Chemistry • Year 12 • Module 5 • Lesson 13
Temperature & Keq, Colourimetry
Lock in core vocabulary, the temperature–Keq relationship, Beer–Lambert law fundamentals, and the colourimetry procedure for measuring equilibrium constants.
1. Term–definition match 10 marks
Match each definition to its correct term. Write the term in the right-hand column. Choose from: Keq, Beer–Lambert law, absorbance, molar absorptivity (ε), calibration curve, colourimetry, van’t Hoff relationship, path length (l), equilibrium constant expression, coloured species.
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficient, at equilibrium at a given temperature. | |
| 1.2 | The law stating that absorbance equals the product of molar absorptivity, path length, and concentration: A = εlc. | |
| 1.3 | A dimensionless quantity that measures how much light is absorbed by a solution; proportional to concentration under Beer–Lambert conditions. | |
| 1.4 | A material-specific constant (units: L mol−1 cm−1) indicating how strongly a species absorbs light at a given wavelength. | |
| 1.5 | A graph of absorbance vs known concentration of a coloured species, used to read unknown concentrations from measured absorbance values. | |
| 1.6 | An analytical technique that uses the absorption of visible light by a coloured solution to determine its concentration. | |
| 1.7 | The qualitative (and quantitative) relationship linking the change in Keq to a change in temperature, governed by ΔH° of the reaction. | |
| 1.8 | The distance (in cm) that light travels through the sample in the cuvette; held constant in colourimetry experiments. | |
| 1.9 | A mathematical expression written from a balanced equilibrium equation showing which species appear in the numerator and denominator of Keq. | |
| 1.10 | In the Fe3+ / SCN− equilibrium system, this is FeSCN2+ — the only species that absorbs visible light significantly, allowing its concentration to be measured by colourimetry. |
2. True or false — with correction 12 marks
Circle T or F. If false, write the corrected statement on the line provided. 2 marks each: 1 for T/F, 1 for the correction where needed.
2.1 Temperature is one of several factors that change the numerical value of Keq. T / F
2.2 For an exothermic forward reaction, increasing temperature causes Keq to decrease. T / F
2.3 For an endothermic forward reaction, decreasing temperature causes Keq to increase. T / F
2.4 The Beer–Lambert law states that absorbance is proportional to concentration, provided path length and molar absorptivity are held constant. T / F
2.5 A calibration curve in colourimetry converts a measured absorbance value directly into a Keq value without further calculation. T / F
2.6 A reaction with Keq = 0.001 strongly favours reactants at equilibrium, consistent with a large positive ΔG°. T / F
3. Fill the blanks — colourimetry procedure 8 marks
Complete the paragraph using words from the box below. Each word is used once only.
The analytical technique known as (1) _________________________ uses the (2) _________________________ law to relate the (3) _________________________ of light by a coloured solution to its (4) _________________________. In practice, a (5) _________________________ is prepared by measuring absorbance for a series of solutions with known concentrations of FeSCN2+. This graph is a straight line through the origin because, for fixed path length and (6) _________________________, A is directly proportional to concentration. The (7) _________________________ concentration of FeSCN2+ is read from this graph and then entered into an (8) _________________________ to calculate all remaining equilibrium concentrations and hence Keq.
4. Function recall 10 marks
Answer each in 1–2 sentences using precise lesson vocabulary. 2 marks each.
4.1 Why is temperature the only factor that changes the value of Keq?
4.2 What is the role of the calibration curve in a colourimetry experiment?
4.3 Why is FeSCN2+ (and not Fe3+ or SCN−) measured by colourimetry in the iron thiocyanate equilibrium?
4.4 What does a very large positive ΔG° value tell you about the value of Keq for that reaction?
4.5 What is the function of the ICE table in calculating Keq from colourimetry data?
5. Build a concept map 5 marks
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “determines”, “is measured by”, “converts to”). Aim for at least 5 labelled arrows.
Supplied terms: temperature • Keq • absorbance • calibration curve • [FeSCN2+]eq • ΔH° of reaction
Q1 — Term–definition match
1.1 Keq • 1.2 Beer–Lambert law • 1.3 absorbance • 1.4 molar absorptivity (ε) • 1.5 calibration curve • 1.6 colourimetry • 1.7 van’t Hoff relationship • 1.8 path length (l) • 1.9 equilibrium constant expression • 1.10 coloured species.
Q2 — True or false with correction
2.1 False. Temperature is the only factor that changes the numerical value of Keq; concentration, pressure, and catalysts change the position of equilibrium but not Keq.
2.2 True.
2.3 False. For an endothermic forward reaction, decreasing temperature causes Keq to decrease (less product favoured at lower temperature). Increasing temperature increases Keq for an endothermic forward reaction.
2.4 True.
2.5 False. The calibration curve converts absorbance into [FeSCN2+]eq; the ICE table and the Keq expression must then be used to calculate Keq.
2.6 True.
Q3 — Cloze paragraph
(1) colourimetry • (2) Beer–Lambert • (3) absorbance • (4) concentration • (5) calibration curve • (6) molar absorptivity • (7) equilibrium • (8) ICE table.
Q4.1 — Why temperature alone changes Keq
Keq is determined by the thermodynamic energy landscape — the stability of products relative to reactants (ΔG° = −RT ln Keq). Because ΔG° = ΔH° − TΔS° contains temperature explicitly, changing temperature changes ΔG° and therefore Keq. No other factor (concentration, pressure, catalyst) changes the relative stability of reactants and products.
Q4.2 — Role of the calibration curve
The calibration curve converts the measured absorbance (a directly measurable instrumental quantity) into [FeSCN2+]eq (the concentration needed for the Keq calculation). Without the curve, the absorbance reading has no direct numerical concentration meaning.
Q4.3 — Why FeSCN2+ is measured
FeSCN2+ is intensely red-coloured and absorbs visible light strongly. Fe3+ and SCN− are essentially colourless at the concentrations used, so they do not absorb at the wavelength selected for the measurement. Only the coloured species can be detected by colourimetry.
Q4.4 — Large positive ΔG° and Keq
A large positive ΔG° means ln Keq is large and negative (ΔG° = −RT ln Keq), so Keq << 1. Reactants are strongly favoured at equilibrium and the forward reaction is non-spontaneous under standard conditions.
Q4.5 — Function of the ICE table in colourimetry
The ICE table uses the known initial concentrations of Fe3+ and SCN− and the measured [FeSCN2+]eq to calculate the equilibrium concentrations of all three species. These are then substituted into the Keq expression to calculate the equilibrium constant.
Q5 — Sample concept map connections
Award 1 mark per valid labelled arrow (minimum 5 required):
- temperature —combined with→ ΔH° of reaction —determines direction of change of→ Keq
- temperature —changing it changes→ Keq
- absorbance —read against→ calibration curve —gives→ [FeSCN2+]eq
- [FeSCN2+]eq —substituted into ICE table to calculate→ Keq
- ΔH° of reaction —sign determines whether Keq increases or decreases with→ temperature
Any biologically valid linking phrases accepted; correct causal direction required.