A-Level Chemistry Practical — Rate of Reaction

Objectives: A-Level Chemistry Practical — Rate of Reaction

A-Level Chemistry Practical — Rate of Reaction

A-Level Chemistry Practical — Determination of Rate Constant

We investigate the reaction between sodium thiosulfate and hydrochloric acid:

Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l)

Notes: This reaction produces sulfur precipitate which causes the solution to become opaque. The time taken for the cross under the flask to disappear is measured as a method to determine the rate of reaction.

Video Demonstrations: Observe disappearance of the cross as sulfur precipitate forms. Measure reaction time accurately.

Practical Question (A-Level)

Given: Various concentrations of sodium thiosulfate reacted with excess HCl. Time (t) for solution to become opaque was recorded:

[Na2S2O3] / MTime t / s
0.10020
0.08025
0.06033
0.04050
0.020100
  1. Determine the rate constant k assuming rate = k[Na2S2O3]^1.
  2. Plot a graph of 1/t vs [Na2S2O3] to verify order.
  3. Explain observations and real-life relevance.

Step-by-Step Answer

  1. Rate ∝ 1/t since HCl is in excess: rate = k[Na2S2O3]^1.
  2. Compute 1/t values:
    3. [Na2S2O3] / Mt / s1/t (s⁻¹)
    0.100200.0500
    0.080250.0400
    0.060330.0303
    0.040500.0200
    0.0201000.0100
  3. Plot 1/t vs [Na2S2O3]. Straight line through origin confirms first order.
  4. Rate constant k = slope of 1/t vs [Na2S2O3] line ≈ Δ(1/t)/Δ[Na2S2O3] = (0.0500-0.0100)/(0.100-0.020) ≈ 0.5 s⁻¹·M⁻¹.
  5. Observations: Higher concentration → faster reaction (smaller t). Sulfur precipitate forms sooner.
  6. Real-life relevance: Understanding reaction kinetics is essential in industry for controlling rates of chemical reactions and in environmental processes like water treatment.

Interactive Graph — 1/t vs [Na2S2O3]

Concentration [Na2S2O3]: -- M | 1/t: -- s⁻¹

More Practical Videos

Observe differences in reaction times with varying concentrations.

Reference Book: N/A

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