📖 In-Depth Theory
What is Rate of Reaction?
The RATE OF REACTION measures how quickly reactants are used up or products are formed.
FAST reactions: explosions, burning, acid + reactive metal.
SLOW reactions: rusting, paint drying, decay.
Rate can be measured by monitoring:
The DECREASE in mass of reactants over time.
The INCREASE in mass or volume of products over time.
The DECREASE in concentration of a reactant.
The INCREASE in concentration of a product.
The amount of GAS PRODUCED per unit time.
The CHANGE IN COLOUR or TURBIDITY (cloudiness) over time.
Calculating Rate of Reaction
RATE OF REACTION formula:
Rate = quantity of reactant used or product formed ÷ time
Units depend on what is measured:
If measuring VOLUME of gas: rate = cm³/s or cm³/min.
If measuring MASS: rate = g/s or g/min.
If measuring CONCENTRATION: rate = mol/dm³/s.
From a GRAPH:
The GRADIENT (slope) of a graph of quantity vs time = the rate at that point.
Steep gradient → fast rate.
Flat gradient → slow rate (or reaction finished).
The gradient DECREASES over time as reactants are used up.
EXAMPLE:
A reaction produces 60 cm³ of gas in 120 seconds.
Rate = 60 ÷ 120 = 0.5 cm³/s
MEAN RATE vs INSTANTANEOUS RATE:
MEAN RATE = total quantity ÷ total time (average over the whole reaction).
INSTANTANEOUS RATE = gradient of a tangent to the curve at a specific point in time.
Measuring Rate Experimentally
Common experimental methods for measuring rate:
GAS COLLECTION:
Collect gas produced over water in an inverted measuring cylinder, or in a gas syringe.
Measure volume of gas collected at regular time intervals.
Plot volume vs time → gradient = rate.
Example: Mg + 2HCl → MgCl₂ + H₂ (measure H₂ collected).
MASS LOSS:
Place reaction flask on a balance.
CO₂ escapes → mass decreases.
Record mass at regular intervals.
Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ (measure mass loss).
COLOUR CHANGE / TURBIDITY:
For reactions involving a colour change — measure colour intensity with a colorimeter.
For precipitation reactions — observe when a mark under the flask becomes invisible.
Example: sodium thiosulfate + HCl → sulfur precipitate clouds the solution.
Time taken for the cross to disappear → inversely proportional to rate.
⚠️ Common Mistake
The rate DECREASES over time in a closed system as REACTANTS ARE USED UP — fewer reactant particles mean fewer collisions. The graph flattens out when the reaction is complete (all reactants used). A steeper initial gradient = faster initial rate.