g) What happens when the pressure is increased? What happens when the pressure is increased? the reaction shifts to the right (lower number of moles of gas) the reaction shifts to the right (lower number of moles of gas)

What Happens if 1. 4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g) + 457KJ Temp increases? Pressure Decreases? Add NO? 2. NaCl(s) +3.88KJ  Na+(aq) +Cl-(aq) Temp decreases? Add Na+? What happens to the Cl-?

What Happens if… 3. C(s) + O 2 (g)  CO 2 (g) KJ Decrease Temp? Decrease Temp? Remove CO 2 ? Remove CO 2 ? Increase Pressure? Increase Pressure? 4. NaOH  Na + + OH KJ Increase Temp? Increase Temp? Decrease Na + ? Decrease Na + ? Add NaOH? Add NaOH?

Catalyst Increases the rate of the forward and reverse reactions Increases the rate of the forward and reverse reactions other than equilibrium may be established faster NO NET CHANGE IN THE EQUILIBRIUM, other than equilibrium may be established faster

Common Ion Effect When an ion is added to a solution, the equilibrium will shift away from that ion When an ion is added to a solution, the equilibrium will shift away from that ion The addition of an ion from a different salt reduces the solubility of the salt The addition of an ion from a different salt reduces the solubility of the salt

Common Ion Effect Example: AgCl(s)  Ag + (aq) + Cl - (aq) If NaCl is added, the concentration of the Cl - ion will increase, therefore the reaction will shift to the left (away from the chloride ion)

Common Ion Effect Example: Na 2 CO 3  2Na + (aq) + CO 3 2- (aq) Na 2 CO 3  2Na + (aq) + CO 3 2- (aq) What will happen if NaCl is added to this solution at equilibrium?