Chemical Reactions PDF Download

Exothermic reactions

Burning

When something burns, a chemical reaction takes place. Burning is a chemical reaction in which a substance combines with oxygen. In a burning reaction, there are energy changes. The substance that reacts with oxygen is called a fuel.

Fuels have a store of chemical energy. Charcoal, wood, coal, natural gas and oil are examples of fuels.

When the fuel burns, the chemical energy is changed to thermal, light and sound energy. The thermal, light and sound energy dissipate (spread out) into the surroundings.

heatoxygen fuel

Burning requires oxygen, fuel and heat (thermal energy)

Combustion is another term for burning.

Look back at the equation in Getting started. You can see that, during the reaction, the atoms of carbon and oxygen join together in new ways. When this happens, chemical energy is changed to thermal energy and the temperature rises.

A chemical reaction in which thermal energy is given out is called an exothermic reaction.

Burning other substances

Hydrogen can be used as a fuel in a model rocket. The combustion of hydrogen is an exothermic reaction. The hydrogen and the oxygen combine to form water.

hydrogen + oxygen water + oxygenhydrogen water

When the atoms of hydrogen and oxygen rearrange themselves and combine together, energy is given out. This chemical energy is changed into kinetic, thermal, sound and light energy.

plastic bottle string to spark generator bent paperclips stopper mixture of hydrogen and air

Burning hydrogen can propel a plastic bottle like a rocket.

In this experiment, a large plastic soda bottle filled with hydrogen and air is attached to a string across the room. The stopper in the bottle has wires that allow a spark to be generated. The hot spark provides the energy to start the reaction. The hydrogen and oxygen react together.

The reaction gives out a lot of energy and the stopper is pushed out. This energy makes the bottle shoot (move very quickly) along the string.

The reactions of other substances burning in air are also exothermic reactions. An example is burning magnesium, which produces magnesium oxide. Energy is given out as heat and light as the magnesium and oxygen atoms rearrange themselves.

+oxygen magnesium oxidemagnesium Mg Mg O O Mg O Mg O

When a substance burns, it combines with oxygen and a new substance called an oxide is formed. Any reaction in which a substance combines with oxygen is an oxidation reaction.

An exothermic reaction with water

This is the equation for the reaction between potassium and water.

+ hydrogenpotassium water potassium hydroxide + K K K K O O H H O O H H H H H H

Water is made up of particles containing atoms of hydrogen and oxygen. In the potassium and water reaction, the bonds between the atoms of oxygen and hydrogen in the water break. The atoms rearrange to form the products potassium hydroxide and hydrogen. Stored chemical energy is changed to thermal energy, which dissipates into the environment.

Burning magnesium ribbon

An exothermic reaction with acid

If you add magnesium to dilute hydrochloric acid the test tube gets hot. This reaction is an exothermic one.

+ +magnesium Mg magnesium chloride Cl Mg Cl hydrogen H H hydrochloric acid H Cl H Cl

Measuring the rise in temperature during a reaction

Sofia and Marcus each measured 10 cm3 of dilute hydrochloric acid into a test tube and measured the temperature. Then they each added an identical piece of magnesium ribbon to their test tube of acid. When the reaction stopped, they each measured the temperature again.

100908070605040302010 0 °C 100 90 80 70 60 50 40 30 20 °C100908070605040302010 0 °C 100 90 80 70 60 50 40 30 20

Measuring the rise in temperature when magnesium reacts with hydrochloric acid.

Sofi a’s results

Start temperature in °C

End temperature in °C

18

42

Marcus’s results

Start temperature in °C

End temperature in °C

21

45

Endothermic reactions

Endothermic reactions

Some chemical reactions absorb thermal energy from their surroundings and change it to chemical energy stored in the chemical bonds. These are called endothermic reactions. When an endothermic reaction takes place, the temperature at the end of the reaction is lower than that at the start of the reaction.

