Exothermic and endothermic reactions

Chemistry

Exothermic and endothermic reactions

Heat is given out in an exothermic reaction. We know this because the surroundings get warm.

In an endothermic reaction, energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls.

An equation can be used to show that some of the bonds in the reactants break during the chemical reaction. New combinations of bonds are made as the products form.

Making and breaking of bonds involves energy. You need to supply energy (heat) to break bonds, so bond breaking is endothermic.

When new bonds are made energy (heat) is released to the surroundings, so bond making is exothermic

Energy level diagrams

We can show these energy changes on an energy level diagram.

The atoms and molecules here have been drawn on levels so that the atoms at the top have more energy than the molecules at the bottom.
Look at the arrows and note that in this reaction, although energy is needed to break up the reactant molecules, even more energy is given out while forming the product molecules.
The length of each arrow is roughly proportional to the amount of energy put in or given out.
Each kind of bond has an energy value called the bond energy. This is the energy needed to break or make a particular type of bond. Bond energies are measured in kilojoules per mole of bonds.

Draw an energy level diagram for an exothermic reaction and an endothermic reaction. Show all the formulae of the reactants, the products and the separated atoms

Questions and Answers

Here is a big question which involves calculation. You need to be able to do this in exam conditions. You will need a calculator.

Methane burns in oxygen to form carbon dioxide and water:

Question 1

How much energy is needed to break a mole of methane into atoms?

Teacher's Note

Work out the energy of the four bonds in methane.

The Answer

There are 4 C -- H bonds in methane, so you have to multiply the C -- H bond energy by 4. The correct answer is 435 x 4 = 1740 kJ.

Question 2

How much energy is needed to break the oxygen bonds in the reactants into atoms?

Teacher's Note

There are two moles of oxygen molecules here. Some people wrongly think that they have to multiply it by two because it's a double bond. You don't because 498 kJ is the bond energy of the double bond.

The Answer

There are two moles of oxygen molecules on the left. The bond energy for a O = O bond is 498 so you multiply this by two to give the correct answer, 996 kJ.

Question 3

How many O -- H bonds are there in the two molecules of water?

Teacher's Note

The 2 in front of the H2O means that there are two moles of water.

The Answer

There are two molecules of water and there are two O -- H bonds in each molecule of water. So the answer is 4 O -- H bonds.

Question 4

What is the energy needed to break the bonds in methane and oxygen?

The Answer

4 C -- H bonds in methane = 1740 kJ and 2 O = O bonds = 996 kJ. Added together = 2736 kJ.

Question 5

What is the energy given out when new bonds are formed in carbon dioxide and water?

The Answer

2 O = C bonds = 805 x 2 and 4 H -- O bonds = 464 x 4. Added together = 1610 + 1856 = 3466 kJ.

Question 6

The energy needed to break the bonds in the reactants is 2736 kJ and the energy given out when new bonds are formed in the products is 3466 kJ. Is energy absorbed or released when the reaction occurs? How much energy?

The Answer

Energy is released and so it is exothermic. Notice that the reaction is about burning methane.

More energy is released when the bonds in the products are formed (3466kJ) than is needed to break the bonds in the reactants (2736kJ). The amount of energy released is 3466 - 2736 = 730 kJ.

Exam Tips

When studying chemistry and calculating the energy needed to break all the bonds in a compound, make sure that it is the bonds that you count and not the atoms. Students commonly count atoms rather than bonds.
Remember when answering chemistry questions that with double and triple bonds, the bond energies you will be given are for all the bonds shown. For example, if O = O --> 498 kJ/ mol, then the figure is for both bonds in oxygen. Do not multiply by 2 as many students do.
For chemistry questions you don't need to learn bond energy values as you will always be given them. They also vary a bit from compound to compound.
When answering chemistry questions you will usually be given the graphical formula of each molecule in these sorts of calculations. However, you should learn to draw some. Write out the following names on a card and test yourself by drawing their formulae: hydrogen, oxygen, water, carbon dioxide and halogens.
If you have a question about chemistry and energy reactions which doesn't show an energy diagram, it may be useful to draw one. Write the total bond energies beside the arrows to help you see whether the reaction is exothermic or endothermic.

