Energy In, Energy Out

Three places to get it right

Almost every mark lost in this topic is lost the same way: the direction is the wrong way round. There are exactly three places it can happen. Which way the temperature moves when a reaction happens. Which side of the reactants the products sit on a reaction profile. And which way round the subtraction runs in a bond-energy calculation. Get those three directions right and the topic is straightforward. This module is built around them.

Energy words for this topic

Six terms, and the third one is the source of most confusion.

The thermometer went up

A student mixes two solutions and the thermometer in the beaker rises from 21 degrees Celsius to 34 degrees Celsius. What does this tell you?

• The reaction is exothermic, because it released energy to the surroundings
• The reaction is endothermic, because it took in energy from the surroundings
• The reaction is exothermic, because a faster reaction always produces more heat
• Nothing can be concluded, because the thermometer measures only the solution

Term to meaning

Two directions, one rule

Everything in this topic falls into one of two columns. The everyday products in the last row are the examples this specification names.

Pick the ones that release energy

Select the TWO processes that are exothermic.

• Burning a fuel in oxygen
• Neutralising an acid with an alkali
• Breaking down a metal carbonate by heating it
• The reaction inside a sports injury cold pack

Work out the change

A reaction mixture starts at 19 degrees Celsius and finishes at 32 degrees Celsius. Work out the temperature change, in degrees Celsius.

The trap: system or surroundings

Here is the sentence that catches people out. In an exothermic reaction, energy is LOST by the reaction but GAINED by the surroundings, so the temperature you measure goes UP. Both halves of that are true at once, and they point opposite ways. Whenever you write about energy in this topic, say which one you mean. A student who writes that an exothermic reaction gets hotter has said something that is not quite right, and an examiner cannot tell whether they understand it or not.

Plot an endothermic profile

This reaction profile shows an ENDOTHERMIC reaction. The reactants are already plotted at 1 across and 30 up, and the activation energy peak at 3 across and 80 up. Plot the product energy level at 5 across, remembering which side of the reactants the products sit on for an endothermic reaction.

Where is the activation energy?

On a reaction profile, which distance shows the activation energy?

• From the reactants up to the top of the curve
• From the reactants across to the level of the products
• From the top of the curve down to the level of the products
• From the bottom of the axis up to the level of the reactants

Describing the profile

A reaction profile is read from left to right. The curve starts at the energy of the _____ and rises to a peak, and the height of that rise is the _____ energy. It then falls to the energy of the _____. If they end up lower than they started, the reaction is _____ and the overall energy change is given a _____ sign.

A bond-energy answer, marked

Higher tier. Question: breaking all the bonds in the reactants takes in 2500 kJ, and making all the bonds in the products releases 2800 kJ. Calculate the overall energy change. Model answer: energy change equals bonds broken minus bonds made, so 2500 minus 2800, which is minus 300 kJ. The answer is negative, so the reaction is exothermic. Look at why the sign works out that way. Breaking bonds always takes energy in, and making bonds always gives energy out. More energy came out on making than went in on breaking, so the reaction gave energy out overall, and a negative sign is what that looks like. If you ever find yourself writing bonds made minus bonds broken, you have the subtraction the wrong way round.

The order of the calculation

Put the steps of a bond-energy calculation into the order you would carry them out.

• List every bond in the reactants and add up the energy needed to break them
• List every bond in the products and add up the energy released making them
• Subtract the bonds-made total from the bonds-broken total
• Look at the sign of your answer to decide exothermic or endothermic
• State the overall energy change with its sign and the unit kJ

Broken minus made

Higher tier. Breaking the bonds in the reactants takes in 1560 kJ. Making the bonds in the products releases 1748 kJ. Work out the overall energy change in kJ, giving your answer with its sign.

Which way is wrong?

Three of these sentences have the direction right and one has it backwards. Select the ONE sentence that is the wrong way round.

• In an endothermic reaction the temperature of the surroundings falls.
• Breaking bonds releases energy, and making bonds takes energy in.
• On the profile of an exothermic reaction the products sit below the reactants.
• A negative overall energy change means the reaction is exothermic.

Explain a temperature fall

A student adds a solid to water in a beaker and the temperature falls from 20 degrees Celsius to 12 degrees Celsius. Explain what is happening, and describe what the reaction profile for this change would look like.

• State the temperature change and say which type of reaction this is
• Explain where the energy has gone, saying clearly whether you mean the reaction or the surroundings
• Describe where the reactants, the products and the activation energy peak would be drawn
• State whether the overall energy change would be positive or negative, and why
• Explain how the bonds-broken and bonds-made totals would compare for this reaction