What a learner can do afterwards
- Writes a rate equation from initial rate data by comparing runs where one concentration changes at a time
- States the order with respect to each reactant and the overall order
- Works out the units of the rate constant for a given overall order
- Explains how a species in the balanced equation can turn out to be order zero
1 · Read
A rate equation links speed to concentration: rate equals k times each reactant raised to its order. The powers are the orders, and their sum is the overall order. The balanced equation never hands you these powers. Only experiment can.
Compare runs where one concentration changes at a time. Doubling that reactant doubles the rate for first order, quadruples it for second, and leaves it flat for zero. Measure starting slopes before products interfere.
Take rate equals k times A times B squared: first order in A, second in B, third overall. The units of k follow the overall order: per second for first, decimetres cubed per mole per second for second, and moles per decimetre cubed per second for zero.
A species in the equation can still be order zero, usually because it reacts after the slow step or sits in such excess that its level barely moves. Rate cares about the slow step, not the paper balance.
Read orders from initial rate runs, add them for the overall order, and set the units of k to match.
2 · Watch
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Where it sits
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.