Equilibrium Constant

• Temp change, a solid or bubbles forming in a solution indicates that reaction is not at equilibrium.

• The equilibrium constant expresses the ratio of products to reactants at equilibrium.

• K can be expressed in terms of concentrations or partial pressures. If concentration K(sub)c, partial pressures K(sub)

ICE Tables

• Used to compare Q and K(sub)eq, to find if reaction proceeds in forward or reverse direction.

• I: Initial concentrations or pressures are determined before the reaction starts

• C: Change in concentrations (pressures) for reactants as the reaction moves toward equilibrium.

• E: equilibrium concentrations (Pressures) of reactants and products when the system is at equilibrium

• If you know the initial concentration and the equilibrium constant, you can find how much reactant and product there are at equilibrium by making an ICE table.

• Start with balanced equation;

• 

• The initial concentration of HI is 0.80 M because 0.40 moles are initially present in the 500.0 mL flask. The initial concentration of each of the products is 0 M. How would you describe the change in concentrations?

  Sample Response: 2 HI molecules will be used up for each hydrogen or iodine molecule made. You could write the change as a loss of 2 x from the HI concentration and a gain of x for the hydrogen and iodine concentrations.

• Then create the equilibrium expression (K = Products / reactants)based on the stoichiometry of the reaction, which will allow us to relate the concentrations of the reactants and products at equilibrium.

• When the equilibrium values from the table are substituted, this equation is
  

  

  In order to solve the equation for x, what is the next step?
  

  Sample Response: Both the top and bottom of the equilibrium expression are squared, so taking the square root of both sides of the equation is the easiest way to solve for x.

• 

When there is an increase in pressure, the equilibrium will shift towards the side of the reaction with fewer moles of gas. When there is a decrease in pressure, the equilibrium will shift towards the side of the reaction with more moles of gas

a reaction with an equilibrium constant much less than one will not proceed spontaneously in the forward direction; it will favor the reactants, meaning the reaction will tend to proceed spontaneously in the reverse direction instead. 

AP Daily

7.3

Reaction Quotient, Q

• You can represent the relative quantities of reactants and products at any point in time as a ratio of the concentrations of products to concentrations of reactants .

• That is the reaction quotient, Q: Products / reactants

• Solids and pure liquids are not included in the reaction quotient. Only gases (g) and aqueous solutions (aq) are included.

• Q(sub)c [for aqueous] and Q(sub)p [for gases] are not interchangeable, READ CAREFULLY

• If there are no products that are applicable to the quotient, then put a 1

Equilibrium Constant, K

• When a system is at equilibrium, the constant K is used instead of Q.

• 

• K tells us we are at equilibrium, and Q tells us we are not.

How They Are Related and Significance

• The value of Q tells whether the reaction will happen in the forward or reverse direction to reach equilibrium.

• The value of K will tell us whether there are more products or reactants at equilibrium.

Temperature and K(sub)EQ / just K

• Problems often have a specified temp, due to those values being calculated at specific conditions and temps.

• But temp is not needed to calculate K, just concentrations, or pressures.

• If temp changes then K will as well.

Extra Info

• If trying to find the pressure, before equilibrium, use the ideal gas law: PV = nRT and convert it to P = nRT/V.

• n = number of moles

• R is 0.08206 L atm mol^-1 K-1

• T is temp

• and V is volume

Magnitude of the Equilibrium Constant

• In the equation for the equilibrium constant, the products (numerator) are directly proportional to K, meaning that if products are favored, K will be larger.

• Reactants (denominator) are inversely proportional to K, meaning that if reactants are favored, K will be smaller.

• If K > 1, there is a higher concentration of products than reactants at equilibrium, so products are favored.

• When K (approximately = ) 1 the concentrations of reactants and products are in the same order of magnitude

• When K < 1 there are more reactants than products.

7.7: Solving equilibrium problems

• If Q is larger than K, the numerator will decrease (and/or denominator increase) until Q =K. Reverse reaction is favored and the concentration of products must decrease, or concentration of reactants increase.

• If K > Q numerator must increase. concentration of products must increase, or reactants decrease, forward reaction is favored.

When Q < K, the reaction will proceed in the forward direction