Chemical Reactions

What is a chemical reaction?

A chemical reaction is a reaction between 2 or more substances, where the original substances change completely during the reaction, and at the end new products are formed.

The substances that are present at the beginning of the reaction are called _______________.

The substances that are present at the end of the reaction are called the _____________.

Example: If we take a paper, the reactant is the paper. If we burn the paper the reaction is burning. At the end of the reaction i.e. when the paper completely burns, the product is ash. Ash is completely different from the initial paper. Therefore, because there was a complete change between the reactant and the product, the reaction is called a chemical reaction.

A physical change is a change where the reactant doesn’t form a completely new product.

For example, if a piece of paper is cut up into small pieces it still is paper. This would be a physical change in the shape and size of the paper.

Chemical Change Physical Change

The reaction cannot be reversed

The reaction can be reversed

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

A new product is formed A new product is not formed

More examples of chemical and physical reactions.

Experiment:

Aim: To burn magnesium ribbon.

Apparatus: magnesium ribbon, tongs, Bunsen burner, matches, safety glasses, watch glass.

Method:

1. Cut a piece of magnesium ribbon and hold it in a tongs.2. Observe its colour and shape

3. Burn over a lighted Bunsen burner

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

4. Observe the flame

5. Observe the reaction.

Results:

Initial colour _______________________________

Shape ____________________________________

Colour of flame _____________________________

Final colour ________________________________

Shape ____________________________________

Precautions:

1. Wear protective safety glasses to protect your eyes

2. wear a lapcoat or apron to protect your uniform

3. do not touch the fire

4. do not smell the burning magnesium

5. do not bring the tongs towards your body while holding hot magnesium.

Conclusion: Was there a complete change between the reactant and the product? ______________________________________________________

Do you think we can reverse the reaction?_______________________________________________________

Do you think this was a physical or a chemical reaction? Why?______________________________________________________

Equation: magnesium + oxygen magnesium oxide.

Diagram:

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

Answer: Tick the correct answer

Physical change Chemical Change

Baking a cakeStriking a matchMaking ice cubesDissolving salt in waterBurning some toastBaking clay

Physical properties:1)  Has no colour or smell.

2)  Has no effect on moist litmus paper or moist universal indicator paper - it is neutral.

3)  Burns with a characteristic 'pop'.4) Hydrogen burns in air with a pale blue, almost invisible flame.

Specific test.Hydrogen gas is recognized by the 'pop' when it burns. The 'pop' is the sound of a small explosion, hydrogen is extremely flammable!Experiment:

To produce hydrogen mix some metal example magnesium with acid example hydrochloric acid.

Metal + acid hydrogen gas.

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

Metal + Acid Salt + Hydrogen.

1. Fill in the blanks. The following words might help you.light chemical used up heat irreversibl

enew

Burning a match is a ________________ reaction. It is an ________________ change. The wood is ______________ and __________ substances are made. Energy is given out as __________ and __________.

2. Fill in the blanks. The following words might help you

pop chemical Hydrogen acids corroded

Some metals react with __________ to make hydrogen. We test for ______________ using a lighted splint. A __________ means it is hydrogen. We say that the acid has _____________ the metal, because the metal is used up in the _________________ reaction.

Combustion is a reaction that occurs when burning an organic material in air. For example burning a paper, or burning wood etc. . . This releases a gas called carbon dioxide.

Physical properties of carbon dioxide:1. appearance – colourless , invisible gas

2. present in air (0.03%) – responsible for greenhouse effect3. does not burn

Producing carbon dioxide

1. heating a carbonate limestone can be used here as the carbonate.

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

2. breathing out – when we breathe out (exhale) we release carbon dioxide. If we breathe out in lime water, as shown in this diagram, lime water turns milky.

3. reacting a carbonate with an acid

Carbon dioxide turns lime water milky.

Complete these sentences:

All carbonates react with ____________ to produce ____________________ gas. We can test this gas by bubbling it into ___________ which turns ___________.

Other reactions that release carbon dioxide gas.

This diagram shows what happens when yeast reacts with sugar solution.

a. This process is called ____________ ____________. Yeast use sugars to produce energy. They also produce alcohol and carbon dioxide.

