Rates and Orders

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Rate of reaction

Definition: is the change in concentration of a reactant or product per unit time

Equation= change in concentration of reactan or product / time taken for change to take place

As a reaction proceeds, the concentrations of the reactants decrease:

  • fewer collisions take place per second between reactant particles
  • the rate of the reaction slows down.

Initial rate: the change in concentration of a reactant, or product, per unit time at the start of the reaction when t=0

Measuring rates

The rate of the reaction can be determined by measuring the concentrations at time intervals during the course of a reaction. Results are plotted on a concentration-time graph

  • at any instant of time, the rate is equal to the slope of the graph
  • the slope is measured by drawing a tangent to the curve at this time
  • the gradient of the tangent is then calculated .
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Experimental techniques to measure reaction rates

for reactions involving acids or bases, measure:

  • pH changes via titrations or by using a pH meter

For reactions that produce gases, measure:

  • the change in volume or pressure, or the loss in mass of reactants

For reactions that produce visual changes, observe:

  • the formation of a precipitate, or a colour change
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