DPST1031 Chap.8 Equilibrium Constants, Reaction Quotients and ICE Tables
Equilibrium Constants, Reaction Quotients and ICE Tables
Chemical equilibrium is the quantitative core of the second half of the course. This chapter describes equilibrium as equal forward and reverse rates, writes K expressions for homogeneous and heterogeneous reactions, and uses the reaction quotient Q to predict the direction of change.
It shows how K changes when an equation is reversed, scaled or added to another, converts between Kc and Kp, and sets out the ICE table method for finding equilibrium concentrations, with a clear test for when the small x approximation is valid. Le Chatelier's principle then predicts the response to changes in concentration, pressure and temperature, which sets up the acid and base chapter.
What this chapter covers
- 01
Dynamic equilibrium and equal rates
- 02
Writing Kc for homogeneous and heterogeneous reactions
- 03
The reaction quotient Q
- 04
K for reversed, scaled and combined reactions
- 05
Converting Kc and Kp
- 06
ICE tables
- 07
The small x approximation
- 08
Le Chatelier's principle and temperature
Worked example · free
Reading K from a concentration against time graph
- +1Equilibrium is reached where both curves are flat, so use the plateau values.
- +1K = [B] ÷ [A] = 0.75 ÷ 0.25 = 3.0.
- +1For the new mixture, Q = 0.60 ÷ 0.40 = 1.5.
- +1Q is less than K, so the mixture moves forward, converting A into B.
Key terms
- Dynamic equilibrium
- A state in which forward and reverse reactions continue at equal rates, so concentrations stay constant.
- Equilibrium constant
- The value K of the equilibrium expression for a reaction at a given temperature.
- Heterogeneous equilibrium
- An equilibrium involving more than one phase, in which pure solids and liquids are omitted from K.
- ICE table
- A table of initial, change and equilibrium concentrations used to solve equilibrium problems.
- Small x approximation
- Treating a change x as negligible beside an initial concentration when it is under 5% of it.
- Le Chatelier's principle
- The rule that a disturbed equilibrium shifts to partly counteract the disturbance.
Equilibrium Constants, Reaction Quotients and ICE Tables FAQ
What is the difference between Q and K?
Both use the same expression. K uses equilibrium concentrations and is a constant at a given temperature; Q uses the concentrations present at any moment. If Q is smaller than K the reaction moves forward, if larger it moves backward, and if equal the system is already at equilibrium.
Why are solids and liquids left out of K?
The concentration of a pure solid or pure liquid is fixed by its density and does not change as the reaction proceeds, so it is absorbed into the constant. Only gases and dissolved species, whose concentrations do vary, appear in the expression.
When can I use the small x approximation?
Use it when K is small compared with the initial concentrations, then check that x is less than 5% of the value it was subtracted from. If the check fails, solve the full quadratic. Never assume it for a large K starting with no products.
Does adding a catalyst change the equilibrium?
No. A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster, but K and the equilibrium concentrations are unchanged. Only a change in temperature alters the value of K itself.
What happens to K if I reverse a reaction?
The equilibrium constant of the reverse reaction is the reciprocal of the forward one, so K becomes 1/K. If a reaction is also multiplied by a factor, K is raised to that power. When reactions are added together, their K values are multiplied to give K for the overall reaction.
Exam move
Practise writing K expressions for twenty reactions, including heterogeneous ones and reactions in water, before attempting calculations. Then work ICE tables in three types: finding K from equilibrium data, finding equilibrium concentrations from K with an exact solution, and the same with the small x approximation plus its 5% check.
Add Q comparisons and Kc to Kp conversions with Δn, then rehearse Le Chatelier predictions in words for concentration, volume and temperature changes. Mastery questions combine these, so finish with problems that disturb an equilibrium and ask for the new concentrations.
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