Chapter 8 of 8

All 8 chapters
  1. 1Chemical formulae, readunderstood
  2. 2Balancing equations, readunderstood
  3. 3The mole, readunderstood
  4. 4Reacting masses and yield, readunderstood
  5. 5Acids, bases and salts, readunderstood
  6. 6The reactivity series, readunderstood
  7. 7Electrolysis, readunderstood
  8. 8Energy changes and rates, readunderstood

S4.7.8 Chapter 8

Energy changes and rates

Why does a fire warm a room, but baking soda in vinegar makes the cup feel cold?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Sort reactions into exothermic and endothermic
  • Explain how temperature, concentration and surface area change reaction rate

Every reaction moves energy. To react, bonds between the atoms of the reactants must break, and that takes energy in. Then new bonds form in the products, and that gives energy out.

If more energy is given out than was taken in, the reaction is exothermic. The extra goes to the surroundings, which get warmer. Burning, neutralisation and respiration in our cells are exothermic. Some hand warmers use iron rusting fast.

If more energy is taken in than is given out, the reaction is endothermic. The energy comes from the surroundings, which get cooler. Thermal decomposition and photosynthesis are endothermic. So is baking soda with vinegar, which is why the cup feels colder.

An energy profile of an exothermic reaction. A line labelled energy goes up the side and a line labelled progress of reaction goes along the bottom. The reactants are a flat line high on the left. A hump rises and falls to the products, a flat line lower on the right. An arrow up to the hump is labelled activation energy. An arrow down between the two levels is labelled energy given out, and under the products is ΔH negative.energyactivation energyenergygiven outΔH negativereactantsproductsprogress of reaction
An exothermic reaction. The products are lower in energy than the reactants. The difference is given out.
An energy profile of an endothermic reaction. A line labelled energy goes up the side and a line labelled progress of reaction goes along the bottom. The reactants are a flat line low on the left. A hump rises and falls to the products, a flat line higher on the right. An arrow up to the hump is labelled activation energy. An arrow up between the two levels is labelled energy taken in, and over the products is ΔH positive.energyactivation energyenergytaken inΔH positivereactantsproductsprogress of reaction
An endothermic reaction. The products are higher in energy than the reactants. The difference is taken in.

Both graphs have a hump. Particles must have a minimum energy to react. This is the activation energy. It is the height from the reactants to the top of the hump.

See it three ways

As a picture

An energy profile with an axis labelled energy (kJ) and a line along the bottom labelled progress of reaction. The reactants are at 350, the products at 150 and the top of the hump at 450. An arrow up to the hump is labelled activation energy. An arrow between the levels is labelled energy given out and the change is ΔH = −200 kJ.0100200300400500600energy (kJ)activation energyenergygiven outΔH = −200 kJreactantsproductsprogress of reaction

In words

The reactants are at 350 kJ and the products at 150 kJ, so 200 kJ is given out. The energy change is written ΔH. It is negative when energy is given out.

In symbols

ΔH = 150 − 350 = −200 kJ, so 200 kJ is given out.

Now think about speed. The rate of reaction is how fast reactants are used up or products are made. We can measure it by collecting a gas and timing it.

A conical flask with a delivery tube to a gas syringe. The plunger has been pushed out by the gas collected. The parts are labelled conical flask, delivery tube and gas syringe.020406080100cm³conical flaskdelivery tubegas syringe
Collecting a gas in a gas syringe. Read the volume every 10 seconds.

Worked example

A reaction makes 24 cm³ of gas in 8 seconds. What is the mean rate of reaction?

  1. mean rate = amount of product ÷ time taken.
  2. mean rate = 24 ÷ 8 = 3 cm³/s.

Answer: 3 cm³/s

A graph of gas collected against time. The bottom is labelled time (s) and runs from 0 to 60. The side is labelled gas (cm³) and runs from 0 to 70. The line rises steeply at first, then more gently, and is flat from 50 seconds at 65 cm³.0102030405060010203040506070time (s)gas (cm³)
The line is steepest at the start, so the reaction is fastest then. It flattens when a reactant is used up.

Worked example

Use the graph. What is the mean rate of reaction between 10 s and 30 s?

  1. At 10 s the graph reads 30 cm³. At 30 s it reads 58 cm³.
  2. Gas made = 58 − 30 = 28 cm³.
  3. Time taken = 30 − 10 = 20 s.
  4. mean rate = 28 ÷ 20 = 1.4 cm³/s.

