6a Physical and chemical change, rates | IGCSE Chemistry 0620

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🎯What you need to be able to do

Identify physical and chemical changes and describe the differences.
Describe the effect on rate of concentration, gas pressure, surface area, temperature and catalysts (including enzymes).
State that a catalyst increases the rate and is unchanged at the end; that it lowers the activation energy EXTENDED.
Describe practical methods for measuring rate (mass loss, gas volume) and interpret rate data and graphs; evaluate the methods EXTENDED.
Explain the effects using collision theory: particles per unit volume, collision frequency, kinetic energy and activation energy EXTENDED.

📚The chemistry

Physical and chemical changes

A physical change (melting, dissolving, boiling) forms no new substance and is usually easy to reverse. A chemical change forms one or more new substances, is usually hard to reverse, and is often accompanied by an energy change, a colour change, a gas or a precipitate.

What speeds a reaction up

higher concentration of a solution

higher pressure of a gas

larger surface area (smaller pieces, powder)

higher temperature

adding a catalyst (enzymes are biological catalysts)

EXTENDED Collision theory explains all five. Particles must collide, with at least the activation energy, to react.

Concentration, pressure and surface area increase the number of particles per unit volume (or exposed at the surface), so collisions are more frequent.
Temperature gives particles more kinetic energy: they collide more often and a larger proportion of collisions have energy greater than E_a. The second effect is the bigger one.
A catalyst provides a pathway with a lower activation energy, so more collisions are successful.

A reaction pathway diagram with two humps from the same reactants to the same products: a tall hump without a catalyst and a lower hump with a catalyst. The activation energy with the catalyst is smaller; the enthalpy change is the same for both. — A catalyst lowers E_a but does not change \( \Delta H \) — the reactants and products stay at the same levels. EXTENDED

Measuring the rate

For a reaction that gives off a gas, such as calcium carbonate with hydrochloric acid, either

collect the gas in a gas syringe and record its volume at regular times, or
stand the flask on a balance (with a cotton wool plug to let gas out but stop acid spray) and record the loss in mass.

A conical flask containing marble chips in hydrochloric acid, sealed with a bung and delivery tube connected to a horizontal gas syringe. The plunger moves out as carbon dioxide is collected; a stop-watch measures the time. — Gas volume against time. A gas syringe measures the volume directly; collecting over water would let some CO₂ dissolve.

Reading rate graphs

The gradient of a volume–time (or mass-loss–time) graph is the rate. It is steepest at the start, when the reactants are most concentrated, and becomes zero when a reactant has been used up. The final volume depends on the amount of the limiting reactant, not on how fast the reaction went. EXTENDED The rate at a particular time is the gradient of the tangent drawn there.

A graph of volume of carbon dioxide against time. The original experiment with marble chips rises steeply at first and levels off at 80 cubic centimetres. The same mass of powdered marble rises more steeply and levels off sooner, at the same final volume of 80 cubic centimetres. A tangent at time zero on the original curve has gradient 2 cubic centimetres per second. — Powder (larger surface area) reacts faster but gives the same 80 cm³, because the amount of marble is unchanged. Initial rate of the chips: 2 cm³/s. (Illustrative data.)

✏️Worked example

Marble chips (calcium carbonate) react with excess dilute hydrochloric acid; the volume of CO₂ is recorded (graph above). (a) Explain why the graph becomes horizontal. [1] (b) The experiment is repeated with the same mass of powdered marble. Describe and explain the difference in the curve. [3] (c) EXTENDED Explain, using collision theory, why the rate is faster at a higher temperature. [3]

(a) The marble (the limiting reactant, since the acid is in excess) has been used up, so no more gas is made.

(b) The curve is steeper at the start and levels off sooner, but reaches the same final volume (80 cm³). The powder has a larger surface area, so collisions between acid particles and the solid are more frequent; the same mass of marble still produces the same amount of gas.

(c) The particles have more kinetic energy and move faster, so they collide more frequently, and a greater proportion of the collisions have energy equal to or greater than the activation energy, so more collisions are successful.

Check it. Any change that only affects speed (surface area, temperature, catalyst, concentration of the excess reactant) leaves the final volume unchanged. Only changing the amount of the limiting reactant changes it.

“More collisions.” Collision theory marks need frequency: “more collisions per second” or “more frequent collisions”. And for temperature, the energy part (≥ E_a) is essential.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which is a physical change? A: burning paper. B: rusting iron. C: melting ice. D: cooking an egg.

C. No new substance forms, and it is easily reversed.

2. (Theory.) State two ways of increasing the rate of reaction between zinc and dilute sulfuric acid without changing the amounts of reactants. [2]

Any two: use zinc powder instead of lumps; warm the acid; add a catalyst (such as a little copper(II) sulfate). (Using more concentrated acid changes the amount unless the volume is reduced to match.)

3. (Theory.) Using the graph above, state the total volume of gas produced by the marble chips and the approximate time when the reaction finished. [2]

80 cm³; the curve becomes flat at roughly 180–200 s. (From a smooth curve, any reading where it visibly levels off is accepted.)

4. (Practical.) In the mass-loss method, a flask of marble chips and acid is placed on a balance with a cotton wool plug in the neck. Explain why the mass decreases, and why the plug is used. [2]

Carbon dioxide gas escapes from the flask, so the mass falls. The plug lets the gas out but stops acid spray escaping (which would also lower the mass).

5. (Practical.) In the first 20 s, 48 cm³ of gas is collected. Calculate the mean rate of reaction in this time, with units. [2]

\( 48 \div 20 = 2.4 \) cm³/s.

6. (Theory.) EXTENDED Explain why increasing the concentration of the acid increases the rate. [2]

There are more acid particles per unit volume, so collisions between reacting particles are more frequent.

7. (Theory.) EXTENDED Explain how a catalyst increases the rate of a reaction. [2]

It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react.

8. (Practical.) EXTENDED Suggest one advantage of collecting carbon dioxide in a gas syringe rather than over water in an upturned measuring cylinder. [1]

Carbon dioxide is slightly soluble in water, so some would dissolve and the measured volume would be too low; the syringe measures all the gas. (Also: readings are easier and more precise.)

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

PhET “Reactions & Rates” — change temperature and concentration and watch the collisions
Royal Society of Chemistry — the disappearing-cross (sodium thiosulfate) rate practical