Reactions and Mixtures (Year 8) Mike Turner, May. 2004 Click to move on

Mike Turner, May. 2004

Reactions and Mixtures (Year 8)

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Pure Substances

Pure substances contain only one sort of particle. Many substances that in everyday language we call pure are really mixtures.

For example, a bottle of pure mineral water has, according to the label, the following contents :

•Calcium•Magnesium•Sodium•Potassium•Bicarbonate•Chloride•Sulphate•Fluoride!

If it was really pure water, in the scientific meaning of the word, it would contain only water particles, H20.

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When we cook, we nearly always make use of mixtures. Mixtures often make very useful materials. They have different properties from the pure substances that make them.

The next two slides go over work from the ‘Building Blocks’ topic. It is important that you know the work from that topic well if you are to understand this one.



Elements, Molecules and Compounds

Some elements, such as the noble gases, exist as individual atoms.

Often, elements consist of two or more atoms joined together. We call these molecules.

If more than one type of atom is in a molecule, we call it a compound.

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Element or Compound?

methane, CH4

(Click for answer)

compound

sulphur, S8

(Click for answer)

element

hydrogen peroxide, H2O2

(Click for answer)

compound

sulphuric acid, H2SO4

(Click for answer)

compound

oxygen (ozone), O3

(Click for answer)

element

carbon (buckyball), C60

(Click for answer)

element

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Mixtures

This is a mixture of elements made up of atoms, elements made up of molecules, and compounds.

In a mixture, the particles are not joined together.

Atom, molecule or compound?(click for answer)



atom

molecule

compound

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Chemical Reactions

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Chemical reactions, or chemical changes, happen when the atoms in one or more chemicals split up and join together in new ways.

For example, hydrogen can react with oxygen :

Before the reaction the oxygen atoms go round in pairs, and the hydrogen atoms go round in pairs. When they react we get two new molecules, of the compound water.

Each of these molecules has three atoms, two hydrogen atoms joined to one oxygen atom.We can write this as H2O.

hydrogen + oxygen

water



Types of Reaction

There are several different groups that we can put reactions into. These include :

combustiondisplacementenzymeneutralisationoxidationprecipitationthermal decomposition

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When a reaction takes place, things happen. There may be a gas given off, a colour change or a solid material appearing. There could also be changes we can’t see, such as a temperature change.

One important thing to remember about chemical reactions is that the mass of the reactants (the chemicals we start with) is always the same as the mass of the products (the chemicals we end with).

The reaction of sugar with concentrated sulphuric acid.



Combustion

This is the name we give to reactions when things burn in air. Combustion is a special sort of oxidation reaction. It happens quickly and a flame is produced which heats the surroundings.

As we have already seen, hydrogen will react with oxygen – it burns to form water.

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Most fuels burn in air to form water and carbon dioxide :

methane + oxygen carbon dioxide + water



Oxidation

In oxidation reactions, oxygen combines with a substance to make an oxide.

For example iron will go rusty :

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iron + oxygen iron oxide

Adding pure oxygen makes reactions go faster than they would in air. Here is a picture of a biscuit burning!



Thermal Decomposition

In this type of reaction, a substance breaks down into simpler parts when it is heated.

For example, copper carbonate will decompose when heated :

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copper carbonateheat

copper oxide + carbon dioxide

If we measure the mass of copper carbonate before the reaction, and then the mass of copper oxide left after the reaction, the mass of the copper oxide will be lower.The mass has not disappeared. The missing mass is that of the carbon dioxide gas which has escaped into the air.



Displacement

In a displacement reaction, one metal ‘kicks out’ a less reactive one.

For example, when iron filings are added to copper sulphate solution, copper metal starts to appear.

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iron + copper sulphate copper + iron sulphate

The iron wants to be with the sulphate more than the copper does. We say that iron is more reactive than copper.

When the iron atoms are mixed with the copper sulphate atoms, the iron atoms ‘kick out’ the copper atoms and join on to the sulphate ions (particles).

We see the red / brown copper metal start to appear in place of the grey iron, and the solution changes colour from blue to green, as copper sulphate is blue but iron sulphate is green.



Precipitation

If an insoluble product is formed in a reaction, it suddenly appears. We say it precipitates, and the solid is called the precipitate.

This reaction happens if we blow into lime water (calcium oxide solution). The carbon dioxide from our breath joins up with the calcium oxide to form calcium carbonate, which is a white, insoluble material.

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calcium oxide + carbon dioxide calcium carbonate



magnesium + oxygen (click for answer)magnesium + oxygen magnesium oxide

Word Equations

If we heat copper in air, an oxidation reaction takes place and a new compound is made. We call this compound copper oxide.

We can write a word equation for this :

copper + oxygen copper oxide

Most metals will react with oxygen to form an oxide. For example :

If we burn 24 grammes of magnesium, we will end up with more than 24 grammes of magnesium oxide. This is because the magnesium atoms are now joined to oxygen atoms which have come from the air, so more atoms are in the compound than we started with.

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Valency

Valency is a word we use to describe the joining power of atoms and ions. Hydrogen has a valency of one, oxygen has a valency of two, so two hydrogen atoms can join on to one oxygen atom.

A useful way to work out the valency of atoms is to check which group they are in on the periodic table.

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Group 1 2 3 4 5 6 7 0

Valency 1 2 3 4 3 2 1 0

A good way to think of valency is to imagine particles as having a number of hands. The number of hands is equal to the valency.



Chemical Recipes

Molecules always have a certain number of atoms. For example water always has two hydrogen atoms and one oxygen atom, methane always has one carbon atom and four hydrogen atoms.

We can write these as chemical formulae.

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Water two hydrogen atoms, one oxygen atom H2O

Methane one carbon atom, four hydrogen atoms CH4

Sodium hydroxide one sodium atom, one hydroxide ion NaOH

Magnesium oxide one magnesium atom, one oxygen atom MgO

Copper sulphate one copper atom, one sulphate ion CuSO4

Calcium carbonate one calcium atom, one carbonate ion CaCO3

A carbonate ion contains one carbon atom and three oxygen atoms.A sulphate ion contains one sulphur atom and four oxygen atoms.A nitrate ions contains one nitrogen atoms and three oxygen atoms.

The ‘ate’ tells you that the ion contains oxygen.



Weighing Atoms

If you look at a periodic table, as well as having a symbol and an atomic number, every element has a mass number. We can use this to find the mass of compounds.

Hydrogen has a mass number of one, oxygen has a mass number of 16. What is the mass of a water molecule?

H2O – two atoms of hydrogen and one atom of oxygen.

Total mass of hydrogen = 2 x 1= 2

Total mass of oxygen = 16

Mass of water = 18

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Example

What is the mass of a copper sulphate molecule? (Copper has a mass of 63.5, sulphur 32 and oxygen 16).

CuSO4 – one atom of copper, one atom of sulphur and four atoms of oxygen.

Total mass of copper = 63.5Total mass of sulphur = 32Total mass of oxygens = 64

Mass of copper sulphate = 159.5



You have now reached the end of the revision presentation for the ‘Reactions and Mixtures’ topic.

I hope you have found it useful. If you have any ideas to improve this presentation please let Mr. Turner know.

Thank you for looking at this work, and good luck in your test.

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Reactions and Mixtures (Year 8) Mike Turner, May. 2004 Click to move on