Exam Tips

When studying chemistry and calculating the energy needed to break all the bonds in a compound, make sure that it is the bonds that you count and not the atoms. Students commonly count atoms rather than bonds.
Remember when answering chemistry questions that with double and triple bonds, the bond energies you will be given are for all the bonds shown. For example, if O = O --> 498 kJ/ mol, then the figure is for both bonds in oxygen. Do not multiply by 2 as many students do.
For chemistry questions you don't need to learn bond energy values as you will always be given them. They also vary a bit from compound to compound.
When answering chemistry questions you will usually be given the graphical formula of each molecule in these sorts of calculations. However, you should learn to draw some. Write out the following names on a card and test yourself by drawing their formulae: hydrogen, oxygen, water, carbon dioxide and halogens.
If you have a question about chemistry and energy reactions which doesn't show an energy diagram, it may be useful to draw one. Write the total bond energies beside the arrows to help you see whether the reaction is exothermic or endothermic.

Periodic Table

Things you should know

Before you start this task you should understand the meaning of element, compound, atom and molecule.
You should know that each element is represented by a symbol.

Table Elements

In the periodic table the chemical elements can be arranged in order of increasing atomic number (number of protons). This list can be arranged in rows so that elements with similar properties fall in the same columns.

The rows are called periods and the columns are called groups.

Exam Tips

Identify the correct period in the periodic table. Remember that the first period only contains two elements, hydrogen and helium.
Take care when interpreting questions about boiling points and melting points in °C. It is a common mistake to think that -200°C is a higher temperature than -100°C. Look for the negative sign!

Things you should know

Take care that you identify the correct period, or row, in the periodic table. Remember that the first period only contains two elements, hydrogen and helium.
The chemical elements are arranged in the periodic table in order of their atomic number.
The rows in the periodic table are called periods; the columns are called groups.
Elements in the same group have atoms with the same number of electrons in their outer electron shell. This gives them similar chemical properties.
The further the outer electrons are from the nucleus, the easier it is to lose an electron from the atom and the harder it is for an atom to gain an electron.
You should understand the meaning of element, compound, atom and molecule.
You should understand the properties that distinguish metals from non-metals.
You should understand how the periodic table is arranged into rows called periods and columns called groups.
You should try to learn the names of the elements in group I (the alkali metals), group VII (the halogens) and group 0 (the noble gases). You will also need to know that within each of these groups the properties of the elements are similar.

Questions and Answers

Question 1

Identify an element in this periodic table which has two electrons in its outer electron shell. [1 mark]

The Answer

Ca (calcium)

Examiner's Note

Well done. The group number (the figure at the top of each column) is the same as the number of electrons in the outer shell of the atoms in that group. For this reason, you could also have answered Mg (magnesium).

Question 2

Identify an element which combines with hydrogen to form a colourless, acidic gas. [2 marks]

The Answer

Cl (chlorine)

Examiner's Note

Well done. Chlorine is a halogen, ...

This is a preview of the whole essay

Questions and Answers

Question 1

Identify an element in this periodic table which has two electrons in its outer electron shell. [1 mark]

The Answer

Ca (calcium)

Examiner's Note

Question 2

Identify an element which combines with hydrogen to form a colourless, acidic gas. [2 marks]

The Answer

Cl (chlorine)

Examiner's Note

Well done. Chlorine is a halogen, in group VII of the periodic table. All of these elements react with hydrogen to form hydrogen halides, gases which dissolve in water to make acids. F (fluorine), Br (bromine) and I (iodine), are all halogens, and so would also have been correct answers. Chlorine reacts with hydrogen to form hydrogen chloride. A 'solution' of hydrogen chloride gas in water is called hydrochloric acid.

Question 3

Identify a halogen (group VII element) that can react with an aqueous solution of sodium bromide.

