Rate of reaction.

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Rate of chemical reactions

Rate of the reaction- defined as the change in concentration (of any of the reactants of products) with unit time.


To find the rate of change (c) at a particular instant we draw a tangent to the curve at that time and then find its gradient.

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Reaction Rates are measured in:

mol dm-3 s-1

There are 3 ways of recording the rate of reaction:

  • Time how long it takes for the product to disappear,
  • Calculate the volume of gas collected over time,
  • Weigh the loss of gas from the product.

Rate = Change in CONCENTRATION / TIME for change to occur

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Rate expression and order of reaction

Rate expression- tells us about the contributions of the species that do affect the reaction rate.

It is an equation that describes how the rate of the reaction at a particular temperature depends on the concentration of species involved in the reaction. It's quite possible that 1 or more of the species that appear in the chemical equation will not appear in the rate expression. This means they do not affect the rate.

Order of reaction- the experimentally determined POWER to which a reactant concentration is raised in a rate equation.

Rate = K (A)p

K = a constant for a particular reaction at a given temperature (the rate constant)

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Order of reaction

Zero Order

If doubling (A) has no effect on the rate then the reaction is said to be zero order w.r.t A.


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First order

If doubling (A) doubles the rate then the reaction is said to be first order w.r.t A.


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Second order

If doubling (A) increases the rate fur fold then the reaction is said to be second order w.r.t A.


It is unusual for reactions to be mre than second order with respect to any reactant.

to get overall order add all the powers together.

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