Answer: 1.4 cm³/s

Reactions happen when particles collide with enough energy. A faster reaction has more successful collisions each second. Three things raise the number.

Temperature: warmer particles move faster. They collide more often, and more of the collisions have enough energy. The reaction goes faster. Concentration: more particles in the same volume collide more often. Surface area: a finer solid exposes more of its particles to the other reactant.

Draw it first

Three ways to make more collisions

  1. A box of particles drawn three times in the same place. Step 1: few particles moving slowly, shown with short trails. Step 2: three times as many particles, still slow. Step 3: the same number of particles moving fast, shown with long trails.

    1 A few particles of two kinds, open circles and dark dots, move slowly in the box. Short trails show slow movement. They collide rarely.

  2. A box of particles drawn three times in the same place. Step 1: few particles moving slowly, shown with short trails. Step 2: three times as many particles, still slow. Step 3: the same number of particles moving fast, shown with long trails.

    2 Put in three times as many particles in the same volume. This is a higher concentration, and there are more collisions.

  3. A box of particles drawn three times in the same place. Step 1: few particles moving slowly, shown with short trails. Step 2: three times as many particles, still slow. Step 3: the same number of particles moving fast, shown with long trails.

    3 Keep them, and make them move faster. This is a higher temperature. A longer trail means a faster particle. There are even more collisions.

A cube drawn whole and labelled side 2 cm, 1 cube and surface area 24 cm². Beside it the same cube cut into small cubes, labelled side 1 cm, 8 pieces and total surface area 48 cm².side 2 cm1 cubesurface area24 cm²side 1 cm8 piecestotal surfacearea 48 cm²
Cutting a cube into 8 pieces doubles the surface area. A reaction can happen only at a surface.

Worked example

A cube has sides of 2 cm. It is cut into 8 small cubes. What is the total surface area of the 8 small cubes?

  1. The whole cube has 6 faces of 2 × 2 = 4 cm². Its surface area is 6 × 4 = 24 cm².
  2. Each small cube has a side of 1 cm. One face is 1 cm², so one small cube has 6 cm².
  3. There are 8 small cubes. Their total surface area is 8 × 6 = 48 cm².
  4. The area is twice as big, 48 cm² instead of 24 cm².

Answer: 48 cm²

A catalyst speeds up a reaction and is not used up. It gives the particles an easier path, so more collisions are successful. The enzymes in your body are catalysts.

Try it

Warm and cold vinegar

You need: two cups, two bowls, vinegar and baking soda, cold tap water, and tap water that is warm but not hot, a spoon, and a clock or watch with a seconds hand.

Be safe: Use only tap water, never hot water from a kettle. Keep your face and eyes away from the cups, and taste nothing.

  1. Put the same amount of vinegar in each cup, about a finger deep.
  2. Stand one cup in a bowl of cold tap water, and the other in a bowl of warm tap water. Wait 5 minutes.
  3. Add one level spoon of baking soda to each cup at the same moment. Do not stir.
  4. Time how long each cup takes to stop fizzing.

What you will see: The warm cup usually fizzes faster and stops sooner. The same amount of gas is made in both, but the warmer particles collide harder and more often. You may also feel that the cups get cooler, because the reaction is endothermic.

If you cannot try it, this is what happens.
A table with the columns Vinegar and Fizzing stops. Cold: later. Warm: sooner.VinegarFizzing stopscoldlaterwarmsooner

Try it

A whole piece and a powder

You need: two pieces of eggshell of the same size, or chalk that fizzes in vinegar, vinegar and two cups, paper and a spoon.

Be safe: Crush the eggshell or chalk on paper and do not breathe the dust. Keep your face and eyes away from the cups, and taste nothing.

  1. Put a whole piece of eggshell, or chalk, into the first cup. Pour in vinegar.
  2. Wrap the second piece in paper. Press it with the back of the spoon until it is a powder. Tip the powder into the second cup and pour in the same amount of vinegar.
  3. Watch both cups for a minute. Look at the bubbles.

What you will see: The powder makes bubbles much faster than the whole piece. Eggshell, and some chalk, is calcium carbonate. Vinegar is an acid, so they make carbon dioxide. Some chalk is calcium sulfate and does not fizz: use eggshell then. The powder has far more surface in contact with the acid, so there are more collisions.

If you cannot try it, this is what happens.
A table with the columns Solid and Bubbles. Whole piece: slowly. Powder: quickly.SolidBubbleswhole pieceslowlypowderquickly

A wrong idea many people have

In an exothermic reaction energy is made, and in an endothermic reaction energy is destroyed.