The Answer

F (fluorine)

Examiner's Note

Well done! As you go up group VII in the periodic table, the halogens become increasingly reactive. A more reactive halogen will displace a less reactive one from a solution of its salt. e.g. Bromine is higher up the group than iodine, and so bromine + potassium iodide --> iodine + potassium bromide.

The halogens that are higher up group VII than bromine are Cl (chlorine) and F (fluorine). Either one of these would have been the correct answer. Iodine is further down the group than bromine, and so does not displace it from its salt.

Question 4

Find an element in this table which forms ions with a 2- charge.

The Answer

O (oxygen)

Examiner's Note

Well done! The periodic table can tell you what type of ions an element can form. For metals in groups I to III the charge on the ion is + and is the same as the group number e.g. Al3+. For non-metal ions in groups V to VII, the charge is the group number minus 8. Therefore group 6 elements must have 2 - ions. (6 - 8 = 2-). Non metals form ions by gaining electrons. Metals form ions by losing electrons.

Question 5

Which noble gas in this periodic table has the lowest boiling point?

The Answer

He (helium).

Examiner's Note

That's right. For elements of group VII (the halogens) and group 0 (the noble gases), the boiling point increases as the size of the atoms increases. In other words, the boiling point increases as you go down the group. Helium is at the top of group 0, and so must be the noble gas with the lowest boiling point.

Question 6

Identify a metal, M, which forms a chloride with the formula MCl2.

The Answer

Mg (magnesium).

Examiner's Note

Well done! The two possible correct answers are Mg (magnesium) or Ca (calcium). Chlorine forms chloride ions, which have a charge of 1- . There are 2 chloride ions in MCl2, so the charge on these is

2 (1-) = 2 -

To balance these charges, the metal ion must have a charge of 2+. Now look at the periodic table. You can find out the charge on a metal ion by finding the group that the metal is in. The group number is the same as the charge on a metal's ions, and so the element must be in group II - meaning it is either Mg (magnesium) or Ca (calcium). Sodium is in group I, and so its ions have a charge of 1+.

This only applies to metals near the top of the periodic table.

Question 7

Sodium reacts slowly with chlorine at room temperature. Select an element from this periodic table that will react more vigorously with chlorine.

The Answer

K (potassium).

Examiner's Note

That's right. The further that an element is down group I, the more vigorous its reaction with chlorine or oxygen. Potassium is further down group I than Sodium, and so will react more vigorously.

Exams often contain statements where you have to choose the correct word from a choice of two. Here are two examples:

Question 8

As you go down group I, the reactivity of the elements increases/decreases because an electron is lost more easily/less easily.

The Answer

The reactivity of the elements increases because an electron is lost more easily.

Question 9

As you go down group I, the boiling point of the elements increases/decreases because the size of the atoms increases/decreases. Look up which are the correct words for these statements and write them on a revision card.

The Answer

The boiling point of the elements decreases because the size of the atoms increases.

Write out similar statements for the group VII elements (the halogens), and the group 0 elements (the noble gases), using their boiling points and reactions with sodium.

Further things to do

Write out all the reactions of the halogens that you know on a filing card. On another card, do the same for the reactions of the group I metals. Get a friend to test you by asking what products are formed in each reaction.

Take a filing card and write the properties of the first 3 elements of group I on it like this:

Rates Of Reaction

Collisions and reactions

For a chemical reaction to occur, reactant particles must collide.

Increasing the temperature of a reaction increases the rate of a reaction. At the higher temperatures reactant particles move faster and collide more often and more violently.
Increasing the concentration of reactants increases the rate of a reaction. This is because there are more particles in the same volume so more collisions are possible every second.
Breaking up solids into smaller pieces increases reaction rate because this increases the total surface area. Increasing the surface area must increase the number of particles of the solid in contact with the other reactant in solution.
Using a catalyst increases the rate of a reaction. Catalysts are not used up in a reaction

Questions and Answers

Question 1

Ammonia is made industrially by reacting nitrogen with hydrogen. The reaction is carried out in a closed reaction vessel. An iron catalyst is used.

N2(g) + 3H2(g)-->2NH3(g)

Use ideas about moving particles to explain how increasing the pressure in this reaction affects the rate of reaction.

