Rates of reaction

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Big idea: why do some reactions race and others crawl?

Big idea (key concept): Change. Chemical reactions happen at very different speeds, and we can change that speed on purpose.
Related concept: Energy. Particles need enough energy when they collide, so temperature and energy decide how fast a reaction goes.
Global context: Scientific and technical innovation. Industry, medicine and food safety all depend on controlling reaction speed: making fertiliser quickly, keeping food fresh, or cleaning car exhausts.
The rate of a reaction is how fast reactants are used up or products are made. A firework is over in a moment, but rusting takes weeks.
mean rate = amount of reactant used or product formed ÷ time taken. For a gas the unit is cm³/s, and for mass loss it is g/s.
Four things change the rate: concentration, temperature, surface area and a catalyst. For gases, pressure acts like concentration.

Collision theory, concentration and surface area

Collision theory: for a reaction to happen, particles must collide, and the collision must have at least the activation energy. A collision that does this is a successful collision.
Most collisions are not successful. Particles bounce off each other with too little energy to break bonds.
Concentration: more particles in the same volume means more frequent collisions, so a higher rate. Pushing gas particles into a smaller volume (higher pressure) works the same way.
Surface area: only the particles on the outside of a solid can be hit. Breaking a lump into powder exposes far more surface, so there are more collisions each second.
Surface area changes how fast the product forms, not how much. The same mass of reactant gives the same total product.
Fine dust such as flour can burn explosively because its surface area is huge. Flour mills therefore control the dust in the air.

Higher concentration means more particles in the same volume

Temperature and catalysts

Temperature: at a higher temperature the particles move faster. They collide more often, and a larger fraction of collisions have at least the activation energy. The second effect matters most.
As a rough guide, a 10 °C rise roughly doubles the rate of many reactions. Food kept in a fridge goes off more slowly for this reason.
A catalyst speeds up a reaction without being used up. It gives the reaction an alternative pathway with a lower activation energy, so more collisions are successful.
A common example is manganese dioxide, which speeds up the breakdown of hydrogen peroxide: hydrogen peroxide → water + oxygen, or 2H₂O₂ → 2H₂O + O₂.
A catalyst does not change the total amount of product made. It only changes how fast it forms.
Catalysts are used in industry to save energy and money, in catalytic converters to clean car exhausts, and as enzymes in our bodies.

A catalyst lowers the activation energy

Measuring the rate of a reaction

Measuring gas volume: collect the gas in a gas syringe or an upturned measuring cylinder over water, and record the volume at regular times, such as every 10 seconds. For example calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide, or CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Measuring mass loss: stand the flask on a balance with a loose plug of cotton wool. When a gas escapes, the mass falls. This works best with a heavy gas such as carbon dioxide.
Timing a change: in the disappearing-cross method, sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur + sulfur dioxide + water. The yellow sulfur makes the mixture cloudy until the cross cannot be seen. A shorter time means a faster rate.
The cross method gives off sulfur dioxide, which irritates the lungs, especially for people with asthma. A teacher runs it in a well-ventilated room, and everyone wears eye protection.
Always put the bung in straight away when collecting gas, because gas lost at the start makes the results too low.
Choose the method that suits the reaction: gas volume or mass loss if a gas forms, and timing a colour or cloudiness change if it does not.

Collecting a gas over water in a measuring cylinder

Reading rate graphs and calculating rates

On a graph of total volume of gas against time, the line is steepest at the start when the reactants are most concentrated. It gets less steep, then becomes horizontal when a reactant has been used up and the reaction has stopped.
A steeper line means a faster rate. If two lines finish at the same height, the same amount of product formed.
Mean rate between two times = change in amount ÷ change in time. In one experiment 24 cm³ of gas formed in the first 10 s, a mean rate of 2.4 cm³/s. In the next 10 s only 14 cm³ formed, so the rate fell to 1.4 cm³/s.
You can rearrange the formula: amount = rate × time, and time = amount ÷ rate. At 1.5 cm³/s, 60 cm³ of gas takes 40 s.
To find the rate at one moment, draw a tangent to the curve and find its gradient: rise ÷ run. A tangent through (0 s, 4 cm³) and (40 s, 28 cm³) has a gradient of 24 ÷ 40 = 0.6 cm³/s.
Compare two experiments by comparing the times for the same amount of product. The shorter the time, the faster the reaction.

