Factors Affecting the Rate of Reaction

Five conditions control how fast a reaction goes. Change any one and you change the rate.

Factor	Effect on rate when increased
Concentration of reactants in solution	Faster
Pressure of reacting gases	Faster
Surface area of a solid reactant	Faster
Temperature	Faster
Adding a catalyst	Faster (catalyst is not used up)

Each factor speeds the reaction up in a slightly different way, but all of them work by changing how often, or how successfully, reacting particles meet. That explanation is collision theory, covered in the next lesson. This lesson focuses on recalling each factor and on the experiment that measures one of them.

Increasing concentration, pressure, surface area or temperature all increase the rate. A catalyst also increases the rate without being used up.

Concentration applies to substances dissolved in solution. A more concentrated solution has more reactant particles in the same volume, so reactions happen faster. Doubling the concentration of a reactant often roughly doubles the rate.

Pressure is the equivalent factor for gases. Increasing the pressure of a reacting gas squeezes the same number of particles into a smaller space, so they are closer together, just like a more concentrated solution.

Change	What it does to the particles	Result
Higher concentration (solution)	More particles per unit volume	Faster rate
Higher pressure (gas)	Gas particles pushed closer together	Faster rate

Concentration is the "crowding" factor for dissolved reactants; pressure is the same idea for gases. Both make particles more crowded, so the reaction speeds up.

Surface area matters for solid reactants. Only particles on the surface of a solid can react with a liquid or gas around it. Breaking the solid into smaller pieces, or grinding it to a powder, exposes far more surface. A powder therefore reacts faster than a single large lump of the same mass.

Temperature speeds up every reaction. Heating the reactants makes their particles move faster, so they collide more often and, crucially, with more energy. This is why a raised temperature has such a large effect on rate.

Factor	Practical example
Larger surface area	Powdered calcium carbonate fizzes with acid faster than a marble chip
Higher temperature	Milk sours quickly on a warm day but slowly in a fridge

A powder has a larger surface area than a lump of the same mass, so more of the solid is exposed and the reaction is faster.

Required Practical 5 investigates how changing concentration affects rate. The first method measures the volume of gas given off. A common version uses magnesium ribbon and dilute hydrochloric acid:

$Mg+2HCl\to MgC{l}_2+H_2$

Method:

1. Add a set volume of hydrochloric acid of a known concentration to a conical flask.
2. Add a fixed length (or mass) of magnesium ribbon and immediately connect a gas syringe.
3. Start a stopwatch and record the volume of hydrogen gas collected every 10 seconds.
4. Repeat the whole experiment with different acid concentrations, keeping everything else the same.

Variables:

Type	Variable
Independent (you change)	Concentration of the acid
Dependent (you measure)	Volume of gas collected in a set time
Control (keep the same)	Volume of acid, mass/length of magnesium, temperature

The more concentrated the acid, the more gas is collected in the same time, and the steeper the graph of volume against time. Controlling temperature and the amount of magnesium is essential, because otherwise you could not tell whether concentration alone caused the change.

The second required method follows a colour change / turbidity. Sodium thiosulfate reacts with dilute hydrochloric acid to form a pale yellow precipitate of sulfur, which slowly turns the solution cloudy:

$N{a}_2{S}_2{O}_3+2HCl\to 2NaCl+S{O}_2+S+H_{2}O$

Method:

1. Draw a black cross on paper and place a conical flask on top of it.
2. Measure set volumes of sodium thiosulfate solution and water into the flask (varying the ratio changes the concentration).
3. Add dilute hydrochloric acid, start the stopwatch, and look down through the flask at the cross.
4. Stop the stopwatch when the cloudy sulfur hides the cross from view. Record the time.
5. Repeat at different thiosulfate concentrations, keeping the total volume, temperature and viewing conditions the same.

Variables:

Type	Variable
Independent	Concentration of sodium thiosulfate
Dependent	Time for the cross to disappear
Control	Total volume of solution, temperature, depth of liquid, the same observer

A higher concentration makes the solution turn cloudy sooner, so the time is shorter. Rate is inversely related to time: a shorter time means a faster rate, so plotting 1 ÷ time against concentration gives a straight line rising with concentration.

A hypothesis is a testable prediction made before the experiment. For Required Practical 5 a suitable hypothesis is: "Increasing the concentration of the acid will increase the rate of reaction." It names the independent variable (concentration), the dependent variable (rate) and the expected direction of the change.

The two methods measure rate in opposite-looking ways, so read them carefully:

Method	Higher concentration means...	Because...
Gas volume	More gas collected per unit time	Rate is faster, so more product forms sooner
Cloudy cross	A shorter time for the cross to vanish	The precipitate forms faster

A shorter time in the thiosulfate experiment means a faster reaction, not a slower one. The rate goes up as the time comes down.

Both methods support the same hypothesis: increasing concentration increases the rate.

1. Thinking a shorter time means a slower reaction

In the cloudy-cross method, a faster reaction hides the cross sooner, so the time is shorter. A short time is a fast rate. Rate is 1 ÷ time, not time itself.

2. Confusing concentration with amount

A more concentrated acid is not necessarily a larger volume. Concentration is how crowded the particles are in a fixed volume. Keep the volume of acid controlled and change only the concentration.

3. Forgetting to control variables

If temperature or the mass of magnesium changes between runs, the experiment is unfair and the results cannot be trusted. Only the concentration should change.

4. Saying surface area speeds up a reaction between two solutions

Surface area only matters when a solid reactant is involved. For two dissolved reactants, concentration is the relevant factor, not surface area.

5. Muddling pressure and concentration

Pressure is the crowding factor for gases; concentration is the crowding factor for dissolved substances. Increasing the pressure of a solution is not a valid answer, because solutions are not compressed like gases.