
Reactant and product concentrations stop changing because the forward and reverse rates have become equal
rate(fwd) = rate(rev)



K = (kfwd/krev) = [NO2]^2eq/[N2O4]eq 

Law of Chemical Equilibrium/
Law of Mass Action


At a given temperature, a chemical system reaches a state at which a particular ratio of reactant and product concentrations has a constant value.
Also, for a particular system and temp, the same equilibrium state is attained regardless of how the reaction is run



The particular ratio of concentration terms that is written for a given reaction
At equilibrium: Q = K


Form of Q for an Overall Reaction 

If an overall reaction is the sum of two or more reactions, the overall reaction quotient (K) is the product of the reaction quotients(K or Q) for the steps:
Koverall = K1 * K2 * K3*…


Relation of Q/K fwd and Q/K rev 


Q for a Reaction w/ Coefficients multiplied by a common factor 

If all the coefficients of the balanced equation are multiplied by some factor, that factor becomes the exponent for relating the reaction quotients and the equilibrium constants 

Reactions Involving Pure Liquids or Solids 

concentration of a pure solid and pure liquid is constant
since the concern is only w/ concentrations that change as they approach equilibrium these solid and liquid concentrations are not included in the reaction quotient



The equilibrium constant obtained when all components are present at their equilibrium partial pressures is designated Kp, the equilibrium constant based on pressures 


the exponent of the RT term equals the change in the amount (mol) of gas (delta(n) gas) from the balance equation, 1
Kp = Kc(RT)^(delta(n)gas)


Simplifying Assumption for Finding and Unknown Quantity 

if a reaction has a relatively small k and relatively large initial concentration, the concentration change ‘x’ can often be neglected
– if the assumption results in a change that is less than 5% the original concentration, the error is not significant, and the assumption is justified


Le Chatelier’s Principle Applied 

When a disturbance occurs, we say that the equilibrium; position shifts, which means that concentrations(or pressures) change in a way that reduces the disturbance, and the system attains a new equilibrium position (Q = K again) 

Effect of; Change in Concentration 

Whenever the concentration of a component changes, the equilibrium system reacts to consume some of the added substance or produce some of the removed substance
note: the value of Kc does not change at given temp


Effect of an Inert Gas on Equilibrium Pressures 


Effect of Pressure on Equilibrium 

If pressure increases, reaction shifts to the side with fewer moles of gas
– change in volume results in change in concentration but does not change Kc


Effect of; Temperature on Equilibrium Position 

A temperature increase(adding heat) favors the endothermic direction while a temperature decrease (removing heat) favors the exothermic direction
Temp rise will increase K for a system with a positive deltaH of reaction
Temp rise will decrease K for a system with a negative deltaH of reaction



ln(K2)ln(K1) = (deltaH of reaction/R)*((1/T2)(1/T1)) 


No effect on equilibrium position, just increases rate 
