5.1.1 How Fast: Module 5: Physical Chemistry & Transition Elements: Chemistry OCR A A Level

Define rate of reaction.

Change in concentration of a reactant or a product per unit time.

At a given instant, how could you calculate the rate of reaction?

Rate of reaction = Change in concentration of reactants or products / Time

If the order is zero with respect to a reactant, what does that mean?

Changing the concentration of the reactant has no effect on the rate.

What is the effect on rate in a first order reaction?

Rate is directly proportional to the concentration.

What is the effect on rate in a second order reaction?

Change in rate = Change in concentration squared

Write a generic rate equation and state what each term means.

Do zero order reactants appear on rate equation? Why?

Do zero order reactants appear on rate equation? Why?

No, because they don’t have an effect on the rate.

How is overall order of a reaction calculated?

Sum of individual orders.

When the overall order is 3, what would be the units for rate constant?

dm^6•mol^-2•s^-1

How could you measure the rate of reaction experimentally (different methods)?

Use a colorimeter at suitable intervals if there is a colour change.

If gas is evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture.

What does half life mean?

The time taken for concentration of a reactant to decrease by half.

What is the symbol for half life?

t_1/2

What is the relationship between first order reactions and half life?

First order reactants have constant half lifes.

What is the equation that is used to determine rate constant using half life in a first order reaction?

k = ln2 / (t_1/2)

How would you draw a rate concentration graph?

Plot [A] against time, draw tangents at different values → draw a secondary graph of rate against [A]

What is the relationship between rate and time?

Rate ∝ 1/t

How to determine the rate constant from a rate concentration graph of first order?

k = rate / concentration

What is rate determining step?

The slowest step in a reaction with multiple steps.

How does the rate determining step link to the species involved in the rate equation?

Any species involved in the rate determining step appear in the rate equation. Species only involved after the rate determining step do not appear in the rate equation.

For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?

The order of the reactant.

The rate equation of a reaction is rate = k[NO]^2 . How many molecules of NO will be present in the rate determining equation?

2

What affects the value of the rate constant for a given reaction?

Temperature, nothing else.

What is the effect of a 10℃ temperature increase on the rate of reaction, roughly?

Doubles rate of reaction.