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Understand Exothermic and Endothermic Reactions

Worksheet Overview

QUESTION 1 of 10

During chemical reactions energy transfer takes place to and from the surroundings. A chemical reaction can be either exothermic or endothermic.

Exothermic Reactions

Exo- means 'out; think about words like "exit". Exothermic reactions release heat to their surroundings. The picture shows an example of an exothermic reaction:

 

Explosions are exothermic reactions, but a reaction can be exothermic without exploding. Any increase in temperature indicates an exothermic reaction. If you were to hold a container where an exothermic reaction takes place, you would feel the heat and a thermometer inside the container would show an increase in temperature. Examples of exothermic reactions are combustion (burning) and neutralisation. Hand warming pads contain chemicals which undergo an exothermic reaction when pressed together. Reactions which release energy by other pathways (like light) are also exothermic- the key thing is that, in some way or other, energy is released.

[Shutterstock 1265598505]

Endothermic Reactions  

Endo- means 'inside'; an endoscope is a camera a doctor puts inside a patient's body. Endothermic reactions take in energy from their surroundings. If you were to hold a container where an endothermic reaction takes place, you would feel your hands getting cold, as the reaction would take heat from the container and...even your hands if you kept holding it! A thermometer in the container would show a decrease in temperature.

The energy of the products is higher than the energy of the reactants. Examples of endothermic reactions are electrolysis, photosynthesis and thermal decomposition. In these reactions, we need to keep putting energy into the reaction for it to keep going. In electrolysis the energy source is the battery, in photosynthesis the energy is light,  and in thermal decomposition the energy source is a heat source (like a flame).  Cold pads to relieve injuries can use chemicals which undergo an endothermic reaction.

Reaction profiles

An reaction profile is a sort of graph, comparing the energy of a set of chemicals before, during and after a chemical reaction. Before the reaction is on the left, and after the reaction is on the right. Energy is on the vertical axis; high energy is high up and low energy is low down.

In an exothermic reaction (on the left), the energy of the chemicals starts high and ends lower. The energy difference is released into the surroundings.

In an endothermic reaction (on the right), the energy of the chemicals starts low and ends higher. The energy difference is taken from the surroundings.

Both reaction profiles have a peak in the middle, where the energy of the chemicals is highest of all. This is because nearly all chemical reactions need some energy input so they can begin to happen. This is because chemical reactions need to break some atoms apart before they can be reassembled in a new pattern, and this requires energy. The energy needed to start a chemical reaction is called the activation energy- it's the energy needed to make the reaction active.

Making and breaking bonds

We can predict whether a reaction will be exothermic or endothermic by thinking about the bonds between atoms in a chemical.  Energy must be supplied to break a bond between two atoms- think about two boxes held together by sticky tape. We have to do work (put energy in) to break them apart. When a bond forms between two atoms, energy is released. In an exothermic reaction, the products have more (or stronger) bonds than the reactants. In an endothermic reaction, the products have fewer (or weaker) bonds than the reactants.

If we have data for the energy in a chemical bond, we can be more quantitative than this. For example, think about the combustion of methane. The balanced symbol equation is CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l).

Step 1: Draw the molecules for the reactants and products

Step 2: Count the bonds

On the reactant side, there are 4 C-H bonds in CH4, and 2 O=O bonds in the two O2 molecules. On the product side, there are 2 C=O bonds in CO2, and 4 H-O bonds in the two H2O molecules. (- means a single covalent bond, = means a double covalent bond).

The energies for each of these bonds are:

Bond C-H O=O H-O C=O
Energy (kJ / mol) 412 498 465 532

You don't need to memorise this table- if you need these data in an exam, you will be given the data you need. Different books and websites will give you slightly different values for the energies of these bonds. The O=O is a double covalent bond, which is why there are two lines.

Step 3: Calculate the energy for the reactants and products separately

Start by writing down the bond counts, then substitute in the bond energy data. Brackets will make the calculation more reliable.

Energy of bonds in reactants =  (4 x C-H) + (2 x O=O)

                                           = (4 x 412) + (2 x 498) = 2 644 kJ / mol.

Energy of bonds in products = (2 x C=O) + (4 x H-O)

                                          = (2 x 532) + (4 x 465) = 2 924 kJ / mol.

Step 4: Work out the energy change

Difference = 2924 - 2644 = 280 kJ / mol

Since the reaction is exothermic, the bonds must have less energy at the end than at the start. We write the bond energy change as - 280 kJ / mol.

In an exam, easier versions of this question will tell you what to do step-by-step, but you could be asked remember the order of the steps for yourself. In that case, it's even more important to write down all the stages in your thinking, not just the final answer. 

Reaction profiles help us visualise the difference between exothermic and endothermic reactions, and to compare reactions involving different amounts of energy; for example, an explosion releases a large amount of energy in a short time. Bond energy calculations are a way of predicting what will happen in a reaction- it's really important to label your numbers, so you don't mix up reactants and products.

