Kinetics II – Experimental Methods and Rate Data

30 vocabulary flashcards covering key experimental techniques, terms, and concepts for Edexcel Topic 16 Kinetics II rate experiments.

What is the initial-rate method?

Technique measuring the rate at the very start of several reactions with varied initial reactant concentrations to deduce the rate law.

What is a clock reaction?

Reaction giving a sudden observable change after a fixed amount of product forms, used as an approximation of the initial-rate method (1/t \propto \text{initial rate}).

What is the continuous monitoring method?

Approach in which samples are taken or a property is recorded throughout the reaction, producing concentration-time or volume-time data.

What is quenching in chemical kinetics?

Rapidly stopping a withdrawn sample’s reaction (e.g., by cooling or neutralising) so its composition no longer changes before analysis.

What is colorimetry?

Analytical technique that follows reaction progress by measuring the absorbance or transmittance of a coloured species at a selected wavelength.

What is a calibration curve?

Plot of absorbance versus known concentrations used to convert colorimeter readings into actual concentrations of a coloured species.

What is the mass change method for rate measurement?

Rate measurement that records the decrease in mass—usually due to gas loss—over time to follow reaction progress.

What is the volume of gas evolved method?

Technique that measures the volume of gas produced with a gas syringe or inverted burette to monitor reaction rate.

How is titration used in a rate study?

It involves withdrawing reaction samples at set times and titrating them to determine concentration of a reactant or product for rate calculations.

What is 'mean rate' in chemical kinetics?

It is the overall change in concentration (or amount) divided by the total time interval considered.

How do you determine 'instantaneous rate'?

It is the reaction rate at a specific moment, obtained from the gradient of a tangent to a concentration-time curve.

What is an independent variable in a kinetic experiment?

It is the factor deliberately changed in an experiment (e.g., initial concentration) to observe its effect on rate.

What is a dependent variable in a kinetic experiment?

It is the quantity measured in an experiment that responds to changes in the independent variable, typically reaction rate.

What are control variables in kinetic experiments?

These are all other conditions kept constant (temperature, volume, pH, etc.) to ensure a fair kinetic test.

Why is sodium hydrogencarbonate used as a quench in the iodine–propanone experiment?

It is added to neutralise H^{+} in the iodine–propanone experiment, removing the acid catalyst and stopping the reaction.

What is the role of starch indicator in chemical reactions?

It forms a blue complex with triiodide ions; used to detect the presence of iodine at the end-point of titrations or clock reactions.

What is a triiodide ion (I₃⁻)?

It is an ion produced when I₂ reacts with I⁻; fits into starch helices to give an intense blue colour.

What is thiosulfate titration used for?

It is a redox titration in which S₂O₃²⁻ reduces I₂ to I⁻, allowing quantitative analysis of iodine concentration.

How does iodine (I₂) act as an oxidising agent?

Iodine (I₂) accepts electrons to form I⁻, acting as a mild oxidiser in redox reactions.

What is peroxodisulfate (S₂O₈²⁻) and its role in the iodine clock experiment?

It is a strong oxidising ion that converts I⁻ to I₂ in the iodine clock experiment.

What is a rate-concentration graph used for?

It is a plot of initial rate versus reactant concentration used to deduce reaction order.

What is the order of reaction?

It is the power to which a reactant concentration is raised in the rate equation, determined experimentally from kinetic data.

What is the rate constant, k?

It is the proportionality factor in the rate equation that is temperature-dependent but concentration-independent for a given reaction.

How is the conductivity method used to monitor reaction progress?

It monitors reaction progress by measuring changes in electrical conductance when ionic species are consumed or produced.

What is the pH monitoring method?

It involves tracking reaction progress by recording pH changes where acids or bases are involved.

What is a gas syringe used for in kinetic experiments?

It is an apparatus for accurately measuring the volume of gas evolved during a reaction over time.

Why is temperature control important in kinetic experiments?

It involves maintaining constant temperature so observed rate changes arise only from the variable under study.

What is a 1/t vs concentration plot used for?

It is a graph used in initial-rate or clock experiments where the reciprocal of the time for a fixed event is plotted against reactant concentration to find order and k.

Describe the iodine–propanone reaction and its rate law.

The reaction is I₂ + CH₃COCH₃ \rightarrow CH₂ICOCH₃ + HI. It is acid-catalysed with rate law Rate = k[CH₃COCH₃][H⁺].

What is the role of an

These are protons that increase reaction rate without being consumed, e.g., the catalyst in the iodine–propanone kinetics study.