Le Chatelier’s principle and more...

Feature of Equilibrium StateExplanation Equilibrium is dynamicThe reaction has not stopped but the forward and reverse reaction are still occurring Equilibrium is achieved in a closed system A closed system prevents exchange of matter with the surroundings The concentrations of reactants and products remain constant They are being produced and degraded at an equal rate The macroscopic properties do not change Color and density are properties that depend upon concentrations Equilibrium can be reached from either direction The same equilibrium mixture will result under the same conditions, regardless of the starting point. Quick review of equilibrium

Catalyst the same process still has to happen, catalysts just help out by lowering the activation energy increase the RATE of a reaction….and therefore the decrease the time in which equilibrium is reached – they speed up the forward and reverse reactions equally therefore decreases the time required for the system to achieve equilibrium less time equals $$$ when making chemicals

Le Chatelier’s principle and more... – Nice video- 20 minutes Nice video- 20 minutes – Another good one- 15 minutes Another good one- 15 minutes – states when a system in chemical equilibrium is disturbed by a change, the system shifts in a way that tends to counteract this change of variable – a change imposed on an equilibrium system is called a stress a stress usually involves a change in the temperature, pressure, or concentration the equilibrium always responds in such a way so as to counteract the stress

Stress 1. Temperature change this is the ONLY STRESS THAT WOULD ACTUALLY CHANGE K C increasing temperature – favors the “cold side”/endothermic/the reaction that needs heat – adding heat favors the reaction to the left since it needs +92 kJ – K c decreases  H = + 92 kJ N 2 (g) + 3H 2 (g)   2NH 3 (g)  H = - 92 kJ Haber Process again McGraw Hill Flash animation

decreasing temperature – favors the “hot side”/exothermic the reaction is already giving off heat it doesn’t need so cooling down is good K c increases  H = + 92 kJ N 2 (g) + 3H 2 (g)   2NH 3 (g)  H = - 92 kJ

Stress 2. Pressure change an increase in pressure causes the equilibrium to shift in the direction that has the fewer number of moles – results in a decrease in N 2 and H 2 and an increase in NH 3 an decrease in pressure causes the equilibrium to shift in the direction that has the most number of moles – results in a an increase in N 2 and H 2 and an decrease in NH 3 does NOT affect the equilibrium constant K c N 2 (g) + 3H 2 (g)   2NH 3 (g)  H = - 92 kJ Haber Process again McGraw Hill Flash animation

Stress 3. Concentration change the equilibrium responds in such a way so as to diminish the increase or equalize the ratio – increasing concentration of reactants shifts the reaction to the right (forward, more product) – increasing concentration of products shifts the reaction to the left (reverse, more reactants) does NOT affect the equilibrium constant K c N 2 (g) + 3H 2 (g)   2NH 3 (g)  H = - 92 kJ Haber Process again K c = [NH 3 ] 2 [N 2 ] [H 2 ] 3 McGraw Hill Flash animation

Practice Problem Predict the effect of the following changes on the reaction in which SO 3 decomposes to form SO 2 and O 2. 2 SO 3 (g)   2 SO 2 (g) + O 2 (g) H o = kJ increasing the temperature of the reaction – shifts right increasing the pressure on the reaction – shifts left adding more O 2 when the reaction is at equilibrium – shifts left removing O 2 from the system when the reaction is at equilibrium – shifts right backwards Contact Process

Le Chatelier’s Principle – Summary ChangeEffect on EquilibriumChange in Kc? Increase concentrationShifts to opposite sideNo Decrease concentrationShifts to same sideNo Increase pressureShifts to side with least moles of gas No Decrease pressureShifts to side with most moles of gas No Increase temperatureShifts in endothermic direction Yes Decrease temperatureShifts in exothermic direction Yes Add a catalystNo changeNo

Le Châtelier’s Principle Changes in Concentration ChangeShifts the Equilibrium Increase concentration of product(s)left Decrease concentration of product(s)right Decrease concentration of reactant(s) Increase concentration of reactant(s)right left aA + bB cC + dD Add Remove

If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position. Le Châtelier’s Principle Changes in Concentration N 2 (g) + 3H 2 (g) 2NH 3 (g) Add NH

Le Châtelier’s Principle Changes in Volume and Pressure (Only a factor with gases) A (g) + B (g) C (g) ChangeShifts the Equilibrium Increase pressureSide with fewest moles of gas Decrease pressureSide with most moles of gas Decrease volume Increase volumeSide with most moles of gas Side with fewest moles of gas

For each of the following reactions, predict how the equilibrium will shift as the temperature is increase N 2(g) + O 2(g)  2NO (g) ∆H = +kJ mol -1 2SO 2(g) + O 2(g)  2SO 3(g) ∆H = -kJ mol -1 Right left

Effect of:Position of Equilibrium Value of K c ConcentrationChangesNo change PressureChanges if reaction involves a change in the number of gas molecules No change TemperatureChangesChanges (depends on if exothermic or endothermic) CatalystNo change

Haber Process (don’t think you need to know this anymore/I will not test you on it) N 2 (g) + 3H 2 (g)   2NH 3 (g)  H = - 92 kJ –good catalyst is iron –optimum temp is 450°C good for kinetic theory (molecules moving fast, more collisions with more energy) bad for equilibrium, reaction is exothermic –optimum pressure 250 atm good for kinetics (more collisions) favors the products (only 2 mol vs. 4 in the reactancts)

Contact Process (don’t think you need to know this anymore/I will not test you on it) 2 SO 2 (g) + O 2 (g)   2 SO 3 (g) H o = kJ –good catalyst is V 2 O 5 –optimum temp is 450°C good for kinetic theory (molecules moving fast, more collisions with more energy) bad for equilibrium, reaction is exothermic –optimum pressure is 2 atm not good for kinetics (less collisions) low pressure favors reactants –however, this is a very efficient yield (99%) even at low pressure »therefore, avoids the need for expensive and dangerous equipment