 

 

 

Think about this chemical reaction:

NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l). 

These statements are all true; tick the ones which tell us that this reaction is exothermic

This is a neutralisation reaction.

One of the products is a liquid.

This reaction does not require energy to be input for it to continue.

This reaction makes the water hotter.

This equation is balanced.

The overall equation for the electrolysis of molten sodium chloride is this:

2 NaCl (l) → 2 Na (s) + Cl2 (g).

These statements about the reaction are all correct; which ones show that the reaction must be endothermic?

The sodium chloride must be molten for the reaction to happen.

If we remove the electrical power, the reaction stops.

The reaction is electrolysis.

One of the products is a gas.

One of the products is a solid.

A group of students do two neutralisation reactions. The volumes and concentrations of the acid and alkali are the same each time

Reaction Initial temperature (°C) Final temperature (°C)
1 22 28
2 20 27

Which of these is the correct conclusion?

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

When you burn gas with a Bunsen burner, you need energy from a match or lit splint to make the gas burn. Use a half sentence from each column to explain why this is.

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

The combustion of hydrogen in oxygen makes water:

2 H2 (g) + O2 (g) → 2 H2O (l).

Use the data in the table below to work out the total bond energy for the reactants, for the products, and the resulting bond energy change.

Use these bond energy data:

bond H-H O=O H-O
bond energy (kJ / mol) 436 498 465

 

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

This picture shows three hydrocarbon molecules- methane, ethane and propane

The equation for the combustion of propane is

C3H8 (g) + 5 O2 (g) → 3 CO2 (g) +4 H2O (l)

Calculate the bond energy of the reactants, the products, and the resulting energy change.

Use these bond energies:

bond C-H C-C O=O C=O H-O
bond energy (kJ / mol) 412 368 498 532 465

 

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

This picture shows three hydrocarbon molecules- methane, ethane and propane

The equation for the combustion of ethane is

2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (l)

Calculate the bond energy of the reactants, the products, and the resulting energy change for one mole of this reaction (so 2 moles of C2H6)

Use these bond energies:

bond C-H C-C O=O C=O H-O
bond energy (kJ / mol) 412 368 498 532 465

 

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

 

The equation for the combustion of ethane is

2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (l)

In the last question, you worked out the bond energy change for one mole of this reaction (so 2 moles of C2H6) is -670 kJ / mol.

To compare this fairly with the changes for methane and propane, we need the energy change per mole of C2H6. Select the correct answer for this, and the correct reason.

 

 

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

Hydrogenation is a reaction where we add hydrogen to another compound. We use it to convert liquid oils into solid fats, like margarine.

The equation for the hydrogenation of ethene (also called ethylene) is

C2H4 (g) + H2 (g) → C2H6 (g)

The structures of ethane and ethene/ethylene look like this:

Use these bond energies:

bond C-H C-C C=C H-H
bond energy (kJ / mol) 412 368 602 436

What is the overall change in bond energy for the hydrogenation of ethene?

Reaction 1 is exothermic, reaction 2 is endothermic.

Reaction 1 is more exothermic than reaction 2.

Reaction 2 is more exothermic than reaction 1.

We cannot tell which reaction is more exothermic.

The equation for the neutralisation of hydrochloric acid in sodium hydroxide is

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l).

The change in bond energy per mole is -57 kJ / mol.

If we did an experiment with 100 cm3 of acid and alkali with 0.1 mol / dm3 concentration, how much energy would be released?

570 J

5 700 J

57 000 J

570 000 J

  • Question 1

Think about this chemical reaction:

NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l). 

These statements are all true; tick the ones which tell us that this reaction is exothermic

CORRECT ANSWER
This is a neutralisation reaction.
This reaction does not require energy to be input for it to continue.
This reaction makes the water hotter.
EDDIE SAYS
All neutralisation reactions are exothermic, so they all release energy into the surroundings. As a result, they don't need any ongoing energy input, and make their surroundings (in this case, the water) hotter.
  • Question 2

The overall equation for the electrolysis of molten sodium chloride is this:

2 NaCl (l) → 2 Na (s) + Cl2 (g).

These statements about the reaction are all correct; which ones show that the reaction must be endothermic?

CORRECT ANSWER
If we remove the electrical power, the reaction stops.
The reaction is electrolysis.
EDDIE SAYS
The key rule is that endothermic reactions need continuing energy input, or the reaction stops. In the case of electrolysis, the energy comes from the power supply. Electrolysis is always endothermic.
  • Question 3

A group of students do two neutralisation reactions. The volumes and concentrations of the acid and alkali are the same each time

Reaction Initial temperature (°C) Final temperature (°C)
1 22 28
2 20 27

Which of these is the correct conclusion?

CORRECT ANSWER
Reaction 2 is more exothermic than reaction 1.
EDDIE SAYS
Both reactions cause an increase in temperature, so both are exothermic. Reaction 2 increases the temperature by 7 °C, and reaction 1 increases the temperature by 6 °C. As long as all the other conditions are the same, that means that reaction 2 has released more energy.
  • Question 4

When you burn gas with a Bunsen burner, you need energy from a match or lit splint to make the gas burn. Use a half sentence from each column to explain why this is.