The Answer

Increasing the pressure forces the gas molecules closer together. The closer the particles are, the more they collide. The greater frequency of particle collision, the greater the rate of reaction.

Examiner's Note

Well done. Look out for questions with the word affects. There will usually be a mark for stating whether the rate increases or decreases.

Question 2

What is meant by the term 'catalyst'?

The Answer

A substance which increases the rate of reaction but does not get used up in the reaction. It can be used over and over again.

Examiner's Note

Well done. A good explanation.

Question 3

Explain how a solid catalyst like iron works.

The Answer

Nitrogen and hydrogen molecules are adsorbed onto the surface of the iron. The nitrogen and hydrogen molecules are now close together and in the correct orientation. The bonds in the reactants can be broken and new bonds formed more easily.

Examiner's Note

Excellent answer. Notice that the term adsorb (not absorb) is used to describe the action of the catalyst on the reactants

Exam Tips

Make sure that you can analyse graphs carefully. Remember that in a graph of extent of reaction against time, the steeper the gradient, the faster the rate of the reaction.
Check with your syllabus what you need to revise for each section.

Further things to do

Carry out an experiment to make sure that you understand why cutting up a solid into smaller pieces increases the surface area and increases the rate of reaction. To see this, take a piece of plasticine, make it into a cube then cut it into two - you now have two extra sides. Now try to explain to a friend why this should increase the rate of reaction. Selecting or highlighting key points is a very useful way to summarise what you know.

Glossary

Activation energy

The minimum energy needed to break bonds in the reactants for a reaction to occur

Catalyst

A substance which increases the rate of reaction but is not used up in the reaction.

Enzymes

Protein molecules which are catalysts for reactions in living organisms - they have many practical uses, e.g. in washing powders. Enzymes work better at higher temperatures - but only up to about 40° C, because above that temperature their structure changes (they become 'denatured').

Gas pressure

The pressure due to gas molecules colliding with the walls of the container they are placed in - each molecule hits a tiny area of the wall with a force.

Further things to do

On a revision card opy out these equations and effects of pressure on the equilibrium.

Draw some more seesaw balance diagrams to see in which direction the reaction 'goes' when:

more reactants are added
more products are added
the reactants are removed

Exam Tips

The fact that there are constant concentrations of reactants and products at equilibrium does not mean there are same amounts of products and reactants.
Make sure that you know what is in your science syllabus as the syllabuses do vary slightly. For example, some syllabuses want you to know about the effect of pressure and temperature on equilibrium but others do not.
And finally, if you find something very difficult, look on the bright side. The ideas behind equilibrium are probably the most difficult you'll come across in chemistry. Asking someone for help is always the best way

How we separate crude oil

Crude oil is often transported to oil refineries in very large ships. The crude oil is a mixture of substances. These substances are mostly chemicals called hydrocarbons. Think about why this is a serious problem if the ship sinks or is damaged.

Hydrocarbons are chemical compounds composed of only carbon atoms and hydrogen atoms. Each hydrocarbon has a different boiling point. This is handy because we use the different boiling points to separate them.

There is a link between how many carbon atoms a hydrocarbon has and its boiling point. For example, look at the four hydrocarbons in the table:

Notice that as the number of carbon atoms increases, the boiling point also increases

What happens in an oil refinery

At the oil refinery the crude oil is heated to about 350°C. It is then pumped into the bottom of a fractionating tower.

Most of the substances in the crude oil evaporate. The mixture of vapours then passes up the tower. The tower is steadily cooler towards the top. Because the different hydrocarbons have different boiling points, they condense at different levels up the tower. Special trays collect the condensed liquid and it runs off in pipes.

Hydrocarbons with high boiling points condense first, low down the tower.
Hydrocarbons with low boiling points can get all the way to the top before they condense.
Some hydrocarbons have very low boiling points and so they are gases. They don't condense but are collected as 'fuel gases'.

The hydrocarbons which condense over a temperature range (for example, between 50°C and 100°C), are called a fraction because they are parts of a whole. Each of the fractions has special uses. Separating the mixture of substances in crude oil into fractions by evaporation and condensation is fractional distillation.

