Chemical Kinetics – Measuring Rates of Reaction

18 Question-and-Answer flashcards covering definitions, formulas and practical techniques for measuring reaction rates in chemical kinetics.

How is the rate of a chemical reaction defined?

Rate = change in concentration of a reactant or product per unit time.

What is the general mathematical expression for reaction rate?

Rate = Δ[Reactant or Product] ⁄ Δt, where Δ[ ] is in mol dm⁻³ and Δt is in seconds.

Which SI-derived unit is most commonly used for reaction rate?

mol dm⁻³ s⁻¹.

Give two alternative units that might be used when measuring reaction rate in specific experiments.

cm³ s⁻¹ (for gas volume) or g s⁻¹ (for mass loss).

Name at least four experimental methods for measuring the rate of a reaction.

Measuring gas volume evolved, measuring loss of mass, colorimetry, pH measurement, sampling & quenching with titration, electrical conductivity, precipitation / time to cloudiness.

What apparatus is used to measure rate by the volume of gas evolved?

A gas syringe or an inverted measuring cylinder in water, recording gas volume at regular time intervals.

For which types of reactions is the gas-volume method particularly suitable?

Metal + acid reactions and the decomposition of hydrogen peroxide (H₂O₂).

How is rate measured via loss of mass?

Place the reaction vessel on a balance and record the decrease in mass as gas escapes over time.

Why is the loss-of-mass method advantageous for fast reactions?

Balances give very precise, rapid readings, allowing accurate tracking of rapid mass changes.

How does a colorimeter help determine reaction rate?

It measures light absorbance of a coloured solution, which is proportional to the concentration of the coloured species.

Give one example of a reaction monitored by colorimetry due to a disappearing colour.

The reduction of potassium manganate(VII) (purple colour fading).

How can pH monitoring be converted into concentration data for rate calculations?

Use [H⁺] = 10^(-pH) to convert pH readings to hydrogen-ion concentration.

What are “sampling and quenching” in kinetics experiments?

Removing small reaction samples at set times and immediately stopping the reaction (e.g., by cooling or neutralising) before analysing concentration, often via titration.

Describe the ‘time to cloudiness’ precipitation method using thiosulfate.

Mix sodium thiosulfate with HCl above a marked cross; record the time for the cross to disappear as sulfur precipitate forms – shorter time indicates faster reaction.

How is the initial rate of reaction obtained from a concentration-time graph?

Draw a tangent to the curve at t = 0; the gradient of this tangent equals the initial rate.

What information does a rate–concentration graph provide?

It shows how the reaction rate changes as the concentration of one reactant varies, enabling determination of the reaction order with respect to that reactant.

What is the core idea behind a clock reaction such as the iodine clock?

Measure the time until a visible change (e.g., blue-black colour with starch); for short times, rate ∝ 1 ⁄ time.

List three key practices to improve accuracy and reliability in rate experiments.

Use clean, calibrated apparatus; maintain constant temperature (water bath); take frequent, regular readings and repeat experiments.