Why it feels right: A fire seems to make heat from nothing, and a cold cup seems to lose it.

The test that breaks it: Think of a hand warmer. It gets warm, and when it is finished it cannot warm you again. The energy came from the store in the chemicals, and moved to your hand.

What is true: Energy is never made or destroyed. A reaction moves it between the chemicals and the surroundings.

Key words

exothermic
a reaction that gives out energy and warms its surroundings.
endothermic
a reaction that takes in energy and cools its surroundings.
activation energy
the least energy that particles need to react.
rate of reaction
how fast reactants are used up or products are made.
collide
to hit one another.
catalyst
a substance that speeds up a reaction and is not used up.

Back to your guess

The answer is A: Some reactions give energy to their surroundings, and some take it in from them.

A reaction that gives out energy warms its surroundings. A reaction that takes energy in cools them. Energy is moved, never made or destroyed.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these reactions is endothermic?

    Tap your answer.

    1. Not this one. Burning gives out energy as heat and light. It is exothermic. Try another.

    2. Not this one. Neutralisation warms the mixture. It is exothermic. Go to S3.6.8. Try another.

    3. Yes, that is right. The reaction takes in energy from its surroundings, so the cup gets colder. A reaction that takes energy in is endothermic.

    See the answer

    Baking soda and vinegar making the cup feel cold

    The reaction takes in energy from its surroundings, so the cup gets colder. A reaction that takes energy in is endothermic.

    • If you chose A: Burning gives out energy as heat and light. It is exothermic.
    • If you chose B: Neutralisation warms the mixture. It is exothermic. Go to S3.6.8.

  2. The profile shows a reaction. How much energy does the reaction give out?

    An energy profile with an axis labelled energy (kJ) and a line along the bottom labelled progress of reaction. The reactants are at 350, the products at 150 and the top of the hump at 450. An arrow up to the hump is labelled activation energy. An arrow between the levels is labelled energy given out, and under the products is ΔH negative.0100200300400500600energy (kJ)activation energyenergygiven outΔH negativereactantsproductsprogress of reaction
    Tap your answer.

    1. Yes, that is right. The reactants are at 350 kJ and the products at 150 kJ. 350 − 150 = 200 kJ is given out.

    2. Not this one. That is the top of the hump. The energy given out is the difference between reactants and products. Try another.

    3. Not this one. That is the activation energy, the height from the reactants to the top of the hump. The energy given out is between the two levels. Try another.

    See the answer

    200 kJ

    The reactants are at 350 kJ and the products at 150 kJ. 350 − 150 = 200 kJ is given out.

    • If you chose B: That is the top of the hump. The energy given out is the difference between reactants and products.
    • If you chose C: That is the activation energy, the height from the reactants to the top of the hump. The energy given out is between the two levels.

  3. Why does a reaction go faster when the temperature is raised?

    Tap your answer.

    1. Not this one. The particles stay the same size. Heat makes them move faster. Go to S3.5.4. Try another.

    2. Yes, that is right. Faster particles collide more often, and more of the collisions have at least the activation energy. So more collisions are successful.

    3. Not this one. Some reactions do, but a cold reaction goes faster when it is warmed from outside, and that is because of the particles. Try another.

    See the answer

    The particles move faster, so they collide more often and with more energy.

    Faster particles collide more often, and more of the collisions have at least the activation energy. So more collisions are successful.

    • If you chose A: The particles stay the same size. Heat makes them move faster. Go to S3.5.4.
    • If you chose C: Some reactions do, but a cold reaction goes faster when it is warmed from outside, and that is because of the particles.

  4. Powdered eggshell reacts faster with vinegar than a whole piece of eggshell of the same mass. Why?

    Tap your answer.

    1. Not this one. The same mass has the same number of atoms. The difference is how much surface the acid can reach. Try another.

    2. Not this one. Both start at the same temperature. The difference is surface area. Try another.

    3. Yes, that is right. Acid particles can hit only the surface of the eggshell. Powder has a much larger surface area than a whole piece, so there are more collisions each second.

    See the answer

    More of the eggshell is exposed to the acid, so there are more collisions.

    Acid particles can hit only the surface of the eggshell. Powder has a much larger surface area than a whole piece, so there are more collisions each second.

    • If you chose A: The same mass has the same number of atoms. The difference is how much surface the acid can reach.
    • If you chose B: Both start at the same temperature. The difference is surface area.

