Exam 1 Part 3 - Equilibrium Calculations

What is the reaction quotient Q?

• Infinite number of values as value changes over time when approaching K

What does it mean when Q is less than K?

• Q<K; not enough product, reaction shifts to the right

What does it mean when Q is greater than K?

• Q>K; too much product, reaction shifts to the left

What does it mean when Q is equal to K?

• Q=K; reaction is at equilibrium

How do you calculate Kc when given all equilibrium concentrations (Type A)?

• Plug in equilibrium concentration to equilibrium expression

Easiest

How do you use an ICE table?

• I: Initial concentrations; product side will usually be zero

• C: Coefficients -x (reactants) and +x (prod)

• E: Expression

How do you determine whether to add or subtract an x in the C area of an ICE table if one has an initial concentration = 0?

Shifts toward side with less conc.

• Subtract; higher conc.

• Add; lower conc. 

How do you determine whether to add or subtract an x in the C area of an ICE table when all initial concentrations given?

Find Q and compare to K value given

• Subtract; higher conc. 

• Add; lower conc.

How do you calculate Kc when given the equilibrium concentration of only one reactant/product?

1. Assume initial conc. is 1

2. Plug into ICE table to obtain expression

3. Set expression equal to equilibrium concentration to solve for x

4. Plug x into the rest of the conc.

5. Plug all equilibrium conc. to solve for Kc

How do you find equilibrium concentration when given all the rest of equilibrium concentrations and the Kc value?

• Plug all values in and solve for equilibrium conc.

What formula is used to determine if a Kc value is too small?

• x/[A]₀ = <0.05

where X is solved for by assuming denominator is negligible and [A]₀ is the initial conc.

In the small x approximation, which concentration is used when given two initial concentrations?

• Choose smaller initial concentration; divide it by x

How do you find equilibrium concentration using only Kc and initial concentrations?

1. Complete ICE table

2. Use ICE expressions to plug into Kc expression

3. Assume denominator is negligible

4. Solve for x

5. Plug into small x approx. equation; if larger than 0.05 plug into quadratic equation

Once solving for quadratic equation and solving for two values, which value do you choose?

• Choose that value that when plugged into ICE expression; will result in a POSITIVE value

How do you know for certain that you will have to use quadratic formula?

• When k value is greater than 1 or near 1

otherwise confirm with equation

Do we have to memorize quadratic formula?