1.10 Rates of reaction & the investigation

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Taught 1 Dec – 9 Dec · Booklet pp. 53–57 · Homework week 8

By the end of this, you should be able to:

Determine the rate of a simple acid-base reaction
Identify & describe factors (concentration, temperature, surface area) which affect rate of reaction
Develop causal (qualitative) models to generate detailed & testable predictions
Communicate experimental design set-ups, equipment & key variables with the use of an experimental diagram
Calculate rates (of reaction) from measurements of change in volume per unit time
Appreciate that measurement accuracy can be improved by taking the average of multiple measurements
Identify independent & dependent variables and state their values
Appreciate what is meant by the term 'fair test'
Identify & account control variables in experimental designs
Identify hazards in a lab & suggest suitable precautions to reduce risk (risk assessment)
Collect & record data (by hand) in a table format
Use data processing software (Sheets, Excel, etc.) to organize (by column), process (averages) & present data in table format
Plot scatter graphs by hand showing how a dependent variable varies with an independent variable
Use data processing software (Sheets, LoggerPro, Excel, etc.) to plot scatter graphs showing how a dependent variable varies with an independent variable
Identify & describe plausible trends in data (increasing/up-up, decreasing/up-down, curved, linear, etc.)
Explain trends in data

How fast — and what changes it

What you need to understand

The rate of a reaction is how fast it happens — defined as the change in concentration of a reactant or product over time. A faster reaction uses up reactants and makes products more quickly.

Collision theory explains rate. For a reaction to occur, particles must collide, and the collision must be successful — the particles need enough energy to react. Anything that makes successful collisions happen more often speeds the reaction up.

The main factors are concentration, temperature, surface area and catalysts.

Why each factor works — all through collisions

Why it works

Every factor changes the rate for the same underlying reason: it changes how often successful collisions happen.

This also explains the shape of the graph. At the start, reactant concentration is highest, so collisions are most frequent and the line is steepest. As reactants are consumed, collisions become less frequent, the line flattens, and when a reactant runs out the reaction stops.

Why is the reaction fastest at the very start? At the start the reactant concentration is at its maximum, so particles collide most often. The most successful collisions per second means the fastest rate — which is why the graph is steepest there and then gradually levels off.

Going deeper

Surface area can be dramatic: a solid lump of flour barely burns, but flour dust exposes so much surface that flour mills have exploded. Catalysts are everywhere too — a car's catalytic converter uses them to clean up exhaust gases. In the IB Diploma you will quantify all of this with the Maxwell–Boltzmann distribution and activation energy, but the Year-9 picture — more frequent, more energetic, more successful collisions — already explains every rate change you will meet.

Diagrams and worked examples are in the Knowledge Book PDF below — they don't reproduce well on a page.

Now test yourself

Say it out loud before you tap.

Words you need

rate of reaction define it

the change in concentration of a reactant or product over time

E.G.20 cm³ of gas collected in 10 s → 2 cm³/s

NOTthe total gas produced — that is an amount, not a rate

collision theory define it

particles must collide with enough energy (a successful collision) to react

E.G.heating speeds a reaction: collisions are harder and more frequent

NOT"more collisions always means faster" — they must be successful

successful collision define it

a collision with enough energy for a reaction to occur

E.G.two particles hit hard enough that bonds break and new ones form

NOTparticles that collide and simply bounce apart unchanged

catalyst define it

a substance that speeds up a reaction without being used up

E.G.manganese(IV) oxide added to hydrogen peroxide

NOTa reactant — a catalyst comes back out unchanged at the end

surface area define it

the amount of a solid's surface exposed to react; greater for a powder than a lump

E.G.powdered marble reacts faster than one marble chip

NOTthe mass — same mass of powder and chip, very different surface area

Easy to get wrong

TRAP A catalyst is used up in the reaction. true or false?

TRUTH No — a catalyst speeds the reaction up but is recovered unchanged at the end, so a little goes a long way.

TRAP Any collision causes a reaction. true or false?

TRUTH Only successful collisions do — the particles must have enough energy. Most collisions simply bounce off.

TRAP A flat line on the graph means the reaction is fast. true or false?

TRUTH A flat line means the reaction has slowed or stopped. A steep line means it is fast.

TRAP More concentration means bigger particles. true or false?

TRUTH Concentration is about how many particles are packed into a space, not their size — more particles means more frequent collisions.

Check yourself

Higher temperature makes the rate… A. faster B. slower C. no change answer

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PhET Reactions and Rates opens in a new tab

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Investigation

Rates

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