Questions and Answers

Question 1

Crude oil consists of a large number of different compounds. Explain how fractional distillation is used to produce useful compounds from crude oil. [3 marks]

Teacher's Note

In this type of question you are expected to write in sentences about fractional distillation.

Teacher's Note

There are three marks so you need to make at least three different points in your answer.

The Answer

Crude oil evaporates in a fractionating tower.
Different hydrocarbon compounds in the crude oil have different boiling points.
Their vapours condense at different temperatures in the tower, and are collected as either purer liquids or gases.
Each fraction contains hydrocarbons with different numbers of carbon atoms and each of these have different uses.

Examiner's Note

That was quite a long question but you could get shorter ones. The question is at about grade C and could be on a Higher or a Foundation paper. Here are some points to make about it.

You had to write in sentences.
There was no need to write lots of detail about the tower itself and its construction, nor did you have to write about how oil forms or now it is transported.
There are more scoring points (4) than there are marks available (3). This is sometimes the case when you have to write in sentences about things. You can never score more than the maximum though. In an exam don't take the chance, always write down what you think is relevant to the question asked. The space provided for your answer is usually a reliable guide to the amount you should write.

Shorter questions may be asked like this:

Teacher's Note

The number of marks tells you how much to write. Use scientific language.

Question 2

Name the two elements in a hydrocarbon. [1 mark]

The Answer

Carbon and hydrogen.

Examiner's Note

You can write them in either order.

Question 3

What is a hydrocarbon? [2marks]

The Answer

A compound containing only carbon and hydrogen atoms.

Examiner's Note

It is most important that you say 'only' and also say hydrocarbons are compounds, then you will get both marks.

Now try these short answer questions:

Question 4

What two physical processes take place in distillation? [1 mark]

The Answer

Evaporation and condensation.

Question 5

Why can petrol and diesel be separated by fractional distillation? [1 mark]

The Answer

They each have different boiling points. This is why fractional distillation works.

Question 6

How does the number of carbon atoms in a hydrocarbon affect its boiling point? [1 mark]

The Answer

The more carbon atoms it has, the higher its boiling point It would also be correct to say 'the fewer carbon atoms it has, the lower its boiling point'.

An exam question on fractional distillation may ask you about the uses of the different fractions.

Diesel is used as fuel for lorries, coaches and diesel trains. Naphtha is used in the chemical industry to make other chemicals. Lubricating oil is still used to lubricate bicycle chains, door hinges, garden tools and so on.

With an image like this you may be asked questions such as these:

Question 7

Which of the fractions shown in the diagram is a:

fuel used in cars?
heating gas such as Calor gas (propane)?
fuel used for flying jet air planes?
material used for road surfaces

The Answer

petrol or diesel
fuel gas
kerosene (sometimes called paraffin)
bitumen

Exam Tips

Sometimes more than one answer could be correct but if the question only asks for one then only give one.
Many exam questions on fractional distillation ask you about the uses of the different fractions. It is well worth learning these uses.

Glossary

Evaporation

happens when a liquid turns into a vapour.

Condensation

happens when a vapour turns into a liquid.

Distillation

is when we make a liquid evaporate and then condense the vapour back to a purer liquid

Questions and Answers

You need to have looked at the Revision Bite on Fractional Distillation before doing this one. There you will learn about how we separate crude oil into other chemicals, the hydrocarbons.

Think about this question before you move on

Question 1

Which are the only two elements found in a hydrocarbon?

The Answer

Carbon and hydrogen are the only two elements found in a hydrocarbon.

