The Equilibrium Constant, Kc
Lajoy Tucker
Teacher
Contents
Introduction & Definitions
The equilibrium constant, , tells us the position of equilibrium in a reversible reaction involving gases or solutions. It is calculated using equilibrium concentrations in mol .
General form for a reaction:
Square brackets mean “concentration at equilibrium ()”
are the balancing numbers from the equation
Units of Kc
The units of depend on the particular reaction. To work them out:
1. Replace concentrations in the formula with
2. Cancel units algebraically
3. Simplify
Example 1 – Making Nitric Oxide
Units
/ no unit
In this case, all units cancel.
No answer provided.
Example 2 – Writing a Expression
Unit:
Final unit
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Calculating Kc – Step-by-Step Example
Reaction:
A sealed container of contains:
Initial moles:
At equilibrium:
Step 1 – Use a table:
Initial mol | 0.60 | 0.60 | 0.00 |
Change | -0.40 | -0.40 | +0.40 |
Equilibrium | 0.20 | 0.20 | 0.40 |
Step 2 – Calculate concentrations:
Step 3 – Substitute into expression:
No answer provided.
Top Tips for Kc Calculations
Always check if you need to divide by volume (unless it cancels in a 1:1:1:1 ratio).
Use a table to track changes from initial to equilibrium moles.
Always square or cube concentrations according to the balanced equation.
Include units in your final answer unless they cancel completely.
Don't forget: only depends on temperature – not concentration or pressure.
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Common Mistakes to Avoid
Forgetting to convert moles into concentrations.
Misapplying the mole ratio when working out how much reacted.
Leaving out units or writing the wrong ones.
Squaring numbers incorrectly when using coefficients.
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Example – Calculating
Reaction:
Initially, a container has:
At equilibrium, % of the has reacted. Calculate the value of .
Answer
Step 1 – Work out moles at equilibrium
% of reacted
From the 1:1:1:1 ratio:
reacted
formed
Initial | 1.20 | 1.50 | 0.00 | 0.00 |
Change | -0.48 | -0.48 | +0.48 | +0.48 |
Equilibrium | 0.72 | 1.02 | 0.48 | 0.48 |
Step 2 – Convert to concentrations ()
Divide each by volume ():
Step 3 – Write the Kc expression
Final answer:
How Changing Conditions Affects
A high means the equilibrium lies to the right (more products)
A low means the equilibrium lies to the left (more reactants)
is only affected by temperature.
Changes in pressure or concentration do not affect the value of .
Catalysts have no effect either.
Effect of Temperature on and Equilibrium
For exothermic reactions (ΔH is negative):
Increasing temperature shifts equilibrium left
value decreases
For endothermic reactions (ΔH is positive):
Increasing temperature shifts equilibrium right
value increases
Effect of Pressure on Equilibrium and
Only affects reactions involving gases
Increasing pressure shifts equilibrium to the side with fewer moles of gas
itself stays constant, as pressure does not affect its value
Example:
In this reaction:
There are fewer moles of gas on the right, so increasing pressure shifts equilibrium to the right. But stays the same
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Effect of a Catalyst on
Catalysts speed up the forward and backward reactions equally
They do not affect the position of equilibrium
They do not affect the value of
Using Algebra to Find Equilibrium Moles
Reaction:
Example – Calculating Equilibrium Moles from
Reaction:
Given:
of A and of B are mixed in a container
At equilibrium,
Total volume
Calculate the equilibrium moles of A, B, and C
Answer
Step 1 – Let x = moles of A that react
Because the mole ratio is 1:1:2:
x of A and B react
2x of C are formed
A | B | C | |
Initial | |||
Change | |||
Equilibrium |
Step 2 – Write expression
Because volume , concentrations = moles
Step 3 – Take square root of both sides
Multiply both sides:
Step 4 – Final answers
Moles of
Moles of
Moles of
Practical: Determining the Equilibrium Constant (Kc)
This practical determines the equilibrium constant for an esterification reaction between ethanoic acid and ethanol using a sulfuric acid catalyst. After reaching equilibrium, titration is used to determine the amount of unreacted acid.