Rate graph: higher and lower temperature

Think like a scientist: investigating a rate

A good question to test is: how does the concentration of sodium thiosulfate affect how fast a cross disappears? A teacher runs this in a ventilated room.
The independent variable is the concentration of sodium thiosulfate, changed by mixing different volumes of the solution with water. The dependent variable is the time for the cross to disappear.
The control variables are the total volume of solution, the volume and concentration of the acid, the temperature and the same flask and cross each time.
To make results more reliable, repeat each concentration and use the mean. Look out for an anomaly. Judging when the cross "disappears" is subjective, so the same person should watch each time.
Safe practice: wear eye protection, work in a ventilated room, never smell the gas and let the teacher handle the acid.
Your task: plan an investigation into how temperature affects the rate of a reaction. State your variables, how you will control them, what you will record, and how you will use the results to decide how the rate changes. Then say how you would check your conclusion is reliable.

The disappearing-cross method for concentration

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Übungsfragen

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1.What does the rate of a reaction tell us?
Easy
- AHow much energy a reaction gives out
- BHow many different products form
- CHow quickly reactants are used up or products are made
- DWhether the reaction is safe to carry out
2.Which of these changes has the fastest rate of reaction?
Easy
- AA firework exploding
- BIron rusting
- CMilk slowly going sour
- DA leaf rotting on the ground
3.For two particles to react, what must happen?
Easy
- AThey must change colour first
- BThey must be exactly the same size
- CThey must be far apart from each other
- DThey must collide with enough energy
4.A teacher runs the disappearing-cross experiment, which makes sulfur dioxide gas. Which precaution is sensible?
Easy
- ASmell the gas to check it has formed
- BWork in a well-ventilated room and wear eye protection
- CPour the used mixture down the sink while stirring
- DCarry out the reaction in a closed bottle
5.Raising the temperature makes particles move faster.
Easy
True or false?
6.A catalyst is used up during the reaction.
Easy
True or false?
7.Complete the sentence.
Easy
The minimum energy needed for a collision to be successful is called the ____ energy.
8.Fill in the missing word.
Easy
mean rate = amount of product formed ÷ ____

