Equilibrium Constant Kc and Kp

Active learning works for equilibrium constants because students often confuse the equilibrium position with the value of Kc or Kp. Hands-on activities let them observe that K remains constant at a fixed temperature, even when concentrations or pressures change, which addresses a key conceptual hurdle in this topic.

• Year 12
• Chemistry
• 4 activities
• 20 min–45 min

Learning Objectives

1. Construct equilibrium constant expressions (Kc and Kp) for given homogeneous and heterogeneous reversible reactions, correctly omitting pure solids and liquids.
2. Calculate the numerical value of Kc or Kp from experimental concentration or partial pressure data at equilibrium.
3. Explain the quantitative relationship between the magnitude of Kc or Kp and the relative amounts of reactants and products at equilibrium.
4. Analyze how changes in temperature affect the numerical value of Kc or Kp, relating this to reaction enthalpy.
5. Compare the effect of changes in concentration or pressure on the position of equilibrium versus the value of Kc or Kp.

Problem-Based Learning

⏱ 35 min·Pairs

Pairs Calculation: Fe(SCN)2+ Colorimetry Data

Pairs receive absorbance data from Fe3+ + SCN- equilibrium. They use calibration curves to find equilibrium concentrations, calculate Kc three ways, and compare results. Discuss why values match despite method differences.

Prepare & details

Construct expressions for Kc and Kp for various reversible reactions.

Facilitation Tip: During the Fe(SCN)2+ Colorimetry Data activity, circulate and ask pairs to justify their Kc calculations using their absorbance readings, ensuring they connect the data to the expression.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 45 min·Small Groups

Small Groups: Kp Gas Equilibrium Stations

Set up stations with syringes for N2O4 ⇌ 2NO2. Groups measure volumes at equilibrium, calculate partial pressures and Kp at two temperatures. Rotate to verify peers' calculations and plot ln Kp vs 1/T.

Prepare & details

Explain the significance of the magnitude of Kc or Kp.

Facilitation Tip: For the Kp Gas Equilibrium Stations, provide a reference sheet with the ideal gas law and standard pressures to support calculations at each station.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 30 min·Whole Class

Whole Class Demo: Perturbing Esterification

Project live esterification setup (ethanoic acid + ethanol). Class predicts, observes color shifts with added water or acid, then calculates Kc before/after. Vote on explanations via mini-whiteboards.

Prepare & details

Analyze how changes in conditions affect the value of Kc or Kp.

Facilitation Tip: In the Perturbing Esterification demo, pause after each disturbance to ask the class to predict whether Kc will change, reinforcing the concept that only temperature affects K.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 20 min·Individual

Individual Challenge: Expression Builder Cards

Students draw reaction cards (e.g., 2SO2 + O2 ⇌ 2SO3), sort concentration/pressure terms into Kc or Kp expressions. Swap and peer-check before sharing heterogeneous examples.

Prepare & details

Construct expressions for Kc and Kp for various reversible reactions.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Teaching This Topic

Teach equilibrium constants by starting with simple expressions before introducing heterogeneous systems and the exclusion of pure solids and liquids. Use real-world examples like the Haber process to ground the concept in familiar contexts. Research shows that students grasp the constancy of K better when they manipulate data themselves rather than relying on teacher explanations alone.

What to Expect

Successful learning looks like students accurately writing Kc and Kp expressions, calculating values from data, and explaining why K does not change with perturbations. They should also justify their reasoning using Le Chatelier’s principle and the role of temperature in shifting equilibrium and altering K.

Watch Out for These Misconceptions

Common Misconception

During the Fe(SCN)2+ Colorimetry Data activity, watch for students who recalculate Kc after adding more Fe3+ or SCN- and assume the value changes.

What to Teach Instead

Before students begin the activity, explicitly state that Kc is constant at a fixed temperature, and remind them to record the temperature of their solutions. During the activity, circulate and ask each pair to confirm their Kc value is the same after each addition, reinforcing the concept.

Common Misconception

During the Kp Gas Equilibrium Stations activity, watch for groups including all gases in their Kp expressions, even those not in the balanced equation.

What to Teach Instead

Provide each station with a diagram of the gas mixture and the balanced equation. Ask groups to highlight the gases involved in the reaction before writing their expressions, and facilitate a class discussion to clarify why only reacting gases are included.

Common Misconception

During the Expression Builder Cards activity, watch for students who argue that Kc cannot be compared across reactions due to units.

What to Teach Instead

Guide students to rewrite Kc expressions using activities (dimensionless forms) during the activity. Provide a set of example calculations where Kc values are unitless, and ask students to compare these to reinforce the idea that Kc can be compared across reactions when expressed correctly.

Assessment Ideas

Quick Check

After the Expression Builder Cards activity, provide students with three different balanced chemical equations and ask them to write the Kc or Kp expression for each, clearly indicating which is which and why.

Exit Ticket

During the Perturbing Esterification demo, ask students to predict whether the equilibrium mixture will contain more products or reactants after adding more ethanol, and explain their reasoning in one sentence.

Discussion Prompt

After the Kp Gas Equilibrium Stations activity, pose the question: 'Le Chatelier's principle predicts how equilibrium shifts with changing conditions, but only temperature changes the value of Kp. Why is this the case?' Guide students to connect this to the enthalpy of reaction and the definition of equilibrium constants.

Extensions & Scaffolding

• Challenge: Ask students to design a scenario where Kc and Kp are equal for a gaseous reaction, explaining the conditions required.
• Scaffolding: Provide a partially completed Kc or Kp expression with gaps for students to fill during the Expression Builder Cards activity.
• Deeper exploration: Have students research how equilibrium constants are used in industrial processes, such as the contact process for sulfuric acid production, and present their findings to the class.

Key Vocabulary

Homogeneous Equilibrium	An equilibrium state where all reactants and products are in the same physical state, typically all gases or all aqueous solutions.
Heterogeneous Equilibrium	An equilibrium state involving reactants and products in different physical states, such as a solid reacting with a gas or a liquid.
Equilibrium Constant (Kc)	A value representing the ratio of product concentrations to reactant concentrations at equilibrium, raised to the power of their stoichiometric coefficients, for homogeneous systems.
Equilibrium Constant (Kp)	A value representing the ratio of product partial pressures to reactant partial pressures at equilibrium, raised to the power of their stoichiometric coefficients, for gaseous systems.
Partial Pressure	The pressure exerted by a single gas in a mixture of gases, proportional to its mole fraction in the mixture.