Looking at endothermic reactions

This is the word equation for the reaction between sodium hydrogen carbonate and citric acid:

sodium hydrogen carbonate + citric acid

sodium citrate + water + carbon dioxide

During this reaction, thermal energy is absorbed from the surroundings and stored in the form of chemical bonds. So, if this reaction was carried out in a test tube, the surroundings will have a lower temperature and the test tube will feel cooler.

If you eat sherbet sweets, this reaction takes place in your mouth. The sherbet is a mixture of dry citric acid and sodium hydrogen carbonate. When you eat the sweets, these substances dissolve in your saliva, and react together. This gives a cool ‘fizzy’ feeling in your mouth (the surroundings), which is refreshing.

Another way to cool down

If you place about of water in a beaker and then stir in three spatulas of potassium chloride, you will find that the beaker gets cold. In this case, no chemical reaction has taken place. No new products are formed. The potassium chloride has just dissolved. A solution of potassium chloride has been formed. Potassium chloride is the solute and water is the solvent.

When potassium chloride dissolves in water, thermal energy is absorbed from the surroundings. This is why the beaker feels cold. This is an endothermic process.

Ice melting is another endothermic process. Thermal energy is absorbed from the surroundings as the solid ice changes to liquid water. Think about what happens to the particles when water changes state. The particles in the ice are lined up in rows and can only vibrate about fixed positions – they cannot move around inside the ice. The forces between the particles are strong.

000 °C 100 90 80 70 60 5040 30 20 10 50 potassium chloride thermometer glass rod water energy taken in

As the particles absorb thermal energy from the surroundings, they vibrate more and more. The ice begins to melt. When the particles have enough energy, they can move and overcome the forces holding them in place. The particles can now slide past one another. The water is now in a liquid state.

Endothermic or exothermic?

In exothermic reactions and processes, thermal energy is given out. In endothermic reactions and processes, thermal energy is taken in.

°C10090807060504030201000°C 100 90 80 70 60 50 40 30 20 10 0 boiling water cooling heat given out

°C10090807060504030201000 °C 100 90 80 70 60 50 40 30 20 100 10ice melting heat taken in

Exothermic processes give out energy to the surroundings (left). Endothermic processes absorb energy from the surroundings (right).

Using exothermic reactions

Some exothermic reactions are used to produce self-heating cans of food or drink. For example, a can of self-heating food contains two chemicals, which are in separate compartments. When you open the can, the two chemicals mix with each other and react. The reaction warms up the food or drink.

The chemicals used are calcium oxide and water. When the water and calcium oxide are mixed together they react, and heat is given off.

water

opening system seal for water and calcium oxide compartments

waterproof separator calcium oxide heat insulator

calcium oxide + water calcium hydroxide

These cans can be very useful if you are in a remote area, or in an emergency when there is no power, or when you are camping.

A self-heating can. When the can is opened, the seal between the water and the calcium oxide compartment is broken and the reaction takes place. Thermal energy is given out and transferred to the food.

The cans are expensive to produce because the compartments must be sealed from one another and from the food, so that it does not become contaminated. There have also been problems with the food not being heated evenly.

Using endothermic processes

People sometimes use ice packs when they injure themselves. These packs are stored in a fridge or freezer until they are needed. When the ice pack is placed on the injured area, heat from this area is transferred to the ice pack and the ice melts. This is an endothermic process. not an endothermic reaction as no new substances are formed It means that the injured area is cooled and this often prevents it from swelling up. After it has been used, the ice pack can go back into the freezer to be used again.

Some ‘ice’ packs are made from substances that undergo an endothermic process when they mix together.

A chemical icepack being used to treat an injury.

These packs can be used even when you don’t have fridge or freezer. The pack has two compartments inside, each with a different substance. These are usually ammonium nitrate and water. When you push on the pack and break the compartment containing ammonium nitrate, the water mixes with it and the ammonium nitrate begins to dissolve. This is an endothermic process, so the temperature drops.

