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Rate Laws and Reaction Order

High schoolChemistryEmpirical two-reactant initial-rate law with independently fitted orders and buffered-B integrated traces

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About this experiment

Prepare independent A and B concentrations, measure initial rates and recover the hidden reaction orders from a table of trials. Then follow A depletion while B is held constant.

Learning goals

  • Separate a rate law from stoichiometric coefficients.
  • Design independent concentration comparisons.
  • Fit both reaction orders with enough independent measurements.

How to use it

  1. 1Measure an initial rate at the starting A and B concentrations.
  2. 2Change A alone, then B alone, or run the three controlled comparison trials.
  3. 3Read the fitted orders and reveal the hidden settings to check them.
  4. 4Run a time trace and compare reactant disappearance with product formation.

The model

Dimensionless concentration ratios keep the displayed rate-scale units consistent.
The fitted plane identifies the two orders if the trial settings are independent.
This is the first-order-in-A trace; the zero- and second-order traces use their respective integrated laws.

Assumptions

  • The empirical rate is k times dimensionless concentration ratios raised to integer orders zero, one or two. The reference concentration is one mole per litre.
  • The rate scale therefore has mole-per-litre-per-second units for every selected order. B is maintained by an external buffer reservoir during each trace.
  • Only A is depleted and generic product concentration increases by the same amount. The model is not a closed stoichiometric A-plus-B batch reaction.
  • Initial-rate trials have no added measurement noise. Log-linear fitting requires independent A and B settings; changing the underlying law clears the table.

Try this in class

  1. Q1Why can changing only A fail to identify the order in B?
  2. Q2Why must initial rates be compared before appreciable depletion?

Frequently asked questions

Are the orders always the stoichiometric coefficients?

No. They are empirical kinetic parameters. This activity allows independent order choices.

Why is B not decreasing during the time trace?

A reservoir holds B constant. The activity isolates the effect of a buffered co-reactant rather than modelling a closed two-reactant batch.