Reaction rates

• Zero order: The rate is unaffected by changes to the concentration.
• First order: The rate is directly proportional to the concentration.
• Second order: The rate increases by a factor of the concentration squared.

• Zero order: The concentration decreases at a constant rate, while the half life decreases with time.
• First order: The concentration halves in equal time intervals, therefore the half life is constant.
• Second order: The concentration decreases rapidly, before the rate of decrease then slows down. The half life increases over time.

• The units for rate of reaction are mol dm−3s−1
• Rate = change in concentration / time.
• rate=k[B]1[C]2rate=k[B]1[C]2rate=k[B]1[C]2rate=k[B]1[C]2Rate = k[A][B]
• If a is presented as [A]^2 it means it is second order. If presented as [A]^1 it is first order with respect to A.

• This is the slowest step in the reaction mechanism of a multi-step reaction.

Example

rate = k[NO2]^2

From the rate equation we know there is 2 NO2 reactants in the rate determining step.

O and CO2 must be involved in the second step as they are involved in the overall equation.

• NO2 must be a product of the second step as there is only one molecule of NO2 is in the overall reaction.

• By balancing the second step equation, the other reactant in the second step must be NO3 .
• The products of the first step must be NO and NO3 .

This gives the completed mechanism:

NO3 NO3 s the intermediate as it is generated in the first step and is consumed in the second step.

Information obtained from : https://revise.im/chemistry/eee/rates