Metals and their reactions with oxygen

Metals and oxygen

In Stage 7 you learned about the properties of metals. Now you are going to investigate how different metals react with oxygen.

Heating metals in air

In this activity, you will heat several different metals in air. Air contains oxygen, and some metals will react with it.

Read though the instructions and decide on the safety precautions you will need to take. Discuss these in your group and then with the class before you carry out your investigation.

You will need:

•

safety glasses

•

Bunsen burner

•

heatproof mat • tongs

•

small pieces of metal such as magnesium, zinc, iron and copper

Method

1

Take a small piece of one of the metals.

2

Place it in the tongs and heat it in a Bunsen flame.

3

Record your observations in a table and explain what happened.

4

Repeat steps 1–3 for each of the other metals provided.

Looking at the reactions of metals with oxygen

Many metals react with oxygen if they get hot enough. When you look carefully at the reactions of metals with oxygen, it is possible to identify which metals are more reactive. For example, magnesium is more reactive than iron because magnesium reacts more quickly than iron. This reaction between metals and oxygen is an oxidation reaction.

Some metals react very quickly with oxygen without even being heated. When pieces of sodium, potassium or calcium are taken from their containers, they appear dull. When the pieces are cut, the surface is shiny. The shiny surface soon becomes dull because the metal reacts with the oxygen in the air. The surface becomes covered with a new substance – the oxide of the metal. These metals are so reactive that they have to be stored under oil to prevent them reacting. The layer of metal oxide on the surface prevents any more of the metal from reacting with the air or water vapour.

A scientist cuts a piece of sodium metal with a scalpel.

The general word equation for this reaction is:

metal + oxygen metal oxide

Some metals, such as gold, do not react with oxygen. They are generally unreactive. They are described as inert.

Silver reacts slowly with the air and if a silver object is not cleaned it goes black over time, as silver oxide is formed.

The reaction between iron and oxygen

When iron is left in damp air it rusts. The iron reacts with oxygen to form an orange-brown solid, called iron oxide, otherwise known as rust.

iron + oxygen iron oxide

This is not a very useful reaction because it means that the iron changes and no longer has the same properties. A strong iron girder can become rusted and fall apart. This could mean that a building collapses.

The reaction between iron and oxygen only takes place when both water and oxygen are present. The water is not part of the equation, but it is needed for the reaction to happen. The reaction takes a long time to happen – iron is not very reactive with oxygen.

This new iron spanner, nuts and bolts are shiny.

The iron sheets in this old barn have rusted.

What causes iron to rust?

A new iron nail is placed in each of four test tubes, as in the diagram.

tube 2 tube 3tube 1 tube 4 tube 2 tube 3tube 1 tube 4 After a few weeks the experiment looks like this.

Test tube 1 contains nothing, apart from the nail, and is open to the air.

Test tube 2 contains water and the nail is half in the water. The tube is open to the air. So, this tube has air and water.

Test tube 3 has calcium chloride in the bottom. The calcium chloride absorbs water so the air inside the tube is dry. The tube is stoppered.

Test tube 4 has water that has been boiled to remove as much dissolved gas as possible. On top of the boiled water is a layer of oil. This stops any air entering the water. The tube is stoppered.

Tube number

Contains

Result

1

moist air

nail is rusted

2

water and air

nail is very rusty

3

dry air

no rust

4

boiled water covered with oil, no air

small amount of rust

How can iron be protected?

There are ways that iron can be protected so that it does not rust.

The iron can be painted. This stops the oxygen in the air reaching the iron.

The iron can be galvanised. This means covering the iron with a layer of zinc. This again prevents the oxygen reaching the iron.

Reactions of metals with water

Think like a scientist

Reactions of metals with water

It can be difficult to see how some metals react with water because they may be covered in a surface layer of metal oxide if they have reacted with the oxygen in the air. In the investigations, you may need to use sandpaper to clean the surface of the metals so that the metal can come in direct contact with the water.