CORRECT ANSWER
EDDIE SAYS
The match only supplies energy at the very beginning, which is what we expect for activation energy. Activation energy breaks bonds in the reactants, so that the atoms can reassemble in their new patterns.
  • Question 5

The combustion of hydrogen in oxygen makes water:

2 H2 (g) + O2 (g) → 2 H2O (l).

Use the data in the table below to work out the total bond energy for the reactants, for the products, and the resulting bond energy change.

Use these bond energy data:

bond H-H O=O H-O
bond energy (kJ / mol) 436 498 465

 

CORRECT ANSWER
EDDIE SAYS
Start by counting bonds. In each H2 molecule, there is one H-H bond, so we have two of those bonds. In the O2 molecule, there is one O=O bond. In each H2O molecule, we have 2 H-O bonds, so there are four in total. Reactants = 2 x H-H + 1 x O=O = (2 x 436) + (1 x 498) = 1370 Products = 4 x H-O = 4 x 465 = 1860 Difference = 1852 - 1018 = 490, but write a negative number because the reaction is endothermic.
  • Question 6

This picture shows three hydrocarbon molecules- methane, ethane and propane

The equation for the combustion of propane is

C3H8 (g) + 5 O2 (g) → 3 CO2 (g) +4 H2O (l)

Calculate the bond energy of the reactants, the products, and the resulting energy change.

Use these bond energies:

bond C-H C-C O=O C=O H-O
bond energy (kJ / mol) 412 368 498 532 465

 

CORRECT ANSWER
EDDIE SAYS
In the reactants, you have 2 C-C bonds, 8 C-H bonds and 5 O=O bonds. In the products, you have 6 C=O bonds, and 8 H-O bonds. The bond energy change is negative, because the reaction is exothermic.
  • Question 7

This picture shows three hydrocarbon molecules- methane, ethane and propane

The equation for the combustion of ethane is

2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (l)

Calculate the bond energy of the reactants, the products, and the resulting energy change for one mole of this reaction (so 2 moles of C2H6)

Use these bond energies:

bond C-H C-C O=O C=O H-O
bond energy (kJ / mol) 412 368 498 532 465

 

CORRECT ANSWER
EDDIE SAYS
In the reactants, you have 2 C-C bonds, 12 C-H bonds and 7 O=O bonds. In the products, you have 8 C=O bonds, and 12 H-O bonds. The bond energy change is negative, because the reaction is exothermic.
  • Question 8

 

The equation for the combustion of ethane is

2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (l)

In the last question, you worked out the bond energy change for one mole of this reaction (so 2 moles of C2H6) is -670 kJ / mol.

To compare this fairly with the changes for methane and propane, we need the energy change per mole of C2H6. Select the correct answer for this, and the correct reason.

 

 

CORRECT ANSWER
EDDIE SAYS
If we want the answer for 1 mole of fuel, we need to half the value we calculated for 2 moles, so that becomes the final step in the calculation. We couldn't work out the energy for 1 mole of fuel to start with, because then we wouldn't have complete O2 molecules. By the way, notice that as the molecules get larger, combustion releases more energy (methane, -280 kJ / mol; ethane, -335 kJ / mol; propane - 390 kJ / mol).
  • Question 9

Hydrogenation is a reaction where we add hydrogen to another compound. We use it to convert liquid oils into solid fats, like margarine.

The equation for the hydrogenation of ethene (also called ethylene) is

C2H4 (g) + H2 (g) → C2H6 (g)

The structures of ethane and ethene/ethylene look like this:

Use these bond energies:

bond C-H C-C C=C H-H
bond energy (kJ / mol) 412 368 602 436

What is the overall change in bond energy for the hydrogenation of ethene?

CORRECT ANSWER
EDDIE SAYS
In the reactants, you have 4 C-H bonds, 1 C=C bond and 1 H-H bond. The total bond energy is 2686 kJ / mol. In the products, you have 1 C-C bond, and 6 C-H bonds. The total bond energy is 2840 kJ / mol. Since the bond energy of the products is a higher number, the reaction is exothermic, so we write the change as a negative number.
  • Question 10

The equation for the neutralisation of hydrochloric acid in sodium hydroxide is

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l).

The change in bond energy per mole is -57 kJ / mol.

If we did an experiment with 100 cm3 of acid and alkali with 0.1 mol / dm3 concentration, how much energy would be released?

CORRECT ANSWER
570 J
EDDIE SAYS
If 1 dm3 contains 0.1 moles and we have 0.1 dm3, there will be 0.01 moles, so we will get 1/100 of the stated amount of energy. If an examiner was being really sneaky, they could ask you about the effect of this on the water temperature. Notice that you have 200 cm3 water in the container- 100 cm3 from the acid solution, and 100 cm3 from the alkali solution.
---- OR ----

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