  5. Vinegar is mixed with an equal amount of water, and then poured onto eggshell. What happens to the rate of the reaction?

    Tap your answer.

    1. Not this one. There is more liquid, but the acid particles are spread through it. Fewer of them hit the chalk each second. Try another.

    2. Yes, that is right. The concentration of the acid is halved, so acid particles meet the eggshell less often. Fewer collisions mean a slower rate.

    3. Not this one. The amount of acid is the same, but the volume doubled. The concentration halved, so there are fewer collisions. Try another.

    See the answer

    It is slower, because there are fewer acid particles in the same volume.

    The concentration of the acid is halved, so acid particles meet the eggshell less often. Fewer collisions mean a slower rate.

    • If you chose A: There is more liquid, but the acid particles are spread through it. Fewer of them hit the chalk each second.
    • If you chose C: The amount of acid is the same, but the volume doubled. The concentration halved, so there are fewer collisions.

  6. The graph shows gas collected from a reaction. At what time did the reaction stop?

    A graph of gas collected against time. The bottom is labelled time (s) and runs from 0 to 60. The side is labelled gas (cm³) and runs from 0 to 70. The line rises steeply at first, then more gently, and then goes flat.0102030405060010203040506070time (s)gas (cm³)
    Tap your answer.

    1. Yes, that is right. The line stops rising at 50 s, and stays at 65 cm³. No more gas is made, so the reaction has stopped.

    2. Not this one. At 10 s the line is steepest, so the reaction is fastest, not finished. Try another.

    3. Not this one. The graph ends at 60 s, but the line has been flat since 50 s. No more gas was made after 50 s. Try another.

    See the answer

    50 s

    The line stops rising at 50 s, and stays at 65 cm³. No more gas is made, so the reaction has stopped.

    • If you chose B: At 10 s the line is steepest, so the reaction is fastest, not finished.
    • If you chose C: The graph ends at 60 s, but the line has been flat since 50 s. No more gas was made after 50 s.

  7. What does a catalyst do?

    Tap your answer.

    1. Yes, that is right. A catalyst gives the particles an easier path, so more collisions are successful. It is not changed, so a little can be used over and over.

    2. Not this one. A catalyst changes the speed, not the amount of product that can be made. Try another.

    3. Not this one. A catalyst is not used up. It is there at the end, just as it was at the start. Try another.

    See the answer

    It speeds up a reaction, and is not used up.

    A catalyst gives the particles an easier path, so more collisions are successful. It is not changed, so a little can be used over and over.

    • If you chose B: A catalyst changes the speed, not the amount of product that can be made.
    • If you chose C: A catalyst is not used up. It is there at the end, just as it was at the start.

  8. From an earlier chapter

    A reaction should make 50 g of product, but it makes 45 g. What is the percentage yield?

    Tap your answer.

    1. Not this one. That divides the theoretical yield by the actual yield. The actual yield goes on top. Go to S4.7.4. Try another.

    2. Yes, that is right. percentage yield = actual ÷ theoretical × 100 = 45 ÷ 50 × 100 = 90%.

    3. Not this one. That is the percentage lost, 5 ÷ 50. The yield is the percentage that was collected. Go to S4.7.4. Try another.

    See the answer

    90%

    percentage yield = actual ÷ theoretical × 100 = 45 ÷ 50 × 100 = 90%.

    • If you chose A: That divides the theoretical yield by the actual yield. The actual yield goes on top. Go to S4.7.4.
    • If you chose C: That is the percentage lost, 5 ÷ 50. The yield is the percentage that was collected. Go to S4.7.4.

  9. A reaction makes 30 cm³ of gas in 6 seconds. What is the mean rate of reaction in cm³/s?

    See the answer

    5 cm³/s

    mean rate = amount of product ÷ time = 30 ÷ 6 = 5 cm³/s.

  10. A cube with sides of 4 cm is cut into 8 equal small cubes. What is the total surface area of the small cubes, in cm²?

    See the answer

    192 cm²

    Each small cube has a side of 2 cm, so one face is 4 cm². One cube has 6 × 4 = 24 cm², and 8 cubes have 8 × 24 = 192 cm².

Keep it

Copy this into your exercise book.

Exothermic gives out energy and warms. Endothermic takes in energy and cools. Higher temperature, higher concentration and more surface area give more collisions, so a faster rate.

Draw: Two energy profiles, one for an exothermic and one for an endothermic reaction, with the energy change marked.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (907 KB). It opens on any phone.