You will need:

•

test tubes • test tube rack

•

sandpaper

•

forceps

•

small pieces of metals such as magnesium, zinc, iron and copper

Method

1

Take a small piece of one of the metals you have been given. Use sandpaper to clean the surface of the metal.

2

Place the metal into a test tube of water.

3

Record your observations in a table and explain what happened. You may need to leave the metal to react for some time. If nothing happens, you could try testing the metal again, this time using hot water.

4

Repeat steps 1–3 for each of the other metals you have been given.

Reactions of sodium and potassium with water

Some metals are too reactive for you to test in water. Sodium and potassium react very vigorously. They have to be stored under oil to prevent them from reacting with the water vapour in the air.

Sodium reacts vigorously with water.

Potassium is even more reactive than sodium. So much thermal energy is generated that the hydrogen gas produced in the reaction catches fire.

In these two reactions the metal reacted with water to produce hydrogen and the metal hydroxide.

metal + water metal hydroxide + hydrogen

Reactions of other metals with water

Some other metals react less vigorously with water – for example, calcium and magnesium. In the experiment shown in the diagram, a piece of calcium has been placed at the bottom of a beaker and covered with water. A filter funnel has been placed upside down over the metal. The gas given off is collected in a test tube by the displacement of water.

bubbles of gas metal (calcium)

Some of the metals that do not react with water may react with steam. Even magnesium will react more rapidly with steam than with water.

In the reaction shown here, magnesium is heated. From time to time, the heat is directed at the ceramic wool. The ceramic wool has been soaked in water, which when heated produces steam. In this reaction the magnesium reacts with water, which is in the form of a gas. Magnesium oxide and hydrogen are formed. The hydrogen gas that is given off can be burnt.

The word equation for this reaction is:

hydrogen gas burning ceramic wool magnesium ribbon heat Heating magnesium

magnesium + water (g) magnesium oxide + hydrogen

In the equation above the (g) after water indicates it is water in the form of a gas, in this case steam. Steam is formed by boiling water and is very hot whereas water vapour is made up of water particles in the air at lower temperatures. Some metals, such as gold, do not react with water at all.

Reactions of metals with dilute acids

A familiar reaction

You will probably remember the reaction of magnesium with dilute hydrochloric acid. This is the word equation for this reaction:

magnesium + hydrochloric acid magnesium chloride + hydrogen Magnesium chloride is an example of a salt. When a metal reacts with an acid, the products are a salt and hydrogen.

The general equation for this reaction is:

metal + acid salt + hydrogen

Think like a scientist

An investigation into the reaction of metals in acid

Sofia and Marcus have been asked to investigate the reactivity of metals with acids. They need to decide on which of the equipment and reagents that are available in the laboratory they need, to enable them to carry out the investigation as shown below.

safety screen safety glasses face protector magnesium, calcium, zinc, copper, and iron gloves ?? °C 100 90 80 70 60 50 40 30 20 10 0 00.00 g stop 40 5:00 hr min start 90 50 80 70 8 30 60 20 10 cm3 100 9 5 4 7 3 6 2 1 cm3 10 Metals such as sodium, potassium, in the form of blocks, filings or powder

Part 1: Planning the investigation

Use the information and ideas above to plan the investigation for Sofi a and Marcus. Choose which of the items in the diagrams they need to use. Some of the items are not appropriate to use.

Discuss in your small group how you will answer these questions.

•

What will they change?

•

What will they keep the same?

•

How will they measure the reactivity and decide which is the most or least reactive metal?

•

How will they keep safe?

•

What equipment will they use?

Remember to include a results table and an idea of what they should be looking for in order to identify which are the most reactive metals.

Write your plan and show it to your teacher.

Part 2: Carrying out the investigation

Your task is to find the order of reactivity of the metals you are given, remember to work in a methodical way and keep an accurate record of your results.

Method

1

Follow the plan you have written, once you have had it checked by your teacher. Remember to work carefully and to keep an accurate record of your results.

2

Select the appropriate equipment.