Examiner's Note

If you didn't get this you really must do the Fractional Distillation Revision Bite before you do this one

Hydrocarbons

Hydrocarbons are:

compounds that contain only carbon atoms and hydrogen atoms.
obtained from crude oil by fractional distillation and can be classified into different groups. The two groups that you need to know about for GCSE are called alkanes and alkenes

Properties of Alkanes

Alkanes are called saturated hydrocarbons because they only have single bonds between carbon atoms.
Alkanes burn in a plentiful supply of air to release energy (this is why they are used as fuels).
Burning (properly called combustion) also produces carbon dioxide and water vapour. For example, this equation shows the combustion of methane in a plentiful supply of air:

If the oxygen supply is restricted - meaning that there's not enough air - then the combustion of alkanes produces the poisonous gas carbon monoxide. For example, this equation shows the combustion of methane in a restricted supply of air:

Properties of Alkenes

Alkenes are said to be unsaturated because of the double bond in their structure.

Alkenes undergo combustion (burning) in a similar way to alkanes.

They also undergo many other chemical reactions that alkanes do not - the double bond in alkenes makes them more reactive than alkanes. For example, bromine can be added to the alkene double bond. This is called an addition reaction - a typical addition reaction occurs with bromine. The reaction makes the red colour of 'bromine water' disappear.

Remember that alkanes are saturated, do not have a double bond and so do not do this reaction. As they do not decolourise bromine, this reaction is useful in telling alkenes and alkanes apart. Alkenes quickly turn bromine colourless, while alkanes do not.

Exam Tips

This Revision Bite is designed for the chemistry paper of a 'Double Award' Science syllabus. If you are taking a separate science chemistry paper you will be required to know more about alkanes and alkenes. You may well need to know about 'isomerism' and 'homologous series'. You will need to look these up in your text book.
Make sure in an exam that you write the words alkanes and alkenes clearly so that the examiner is certain about which group you are referring to.
When asked to complete a table of information on alkanes and you are drawing the structural formula, keep the C atoms in astraight line. You will probably lose marks if you forget to put in all of the H atoms.
If you are writing an equation and are unsure of the symbol equation, first write down a correct word equation, then have a go at the symbol equation underneath.
Alkenes could contain several double bonds but it is very unlikely that you will be asked this at GCSE.
When answering questions about alkanes it is common that you will be asked to complete a table of information on alkanes.

Rock textures

There are many different sorts of igneous, sedimentary and metamorphic rock, but each of the three rock types has its own structure:

Igneous rocks tend to be smooth and hard with an irregular crystalline structure. If the molten rock from which they are formed cooled quickly then the crystals will be small. If this rock cooled slowly then the crystals will be much larger.
Sedimentary rocks often contain smaller fragments held together in a cement. Many also even contain fossils.
Metamorphic rocks usually contain regularly aligned crystals, and may be shiny and hard. They sometimes flake into layers.

Plate tectonics

The Earth's crust is made up of separate pieces called tectonic plates.
Earthquake and volcanic activity occurs at the edges of these plates.
It is the movement of the edges of these plates past each other which results in earthquakes.
Below the crust is the mantle, from where magma originates. This combination of crust and mantle is known as the lithosphere.
The edges of these plates are located on or near to mountain ranges, strings of ocean islands or deep ocean trenches.
The movement of the tectonic plate edges nearby dictates the sizes of these mountains, islands and trenches.

Measuring processes in the rock cycle

How are scientists and geologists able to follow the processes occurring within the rock cycle?

By looking at the different layers in the rock, geologists are able to see what type of rocks or particles are in each of the layers. Deeper layers tend to be made up of the oldest materials.

Fossils are often trapped within the layers of rock (what type of rock do you think is going to contain the fossilised remains of dead organisms?). We can then compare the type of fossils found with information we already have about the kind of organisms that existed at a certain period in time.

Because much of the material at the Earth's core is radioactive, many rocks will also contain a certain amount of these radioactive substances. This radioactivity means that some elements are unstable. The nucleus of a radioactive atom will have a tendency to fall apart. As it does so it will release some energy. Both the type of element and the energy it releases can be detected by various methods.

Radioactive substances decay (break down) into simpler substances over time - the older the sample, the less radioactive material there will be left in it. We can compare the amounts of radioactive and non-radioactive material in a rock to get some idea of the rock's age. Common elements to look for are uranium, potassium or rubidium

Exam Tips

Try to draw the diagram of the rock cycle from memory. When you think you have got it right, check the diagram again