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

b. Carbon dioxide is given off during this reaction.c. The word equation for the above reaction is:

___________________________________________________

d. This reaction is used in the production of bread, alcohol (wine, beer, etc)

Making cakes

Baking powder contains bicarbonate of soda and a weak acid. When the powder gets wet, the acid and the bicarbonate of soda react together. One of the products of the reaction is carbon dioxide gas. This helps cake mixture to rise.

Acid RainMarble stone (or limestone) is calcium carbonate. Carbonates dissolves in acids. What do you know about acid rain? What happens to limestone in acid rain? _______________________________________________________

Fizzy drinksWhat happens when you open a bottle of soft drink or fizzy water that has been shaken? Do you know that the gas that makes that ‘fizz’ is carbon dioxide? In fact fizzy drinks are also called carbonated drinks.

The greenhouse effectThe carbon dioxide in the air has the same effect as the glass in a greenhouse. The carbon dioxide stops some of the heat energy from the Earth escaping, and the Earth stays warm. This is called the Greenhouse effect or Global warming. Scientists think that too much carbon dioxide will increase

the temperature of the air.

Answer these questions.

1. Write some sentences about making of bread and alcohol.2. What is making the Earth’s temperature getting hotter?

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

3. Write down the word equation of fermentation. What is the importance of this reaction?

The Fire Triangle

Fire is a chemical reaction that requires three main ingredients: • fuel (carbon) • oxygen• heat

These three ingredients make up the fire triangle. If any one is not present, a fire will not burn.

Fuel generally is available in ample quantities in the forest. Fuel must contain carbon. It comes from living or dead plant materials (organic matter). Trees and branches lying on the ground are a major source of fuel in a forest. Such fuel can accumulate gradually as trees in the stand die.

Oxygen is present in the air. As oxygen is used up by fire, it is replenished quickly by wind.

Heat is needed to start and maintain a fire. Heat can be supplied by nature through lightning. People also supply a heat source through misuse of matches, campfires, trash fires, and cigarettes. Once fire has started, it provides its own heat source as it spreads.

BURNING CANDLES

What happens when a beaker is put over a burning candle?_______________________________Why? _________________________________

Candle wax + oxygen carbon dioxide + water + energy

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

Dangerous fires start when burning gets out of control. Fire produces large amounts of energy as heat and light. Firefighters sometimes have a cylinder of oxygen on their backs, so they can breathe in a burning building.

Preventing a fire using the fire triangle

What are you removing when you try to put off a fire by:

a. using water? _________________________

b. using a fire blanket?_________________________

c. switching off the electricity supply?_________________________ d. cutting down trees?

_________________________

e. using a CO2 fire extinguisher? __________________________

f. using a powder fire extinguisher? __________________________

Physical Properties Oxygen is a colourless gas, without smell or taste,

is slightly heavier than air, is sparingly soluble in water,

It is vital for most living organisms. It is obtained from air. There is 21 per cent by volume or 23 per cent by weight oxygen in the atmosphere.

Combined Oxygen also occurs

in water, in vegetable and animal tissues, in nearly all rocks and in many minerals.

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

Oxygen occurs to a larger extent in the earth's crust than any other element.

Testing for oxygen:

Oxygen rekindles a glowing splint.

Answer the following questions:

1. The chemical name for burning is __________________ . When things burn, they react with ________________ gas in the air. The products formed are called ______. _____________ gas relights a _____________ splint.

2. You have burned magnesium in air. Remember that air is a mixture of gases; so what do you think will happen if we burn magnesium in pure oxygen gas?

________________________________________________

3. You are given three test tubes containing samples of hydrogen, carbon dioxide and oxygen. What tests do you do to find out which is which?

a. Hydrogen – ____________________________________

b. Carbon dioxide - _______________________________

c. Oxygen - _____________________________________

When a metal is attacked by water, air or acids in their environment, they corrode. Corrosion results in the metal become weaker and brittle. The corrosion of iron and steel is specifically called rusting due to the red-brown substance called rust that forms in the presence of water and oxygen. The experiment

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

below shows that an iron nail only rusts if both water and oxygen are present:

How to prevent rusting

1. Paint or grease. This prevents water or oxygen reaching the iron. However, this is only a temporary step since paint can flake off and grease can be rubbed off quite easily. Bikes are often painted of greased to prevent rust, since this is the cheapest method of prevention.