Mehr Fragen

9.Which form of calcium carbonate would react fastest with the same acid?
Easy
- AOne large lump
- BA fine powder
- CSeveral medium chips
- DA single smooth pebble
10.Why is food kept in a fridge?
Easy
- AThe low temperature speeds up the reactions that keep food fresh
- BThe fridge removes oxygen from the food completely
- CThe low temperature makes the food more concentrated
- DThe low temperature slows down the reactions that make food go bad
11.Which statements about catalysts are true? (select all that apply)
Easy
- AA catalyst increases the amount of product made
- BA catalyst is not used up
- CA catalyst is always a reactant
- DA catalyst speeds up a reaction
12.Match each change to the reason it speeds up a reaction.
Easy
- Higher concentration
- Higher temperature
- Crushing a solid into powder
- Adding a catalyst
- Particles move faster and collide harder
- More surface is exposed to the other reactant
- A route with lower activation energy
- More particles in the same volume, so more collisions
13.Put these steps of a gas-collection experiment in a sensible order.
Easy
- Plot a graph of volume against time
- Set up the apparatus and check it for leaks
- Calculate the rates from the results
- Add the acid and start the stopwatch
- Record the volume of gas every 10 seconds
14.Finish the statement.
Easy
calcium carbonate + hydrochloric acid → calcium chloride + water + ____
15.Why does increasing the concentration of an acid increase the rate of reaction?
Medium
- AMore acid particles in the same volume, so collisions happen more often
- BThe acid particles become larger and so are easier to hit
- CThe activation energy of the reaction is reduced a lot
- DThe acid particles move much faster and hit harder
16.Which explains why a higher temperature increases the rate of reaction?
Medium
- AParticles get bigger, so they are more likely to collide with others
- BThe activation energy becomes much lower as the mixture warms up
- CParticles move faster, so collisions are more frequent and more have enough energy
- DMore reactant particles are created by the heat that is added
17.Why does powdered chalk react faster with acid than a lump of chalk of the same mass?
Medium
- AThe powder contains more chalk than the lump does
- BThe powder has a lower activation energy than the lump
- CThe powder is always hotter than the lump is
- DThe powder has a greater surface area, so more particles are exposed
18.How does a catalyst increase the rate of a reaction?
Medium
- AIt raises the temperature of the reactants a great deal
- BIt provides an alternative pathway with a lower activation energy
- CIt increases the concentration of all the reactants
- DIt supplies extra reactant particles to the mixture
19.Which reaction is best followed by measuring the loss in mass?
Medium
- ASodium hydroxide and dilute acid, because a salt forms
- BIron and copper sulfate, because copper is made
- CMarble chips and dilute acid, because carbon dioxide escapes
- DSalt dissolving in water, because it disappears
20.Which expression gives the mean rate of reaction for a gas being collected?
Easy
- AVolume of gas collected ÷ time taken
- BTime taken ÷ volume of gas collected
- CVolume of gas collected × time taken
- DVolume of gas collected − time taken
21.
A teacher collected hydrogen from magnesium and dilute acid in one experiment.
Time (s) Total volume of gas (cm³)
0 0
10 24
20 38
30 46
40 51
50 54
60 54
During which 10-second interval was the rate of reaction highest?
Medium
- A10 to 20 s
- B30 to 40 s
- C0 to 10 s
- D50 to 60 s
22.
Use the data in this table from one experiment.
Time (s) Total volume of gas (cm³)
0 0
10 24
20 38
30 46
40 51
50 54
60 54
At what time had the reaction stopped?
Medium
- A50 s
- B20 s
- C40 s
- D60 s
23.In an experiment on how the concentration of sodium thiosulfate affects the time for a cross to disappear, what is the independent variable?
Medium
- AThe time for the cross to disappear
- BThe temperature of the room
- CThe volume of acid added
- DThe concentration of sodium thiosulfate
24.In the same experiment, which is the dependent variable?
Medium
- AThe concentration of the thiosulfate
- BThe time for the cross to disappear
- CThe size of the flask
- DThe volume of acid added
25.Which changes would increase the rate of reaction between marble chips and dilute hydrochloric acid? (select all that apply)
Medium
- AUsing a more concentrated acid
- BUsing larger lumps
- CCrushing the chips
- DAdding water to the acid
- EWarming the acid
26.Powder reacts more slowly than lumps of the same mass.
Easy
True or false?
27.Adding a catalyst increases the total amount of product made.
Medium
True or false?
28.Fill the gap.
Easy
A ____ speeds up a reaction without being used up.
29.Complete this statement.
Easy
Powders react faster because they have a larger ____.
30.Match each piece of equipment to what it measures.
Easy
- Gas syringe
- Balance
- Stopwatch
- Measuring cylinder
- Mass of the reaction mixture
- Time taken
- Volume of a liquid reactant
- Volume of gas collected
31.Put these steps of a successful collision in order.
Medium
- Bonds in the reactants break
- Reactant particles move and collide
- New bonds form and the products are made
- The collision has at least the activation energy
32.
A teacher collected gas from a reaction in one experiment.
Time (s) Total volume of gas (cm³)
0 0
10 24
20 38
30 46
40 51
50 54
60 54
Calculate the mean rate of reaction between 0 and 20 s in cm³/s.
