The method and the results are on the chemical equilibrium lab. Do the analysis yourself first, then check the answers below.
Quick answer: A reversible reaction can go in both directions, and in a closed system it reaches dynamic equilibrium when the forward and reverse rates are equal and concentrations stay constant. These answers go with the Chemistry 9-10 lab on chemical equilibrium: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Use the sample scores to check your reasoning. Your own colour scores will differ by a point or two, but tubes 2, 3 and 6 should be darker than the control and tubes 4 and 5 paler. Link each change to the shift in equilibrium and the energy sign.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-6
Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq). Extra Fe³⁺ (tube 2) or extra SCN⁻ (tube 3) raises the reactant concentration, so the equilibrium shifts right to use some up, making more red ion and raising the score from 5 to 8 and 7. Shift right.
Question 2 Effect of sodium hydroxide
1. Fe³⁺ + 3OH⁻ → Fe(OH)₃(s). 2. Removing Fe³⁺ shifts the equilibrium left to replace it, so the red ion breaks down and the colour score falls from 5 to 2. Shift left.
Question 3 Temperature effect
Warming (tube 5) gives a paler solution, so the equilibrium shifts left; cooling (tube 6) gives a darker one, so it shifts right. Heating favours the endothermic direction, so the reverse reaction is endothermic and the forward reaction is exothermic: ΔH is negative. Forward reaction exothermic; ΔH negative.
Question 4 Role of the control
Tube 1 is the standard for comparison, so any difference can be linked to the change made. All tubes are scored together under the same light because the equilibrium changes slowly and the colour looks different in different lighting. A control is the reference colour.
Question 5 A dynamic system
Both reactions continue at the same rate, so the particles keep forming and breaking [Fe(SCN)]²⁺ and the net amounts stay constant. This is dynamic equilibrium, not a stopped reaction. Forward rate equals reverse rate.
Question 6 Limits of the method
Judging colour by eye is subjective, so two students may give different scores. A colorimeter measures the absorbance of light and gives a number that depends on the concentration of the red ion. Use a colorimeter.
Question 7 Prediction and conclusion
Adding solid potassium thiocyanate raises the concentration of SCN⁻, so the colour becomes darker red (a shift to the right). Conclusion: adding reactants or cooling shifts this equilibrium right; removing a reactant or heating shifts it left, which agrees with Le Chatelier’s principle. Darker red.
