chapter 1 - rates of reactions

when do reactions occur?

• when molecules collide into one another

in a reaction between molecule A and B, what is the rate proportional to?

• the number of collisions between A and B

  • so, if you double the concentration of B, you should double the collisions between A and B

• so, you would expect the reaction rate to double

what form is the rate reaction normally written in?

• rate = k × [A]^m[B]ⁿ

  • the numbers m and n tell you how the rate depends on the reactants

  • m is the order of reaction with respect to A, while, n is the order of reaction with respect to B

    • k is the rate constant

first order reaction?

• a reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one

second order reaction?

• a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of 2

example: the rate equation and overall order for a reaction

example: rate constant from experimental results

*note= always start where A doubles and B stays the same

example: harder rate constant from experimental results

how to know if reactant is zero, first or second order?

• zero order - no matter what you do to the concentration, the rate does not change

• first order - what you do to the concentration, the rate changes by the same factor

• second order - whatever you do to the concentration, the rate changes by the same factor squared

3 key rules when finding the units of the rate constant?

what is the overall order?

m + n

features of the rate constant?

• it is temperature dependent

• it has different units for different reactions

what do the units of the reaction rate have to be?

• concentration over time

• but the units of the products of the reactants are never that, so you need to assign units to k to sort it out

  • the larger the rate constant, the faster the reaction

calculating the rate constant?

• E.g. the rate equation for the production of I-Cl is: Rate = k[I₂]¹[Cl₂]¹

• Given that the concentrations of I₂ and Cl₂ are 1moldm^-3, and the rate of reaction is 40moldm^-3t^-1, what is the value of k?

• k = 40moldm^-3t^-1 ÷ (1moldm^-3)² = 40mol^-1dm³t^-1

calculating the rate of reactions?

• E.g. if a reaction obeys the equation:

  • Rate = k[A][B], where the rate constant is 0.5dm³mol^-1t^-1, and the reactant concentrations are both 2moldm^-3

  • The rate is 2moldm^-3 × 2moldm^-3 × 0.5dm³mol^-1t^-1 = 2moldm^-3t^-1