Medium
33.
A flask of marble chips and acid stands on a balance in one experiment.
Time (s) Mass lost (g)
0 0.00
60 0.60
120 1.05
180 1.35
240 1.50
300 1.50
Calculate the mean rate of mass loss between 0 and 60 s in g/s.
Medium
34.A reaction gives off gas at a mean rate of 1.5 cm³/s. What volume of gas forms in 40 seconds?
Medium
35.Explain why increasing the temperature increases the rate of a reaction.
Medium
36.Why does the rate of a reaction decrease as the reaction goes on?
Medium
- AThe reactants are being used up, so their concentration falls
- BThe temperature of the mixture always drops to zero
- CThe activation energy increases during the reaction
- DThe catalyst is slowly used up
37.On a graph of total volume of gas against time, what does a steeper line show?
Easy
- AA slower rate of reaction
- BA greater final amount of product
- CA faster rate of reaction
- DA lower temperature
38.
A student compares calcium carbonate lumps and powder (the same mass of each) reacting with excess acid.
Form of calcium carbonate Time to collect 40 cm³ of gas (s)
Lumps 100
Powder 25
Both reactions end with the same volume of gas. What explains this?
Hard
- AThe powder and the lumps have exactly the same surface area
- BLumps are more reactive than powder in the long run
- CThe acid was acting as a catalyst throughout the reaction
- DThe same mass was used, so the same amount of product formed
39.
A class timed how long a cross took to disappear when the concentration of sodium thiosulfate solution was changed.
Volume of sodium thiosulfate solution (cm³) Time for cross to disappear (s)
50 20
40 25
25 40
20 50
10 100
The total volume of solution was kept the same each time. Which conclusion fits the results?
Hard
- AThe rate stayed the same whatever the concentration was
- BHalving the thiosulfate from 50 cm³ to 25 cm³ doubled the time
- CA lower concentration made the reaction go faster
- DThe time got shorter as the concentration was lowered
40.
In one idealised set of results, the time for a cross to disappear was measured at different temperatures.
Temperature (°C) Time for cross to disappear (s)
20 80
30 40
40 20
50 10
Which statement fits these results?
Hard
- AEach 10 °C rise made the time ten seconds shorter
- BThe rate stayed the same as the temperature rose
- CEach 10 °C rise halved the time, so the rate doubled
- DEach 10 °C rise made the rate three times as fast
41.Flour mills must control the dust in the air. Which idea explains why flour dust can burn explosively when a lump of flour does not?
Hard
- AThe fine dust has a huge surface area, so it reacts extremely quickly with oxygen
- BFlour dust has a lower activation energy than lump flour has
- CFlour dust already contains more oxygen than lump flour does
- DFlour dust is a catalyst for the burning of the flour
42.A student says, 'Powder reacts faster than lumps.' Her test used 5 g of powder and 2 g of lumps. What is the problem?
Medium
- APowder cannot be weighed accurately enough
- BOnly lumps should ever be tested
- CThe masses were different, so the test was not fair
- DThe acid should have been left out of it
43.A student collects hydrogen over water in an upturned measuring cylinder. Why might the result for the first few seconds be low?
Hard
- ASome gas escapes before the bung is put in the flask
- BThe acid reacts more slowly at the start
- CThe hydrogen is lighter than the measuring cylinder
- DThe gas is used up by the water
44.A student investigates how temperature affects the rate of reaction between magnesium and acid. Which should be kept the same? (select all that apply)
Hard
- AThe volume of acid
- BThe temperature of the acid
- CThe mass of magnesium
- DThe result for each trial
- EThe concentration of the acid
45.A horizontal part of a volume-against-time graph means the reaction has stopped.
Easy
True or false?
46.The rate of a reaction stays constant from start to finish.
Easy
True or false?
47.Choose the right word to finish the sentence.
Medium
2H₂O₂ → 2H₂O + ____
48.Match each everyday example to the factor that explains it.
Hard
- A flour-dust explosion
- Food kept in a fridge
- A catalytic converter in a car exhaust
- Concentrated cleaner removing limescale faster than dilute
- Temperature
- Catalyst
- Concentration
- Surface area
49.Put these experiments in order from the fastest reaction to the slowest. Each experiment was timed for the same amount of product.
Medium
- Experiment A: 12 s
- Experiment B: 45 s
- Experiment D: 8 s
- Experiment C: 30 s
50.Explain why powdered chalk reacts faster with acid than a lump of chalk of the same mass.
Hard
51.Describe how you would plan a fair test to find out how temperature affects the time for a cross to disappear, using sodium thiosulfate and dilute acid (teacher-run in a ventilated room).
Hard
52.A student concludes that a higher concentration of acid makes the reaction faster after doing the experiment once for each concentration. Evaluate the method and suggest two improvements.
Hard
53.A reaction gives off gas at a mean rate of 1.5 cm³/s. How long does it take to produce 90 cm³ of gas?
Medium
54.A tangent is drawn to a volume-against-time curve at 20 s. It passes through the points (0 s, 4 cm³) and (40 s, 28 cm³). Calculate the gradient, which is the rate at 20 s, in cm³/s.
Hard
55.
One class experiment gave the results below.
Form of calcium carbonate Time to collect 40 cm³ of gas (s)
Lumps 100
Powder 25
Calculate the mean rate for the powder in cm³/s.
Medium

