Determining Rates of Reaction (SQA National 5 Chemistry): Revision Note

Average rate of reaction

• The rate of a reaction tells us how fast a reaction is

• The speed of a reaction changes over time

  • It's fastest at the start and slows down

• So, we often measure the average rate over a specific time period

• The average rate of reaction is the change in the amount of a reactant or product over a period of time

The average rate of reaction formula

• The average rate of reaction is calculated using a formula from the SQA Chemistry Data Booklet:

average rate =

• Average rate is the speed of the reaction over a certain time period

• Δquantity is the change in quantity

  • This is calculated by:

Δquantity = final quantity - initial quantity

• Δt is the change in time.

  • This is calculated by:

Δt = final time - initial time

Working out the units

• The units for the rate depend on the units used in the experiment

• You work them out by simply putting the quantity unit "per" the time unit

• For example:

  • If you measure volume in cm³ and time in s, the rate unit is cm³ s^-1

  • If you measure mass in g and time in s, the rate unit is g s^-1

Calculating average rate from a graph

Rates of Reaction over Time

• The rate of a chemical reaction is not constant

• By calculating the average rate at different stages, we can prove that a reaction slows down over time

Why the rate changes

• A reaction's speed changes as it progresses:

• At the start:

  • The rate is fastest

  • The concentration of reactants is high

  • So, the collisions are more frequent

• In the middle:

  • The rate slows down

  • The reactants are being used up

  • So, collisions become less frequent

• At the end:

  • The rate is zero

  • A reactant has run out

  • So, the reaction stops

Proving the rate decreases

• We can use a reaction-rate graph to calculate the average rate during different periods of the reaction

  • This mathematically proves that the reaction slows down