3.8- equilibrium constants

Dynamic equilbrium

Reversible reaction where the rates of the forward and reverse reactions are equal, so the quantities of each substance stay the same

le chateliers principle

If a system at equilibrium is subjected to a change, the equilibrium tends to shift so as to minimize the effect of that change

Kc and Kp

Kc- the equilibrium constant in terms of concentration and is usually used for reactions in solution

kp- the equilibrium constant in terms of partial gas pressures and is used for reactions only involving gases

Equilibrium constants in solutions

Products over reactants

equilibrium constants in gases

each represent their partial pressure

Products over reactants

Partial pressure

The pressure that the gas would exert if it alone occupied the volume

sum of the partial pressures = the total pressure of the system

Interpretating equilibrium constants

• If Kc is <1: very few products are formed and most of the mixture is starting material, typically the case when free energy change is positive and reaction doesn’t occur spontaneously

• if Kc is >1, then most reactants have been converted into products

equilibrium data

tells us about the relative stability of reactants and products, and the changes that occur between them

reaction rate

Gives us information about the changes that occurred between the reactants and transition state

allows us to deduce what's happening during the reaction and the order in which individual bonds are broken and made- called the reaction mechanism

Changing equilibrium yield

Buy altering concentration pressure or temperature equilibrium constant will allow us to identify which concentration or pressure values favour high yield

the energetics can allow us to identify what temperatures will favourite higher yield

Maximising rates

Buy increasing temperature pressure and adding catalyst

these factors would be altered to increase rate unless the factor decreases yield

Identifying energy calculations

Energy calculations will identify how much energy needs to be the input into the system for reaction to occur minimizing the input of excess energy

the energy generated by an exothermic reaction may be calculated and this allows a company to decide whether it's economical to harness this energy or needs to be removed as waste

Equilibrium constants and temperature

exothermic

Temperature is the only thing that affect equilibrium constants

for an exothermic reaction and increasing temperature will cause the equilibrium to shift in the endodermic direction (left) reducing the amount of products and increases the amount of reactants

decreasing the value of the equilibrium constants

Equilibrium constant and temperature

endothermic

For an endothermic reaction and increasing temperature will cause the equilibrium to shift in the exothermic direction (right) increasing the amount of products and decreasing the amount of reacants

this increases the value of the equilibrium constant

Increase and decrease in Kp and Kc

exothermic and endothermic

• Exothermic reaction: increasing temperature decreases Kc and Kp

• endothermic reaction: increasing temperature increases Kc and Kp