Lernnotizen

## The big idea - Reactions happen at different speeds, and we can control them (**Change**, **Energy**). - mean rate = amount ÷ time. Rearrange: amount = rate × time, time = amount ÷ rate. ## Collision theory - Particles must collide with at least the **activation energy** for a **successful collision**. ## Four factors - **Concentration:** more particles per volume, more collisions. - **Surface area:** smaller pieces expose more surface. - **Temperature:** faster particles, more collisions and more of them successful. - **Catalyst:** lower activation energy, not used up. Does not change the amount of product. ## Measuring and graphs - Gas volume, mass loss, or time for a cross to disappear (teacher-run, ventilated). - A steeper line is a faster rate. Horizontal means the reaction has stopped. - Rate at a moment is the gradient of a tangent: rise ÷ run. ## Investigating - Change one variable, measure one, control the rest and repeat to improve reliability.

rate of reaction: How fast reactants are used up or products are formed.
collision theory: Reactions happen when particles collide with at least the activation energy.
activation energy: The minimum energy particles need when they collide for a reaction to happen.
successful collision: A collision with enough energy to break bonds and form products.
concentration: The number of particles in a given volume of a solution.
surface area: The amount of a solid that is exposed and can be hit by other particles.
catalyst: A substance that speeds up a reaction without being used up.
mean rate: The amount of product formed or reactant used divided by the time taken.
gas syringe: A piece of apparatus that collects a gas and measures its volume.
tangent: A straight line touching a curve at one point, whose gradient gives the rate at that moment.
gradient: The rise of a line divided by its run, which gives a rate on a rate graph.
independent variable: The variable that you change in an investigation.
dependent variable: The variable that you measure in an investigation.
anomaly: A result that does not fit the pattern of the others.

Karteikarten

What is the rate of a reaction?

How fast reactants are used up or products are made.

What is the formula for mean rate?

Mean rate = amount of product formed (or reactant used) ÷ time taken.

What is activation energy?

The minimum energy particles need when they collide for a reaction to happen.

What is a successful collision?

A collision with at least the activation energy, which breaks bonds and forms products.

Why does a higher concentration increase the rate?

More particles in the same volume, so more frequent collisions.

Why does a powder react faster than a lump?

A larger surface area exposes more particles, so more collisions each second.

Why does a higher temperature increase the rate?

Particles move faster, so they collide more often and more collisions have enough energy.

What is a catalyst?

A substance that speeds up a reaction without being used up.

How does a catalyst work?

It gives an alternative pathway with a lower activation energy.

Does a catalyst change the amount of product?

No. It only changes how fast the product forms.

Name two ways to measure the rate of a gas-producing reaction.

Collect the gas volume over time, or measure the loss in mass on a balance.

What is the disappearing-cross method?

Timing how long it takes for a cross under a flask to be hidden by a cloudy product. A teacher runs it in a ventilated room.

What does a steeper line on a volume-against-time graph show?

A faster rate of reaction.

What does a horizontal line on a rate graph mean?

The reaction has stopped because a reactant has been used up.

How do you find the rate at one moment on a curve?

Draw a tangent and find its gradient (rise ÷ run).

How long does 90 cm³ of gas take at 1.5 cm³/s?

90 ÷ 1.5 = 60 s.

Why does the rate fall during a reaction?

The reactants are used up, so their concentration falls and there are fewer collisions.

Why is flour dust a fire risk?

Its huge surface area lets it react extremely quickly with oxygen.

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Time (s)	Total volume of gas (cm³)
0	0
10	24
20	38
30	46
40	51
50	54
60	54

Time (s)	Total volume of gas (cm³)
0	0
10	24
20	38
30	46
40	51
50	54
60	54

Time (s)	Total volume of gas (cm³)
0	0
10	24
20	38
30	46
40	51
50	54
60	54

Time (s)	Mass lost (g)
0	0.00
60	0.60
120	1.05
180	1.35
240	1.50
300	1.50