Chapter 18 Rates of reactions

How do chemists measure the rate of a reaction?

Rate = change in concentration/change in time

What are the units for rate of reaction?

mol dm³ s^-1

What is the rate of reaction proportional to?

The concentration of a particular reactant

What is the order with respect to a reactant?

A measure of how the change in the concentration of that reactant will affect the overall rate.

What is zero order?

When the concentration has no effect on the rate

What is first order?

When the rate depends on the concentration of a reactant raised to the power of one

What is second order?

When the rate depends on the concentration of a reactant raised to the power of two

What does the rate equation do?

It gives the mathematical relationship between the concentrations of the reactants and the reaction rate

What is the rate constant?

The proportionality constant. It is the number that mathematically converts between the rate of reaction and concentration and orders

What is the overall order of reaction?

The value that gives the overall effect of the concentrations of all reactants on the rate of reaction. It is the sum of orders with respect to each reactant.

What is the initial rate?

The instantaneous rate at the beginning of a reaction when t = 0

What is continuous monitoring?

Continuous measurements taken during the course of a reaction. This can be monitored by gas collection and mass loss.

What is a colorimeter?

A device that measures the absorbance or transmittance of specific wavelengths of light by a solution to determine its concentration or quantify its colour

What is the gradient of a concentration-time graph equal to?

The rate of reaction

What does the concentration-time graph for a zero order reaction look like?

A straight line with a negative gradient

What does the concentration-time graph for a first order reaction look like?

Downward curve with a decreasing gradient over time. The time for the concentration of the reactant to halve is constant

What does the concentration-time graph for a second order reaction look like?

A downward curve, steeper at the start but tailing off more slowly

What is the half-life of a reaction?

The time taken for half of a reactant to be used up. Constant half-life is called exponential decay

How can you calculate the rate constant from the half-life?

k = ln2/t_1/2

What are the different shapes of a rate-concentration graph?

What is observed in a clock reaction?

A visual change, and the time it takes for this to happen is measured

What are you measuring in a clock reaction?

The average rate during the first part of the reaction

What assumptions are made in a clock reaction?

The average rate of reaction is constant and is the same as the initial rate

Why are clock reaction results more accurate when there is a shorter time taken for a colour change?

Because that means there are less rate changes over that period of time

What is the reaction mechanism?

The series of steps that make up an overall reaction

Why is the following reaction likely to take place in a series of steps?

Because all of the reactants colliding at once would be an extremely unlikely event

What is the rate-determining step?

The slowest step in a reaction mechanism.

How do you know whether a reaction mechanism is likely to be correct?

• The rate equation only includes reacting species involved in the rate-determining step

• The orders in the rate equation match the number of species involved in the rate-determining step

What is the Arrhenius equation?

k = Ae^-Ea/RT

What does the exponential factor (e^-Ea/RT) represent?

The proportion of molecules that exceed E_a and that have sufficient energy for a reactio