2. Plastic. Plastic is cheap and acts as a cover for the iron, for instance, it stops water or oxygen reaching the metal surface. Garden chairs are often made from iron coated in plastic.

3. Galvanising. This involves the iron been covered, usually in the form of a paint, by zinc.

4. Chromium plating. Works for the same reason as galvanising. Car bumpers are often chrome-plated.

Hydrogen filled balloons

Hydrogen was used to provide lift for the first balloons. The hydrogen gas is dangerous as it can burn with the oxygen in the air and the hydrogen filled balloon can explode.

Modern airships are filled with a gas called

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

helium, and they are a very safe way to travel as helium does not burn at all!

Why did they use hydrogen to lift balloons? ____________________________________________________

Hydrogen is no longer used in balloons. Why?____________________________________________________

Today Helium is used instead of Hydrogen. Why?____________________________________________________

1. Chemical reactions make new materials. They are irreversible changes. Which are chemical reactions and which are physical changes.a) lighting a Bunsen burner _________________b) salt disappearing as it is stirred into a beaker of water ___________c) water droplets forming on a kitchen window __________________d) green copper carbonate powder turning to black copper oxide when it is heated strongly ___________________

2. Burning and rusting are similar because they are reactions where oxides are formed. However, each reaction requires different things.a) Complete these sentences.i. For BURNING to take place, there must be a FUEL, OXYGEN and _________.ii. For RUSTING to take place, there must be IRON, OXYGEN and _________.

3) Many changes take place everyday. Some of them are reversible and others are not.a) What is the meaning of the word reversible?_______________________________________________________

b) Are reversible changes called physical or chemical changes?_______________________________________________________

c) Which changes are reversible.cooling water __________ baking bread _________boiling an egg _________ boiling water __________dissolving salt in water ________ burning a candle __________

4) A burning candle is floating on water and a beaker is placed over the burning candle. What happens to the flame after a short time? Why? ______________________________________________________

What happens to the water in the beaker? ____________________________________________________________ 5) Fire-fighters use this knowledge to put out a fire. Fill in the blanks:

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

Cool things down with water. This removes __________________. Choke a fire with foam. This removes ___________________. Let it burn out. This removes ____________________.

6) a) Give three examples of chemical reactions that take place in everyday life. ____________________________________________________________ b) Many metals react with acid giving off hydrogen gas.

i) Which metal and which acid are reacting together? metal _______________ acid _______________

ii) How can you tell that a gas is given off? ___________________________________________________________

iii) Where is the gas collected? ___________________________________________________________

iv) Explain how you would check that this gas is hydrogen. ________________________________________________________

___________________________________________________________

7) Some materials can burn. Underline the objects that can burn: paper, water, tree, iron, limestone, coal, hydrogen What gas is needed for a material to burn? ________________________ Where does this gas usually come from? ________________________

What two gases are produced when a wax candle burns? ______________________________________________________ Explain why a candle doesn’t burn unless it is lit by a match. ______________________________________________________i) Write one similarity between rusting and burning. _______________________________________________________ii) Write one difference between rusting and burning. ______________________________________________________

8. Write down the equation for burning wax._____________________________________________________

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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Chemical Reactions

9. These word equations show what happens when some chemicals are heated in air. Complete the word equations below:magnesium + oxygen __________________

iron + __________________ iron oxide

zinc + oxygen __________________

__________________ + oxygen Sulphur dioxide

Sharon was doing some experiments to find out what acids can do to metals. She was using magnesium, zinc and copper. She put a piece of each metal in a test tube and added acid to them.Two of the metals gave out bubbles of gas.

a) Which metals gave out bubbles of gas? ________________ ______

Sharon put a lighted splint at the mouth of the test tubes where gas was coming out. She heard a loud ‘pop’.b) What was the gas coming out? ______________________

c) Complete this word equation.acid + _______________--> ________________

d) What did Sharon learn about copper?______________________________________________________

Mrs. J. Ebejer Grech B.Ed.(Hons.)

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