CHEMISTRY RATE EQUATIONS

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Rate of reaction

The change in concentration of a substance per unit time

Rate of reaction units

mol dm^-3 s^-1

How can we measure the rate of a reaction?

By timing an observable change e.g. collecting a volume of gas, observing a colour change, measuring the loss of mass or a solid becoming too small to see

To find rate of reaction from a graph of concentration against time...

Draw a tangent and find its gradient

Rate equations always take the form...

Rate = k [A]^m [B]ⁿ, where k is the rate constant and m and n are the orders of the reactions. The total order of the reaction is m + n.

If a substance in a rate equation isn't in the reaction, it is a...

Catalyst

What is a catalyst

They increase the rate of a chemical reaction but they are unchanged by the chemical reaction itself.

What does the order of a reaction mean?

It’s a number that tells you how the concentration of a reactant affects the reaction rate. It can only be determined by an experiment.

A + B + C = D + E If I was finding the order with respect to [A], what would I control?

The experiment would be repeated for several different concentrations of A. The other reactant concentrations should be in excess.

Zero order reaction

Rate = k [A]⁰
Rate is unaffected by changing the concentration of the reactant

First order reaction

Rate = k [A]¹
Rate is directly proportional to the concentration of the reactant

Second order reaction

Rate = k [A]²
Rate is proportional to the square of the concentration of the reactant.

What is the overall order?

The sum of the powers to which the reactants are raised.

Why is the overall order important?

• Determine the rate determining step for a reaction

• Determine the mechanism for a reaction

• Fast simplified analysis

Reaction order graphs - initial rate against concentration

Zero order - flat line
First order - straight line through the origin
Second order - curve upwards

What would you do if a rate/conc graph shows an unclear order?

Take logs of the rate equation: Rate = k [A]^m —> log Rate = mlog[A] + logk

It is now in the form y = mx + c.

Gradient = order with respect to A

What happens if the concentration of a reactant is much larger than another?

the concentration of excess reactant can be considered to be constant

This means the concentration of the chemical is unlikely to affect the rate of reaction

This excess reactant can be treated as zero order

So this chemical would not appear in the rate equation

Reaction order graphs - concentration against time (Reactants)

Zero order - straight line downwards (gradient = rate constant)
First order - curve downwards (constant half-life)
Second order - steeper curve downwards (increasing half-life)

Gradient for all = rate

What is the rate constant, k?

A constant that links concentration to the rate of a reaction,

The larger the value, the faster the rate of reaction and larger the temperature.

The units of k

Some multiple of mol dm^-3 and s^-1

The only factor which affects k is...

Temperature. If you increase temperature, rate increases and k does too

Rate determining step

The slowest step in a multi-step reaction

The rate equation only contains species...

In or before (if there is an intermediate step) the rate determining step. The order depends on how many times each substance appears before the RDS.

What molecule is more likely to undergo Sn2 mechanism?

Primary halogenoalkanes and tertiary (branched) halogenoalkanes to undergo Sn1 mechanism.

The Arrhenius Equation

k = Ae^(-Ea/RT)

lnk =

lnA - Ea/RT

Ea =

RT(lnA - lnK)

In a graph of lnk against 1/T, the gradient is...

-Ea/R

In a graph of lnk against 1/T, the y-intercept is...

lnA