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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. This transfer is usually in the form of thermal energy, i.e. heat. A chemical reaction can be either exothermic or endothermic.

Exothermic Reactions

Exo- means 'out; think about words like "exit". Exothermic reactions release energy 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.

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.

In an endothermic reaction, the energy of the products is higher than the energy of the reactants. Examples of endothermic reactions are electrolysis, photosynthesis and thermal decomposition. In both 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. 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.

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.

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.

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.

 

 

 

For each statement, decide if it matches exothermic or endothermic reactions.

 ExothermicEndothermic
Release energy into surroundings
Extract energy from surroundings
Making more/stronger bonds
Making fewer/weaker bonds

Classify each of these reactions as exothermic or endothermic.

 ExothermicEndothermic
Combustion
Electrolysis
Thermal decomposition
Neutralisation
Photosynthesis

A group of students perform a chemical reaction. At the start of the experiment, the thermometer reads 22 °C, and at the end of the experiment, the thermometer reads 15 °C.

What type of reaction was that?

Is the chemical reaction in a glow stick exothermic or endothermic? Choose answers from each column to make a complete sentence.

What is the evidence that photosynthesis is an endothermic reaction?

Plant leaves are cold to touch

Plant leaves are green

Plants do not do photosynthesis in the dark

Plants produce oxygen

Fill in the gaps in this paragraph about activiation energy. Use some of these words:

continue, large, most, small, start

Plant leaves are cold to touch

Plant leaves are green

Plants do not do photosynthesis in the dark

Plants produce oxygen

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.

Plant leaves are cold to touch

Plant leaves are green

Plants do not do photosynthesis in the dark

Plants produce oxygen

The combustion of hydrogen in oxygen makes water:

2 H2 (g) + O2 (g) → 2 H2O (l).

How many bonds are there of each type in the reactants and in the products?

Plant leaves are cold to touch

Plant leaves are green

Plants do not do photosynthesis in the dark

Plants produce oxygen

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

 

Plant leaves are cold to touch

Plant leaves are green

Plants do not do photosynthesis in the dark

Plants produce oxygen

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

 

Plant leaves are cold to touch

Plant leaves are green

Plants do not do photosynthesis in the dark

Plants produce oxygen

  • Question 1

For each statement, decide if it matches exothermic or endothermic reactions.

CORRECT ANSWER
 ExothermicEndothermic
Release energy into surroundings
Extract energy from surroundings
Making more/stronger bonds
Making fewer/weaker bonds
EDDIE SAYS
Exothermic reactions release thermal energy into their surroundings; exo is like exit. They do this by creating more or stronger bonds in the chemicals.
  • Question 2

Classify each of these reactions as exothermic or endothermic.

CORRECT ANSWER
 ExothermicEndothermic
Combustion
Electrolysis
Thermal decomposition
Neutralisation
Photosynthesis
EDDIE SAYS
The key rule is that endothermic reactions need continuing energy input, or the reaction stops. Exothermic reactions release energy into their surroundings.
  • Question 3

A group of students perform a chemical reaction. At the start of the experiment, the thermometer reads 22 °C, and at the end of the experiment, the thermometer reads 15 °C.

What type of reaction was that?

CORRECT ANSWER
endothermic
EDDIE SAYS
The thermometer reading has gone down, so energy has gone into breaking chemical bonds- this shows the reaction was endothermic.
  • Question 4

Is the chemical reaction in a glow stick exothermic or endothermic? Choose answers from each column to make a complete sentence.

CORRECT ANSWER
EDDIE SAYS
The "thermic" in exothermic is a bit misleading; the energy usually leaves the reaction by heating the surroundings, but any reaction which gives out energy counts as exothermic.
  • Question 5

What is the evidence that photosynthesis is an endothermic reaction?

CORRECT ANSWER
Plants do not do photosynthesis in the dark
EDDIE SAYS
Endothermic reactions take energy from their surroundings, and if the energy source disappears, the reaction stops. For photosynthesis, the energy input is light.
  • Question 6

Fill in the gaps in this paragraph about activiation energy. Use some of these words:

continue, large, most, small, start

CORRECT ANSWER
EDDIE SAYS
Activation energy is the energy needed to make a reaction begin to happen. This is linked to the energy needed to start breaking atoms apart in the structures of the reactants.
  • Question 7

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. If burning methane was endothermic, we would have to keep using a match (or similar) to keep the reaction going.
  • Question 8

The combustion of hydrogen in oxygen makes water:

2 H2 (g) + O2 (g) → 2 H2O (l).

How many bonds are there of each type in the reactants and in the products?

CORRECT ANSWER
EDDIE SAYS
In an H2 molecule, there is one H-H bond. Since there are two of the molecules in the reactants, that makes 2 H-H bonds. There is only one O2 molecule, so only 1 O=O bond. (The = symbol is because the bond between two oxygen atoms is a double bond). Afterwards, there are 2 H2O molecules, each with 2 H-O bonds, making four in total.
  • Question 9

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
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 10

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.
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