Reaction:
Method
Part 1 – Preparing the Equilibrium Mixture
1. Use burettes to measure out specific volumes of ethanoic acid, ethanol, and dilute sulfuric acid into a boiling tube.
2. Fit a bung to the boiling tube, swirl gently to mix, and leave the sealed tube in a rack for one week to allow equilibrium to be established.
Part 2 – Titrating the Equilibrium Mixture
3. Rinse a volumetric flask with distilled water.
4. Transfer the contents of the boiling tube into the volumetric flask using a funnel. Rinse the boiling tube with distilled water and add these rinsings to the flask.
5. Make up the solution to exactly with distilled water. Stopper the flask and invert it several times to ensure complete mixing.
6. Use a pipette to transfer a portion of the equilibrium mixture into a conical flask.
7. Add 3–4 drops of phenolphthalein indicator.
8. Fill a burette with standard sodium hydroxide solution.
9. Titrate the acid in the conical flask until a pale pink colour persists. Record the volume of used.
10. Repeat the titration until you obtain at least two concordant results (within ).
Key Notes:
neutralises both unreacted ethanoic acid and the sulfuric acid catalyst.
is a diprotic acid, so it releases 2 moles of per mole.
No answer provided.
Processing the Results
There are various calculations that could follow. Here, we calculate the amount of ethanoic acid at equilibrium using titration data.
Step 1 – Calculate Initial Moles of Reactants
You can determine the moles of ethanoic acid and ethanol from their densities and volumes:
The number of moles of sulfuric acid catalyst is usually determined via separate titration against sodium hydroxide.
Step 2 – Use Titre Data to Find Moles of
Example data (updated):
Volume of used
Concentration of
sample used for titration
Initial moles of sulfuric acid
:
Moles of
In total mixture the used):
Total moles of
Step 3 – Subtract From Sulfuric Acid
contributes per molecule:
So, moles of ethanoic acid at equilibrium =
Key points
The pale pink endpoint can fade as reacts with more acid and shifts the equilibrium.
Ensure volumes are accurate, and no solution is lost during transfers.
can now be calculated using initial and equilibrium moles of each species.
No answer provided.
Common Mistakes and Their Consequences in the Practical
1. Rinsing the burette with water instead of sodium hydroxide solution
What happens: The solution is diluted by leftover water.
Consequence: The is less concentrated than expected more volume needed to neutralise the same amount of acid overestimates moles of acid, so value is underestimated.
2. Rinsing the pipette with water instead of the equilibrium mixture
What happens: Water remaining in the pipette dilutes the sample.
Consequence: Fewer moles of acid enter the conical flask lower titre underestimates moles of acid, leading to overestimated .
3. Rinsing the conical flask with
What happens: Extra is present in the flask before titration begins.
Consequence: Less volume is required from the burette titre is too low acid is underestimated is overestimated.
4. Not rinsing the funnel into the volumetric flask
What happens: Some of the equilibrium mixture remains in the funnel.
Consequence: Final volume has less acid/ester/ethanol than intended total moles in solution are incorrect, leading to an inaccurate value.
5. Not swirling the conical flask during titration
What happens: The acid and don’t mix evenly.
Consequence: The endpoint is reached too late or too early titre is unreliable introduces random error.
6. Overshooting the endpoint / inconsistent colour judgement
What happens: The solution turns a deeper pink than necessary.
Consequence: Too much is added overestimates moles of acid underestimates .
7. Not using a bung or allowing evaporation during the week
What happens: Volatile components like ethanol may evaporate.
Consequence: Less ethanol is present at equilibrium shifts equilibrium inaccurate .
8. Not accounting for acid catalyst (sulfuric acid)
What happens: The student includes all ions as if they came from ethanoic acid.
Consequence: Ethanoic acid at equilibrium is overestimated Kc is underestimated.
9. Dirty or wet glassware
What happens: Contamination or dilution alters concentrations.
Consequence: Introduces systematic error across all readings affects titre values and final calculation.
10. Not using concordant titres or averaging poor data
What happens: Unreliable or inconsistent titres are used in calculations.
Consequence: Introduces significant random error, reducing the reliability of .