Chemical Equilibrium

Reversible Reactions liquid + heat → vapor vapor + cooling → liquid Products formed react to produce starting reactants A + B → C + D Forward (right) C + D → A + B Reverse (left) Simultaneous

Reaction Rates Speed w/ which reaction proceeds Varies Temperature Concentration Catalysts Chemical kinetics

What’s going on?

Dynamic Equilibrium Chemical equilibrium Two or more systems Oppose each other (forward and reverse reaction) Same time Same rate When rate of forward reaction = rate of reverse reaction…Dynamic Equilibrium is attained!

Equilibrium

Le Chatelier If stress is applied to a system in equilibrium, the system will respond in such a way as to relieve that stress and restore equilibrium under a new set of conditions.

Le what??? If change is imposed on a system in equilibrium… then the system will adjust to a new equilibrium…

How does this work? What will effect a chemical reaction? Pressure Concentration Temperature Volume Catalyst

Concentration and Equilibrium Concentration effects equilibrium Effect on reactants is determined by experiment What happens if you increase the reactants of the following reaction? H2(g) + I2(g) 2HI(g) Increases the number of reactants available for collisions… What happens to the rate of the reaction? It increases The system will establish a new equilibrium

H2(g) + I2(g) 2HI(g) What happens if we increase the concentration of [H2]? Rate to the right increases Reactants are used faster than products are produced HI is produced faster than it is used Eventually, equilibrium is reached [I2]decreases and [HI] increases

Equilibrium and Volume If one or more of reactants are a gas and the reaction is closed, then you will see significant changes to equilibrium with change to volume How? If you decrease the volume of the gas, what effect does that have on the reaction? Think about the Ideal Gas Law…what would it do the concentration of the gas? Increase!

CaCO3 (s) 825°C CaO (s) + CO2(g) Decrease the volume The reverse reaction speeds up Equilibrium shifts to the left WHY? Decreasing the volume of the container increases the concentration of the carbon dioxide It’s the only gas! What happens to pressure if you decrease volume? Think… Pressure will increase. (Boyle’s Law and Ideal Gas Law)

If volume decreases, pressure increases If a system is all gas; shift in reaction and equilibrium will be toward the side of the reaction that contains the least molecules Why? By shifting this way the system is trying to reduce the number of molecules and thus lower the pressure

Haber Process

Temperature and Equilibrium Increase temperature; increase rate Why? KE increases (what is KE) Forward and reverse increase How will equilibrium shift? Toward the side of the equation that absorbs the heat Greater increase occurs Exothermic reactions favor a left shift Endothermic reactions favor a right shift

Catalyst on Equilibrium Influence rate / affects speed Not consumed during reaction Lowers activation energy DOES NOT SHIFT EQUILIBRIUM

Activation Energy Minimum energy needed for reaction to occur No change to Energy Reactants Products

Equilibrium Constants Reversible reactions Concentration of products and reactants is constant @ equilibrium, rate of forward reaction = rate of reverse reaction Why is this significant? Equilibrium constant

Keq @ particular temperature For the general equation aA + bB ⇄ cC + dD

Significant of Keq Magnitude indicates extent the forward or reverse reaction takes place If Keq > 1, the amount of product at equilibrium is greater than the amount of reactants If Keq < 1, amount of reactants at equilibrium is greater than the amount of products If Keq is large, the reaction will go forward to completion If Keq is small, the reaction will go to the left