Standard Grade Chemistry
Summary Notes
Topic 11. Metals
General
Learning Outcomes
Metals conduct electricity when solid or liquid.
You must be able to relate the specific properties of metals e.g density, electrical and thermal conductivity, malleability and strength to their uses.
Explain the need for recycling metals in terms of the finite nature of metal resources.
Describe the reactions of different metals with oxygen, water and dilute acid.
These reactions give an indication of the reactivity of the metal.
Ores are naturally occurring compounds of metals.
The less reactive metals are found uncombined in the earth’s crust and the more reactive metals have to be extracted from ores.
Give examples of social and industrial factors which resulted in large scale extraction of more reactive metals.
Some metals can be obtained from metal oxides by heat alone; some by heating with carbon.
Iron is produced from iron ore in the blast furnace.
The two key reactions that take place in the blast furnace are :
production of carbon monoxide reduction of iron oxide
an alloy is a mixture of metals or of metals with non-metals
give examples of the important uses of alloys e.g. brass, solder and stainless steel
Credit
Learning Outcomes
The extract of a metal from its ore is an example of reduction.
Explain in terms of the reactivity of the metal why some metals can be obtained from metals by heat alone; and why some metal oxides need to be heated with other substances e.g. carbon or carbon monoxide; and why some metals cannot be obtained by these methods.
Work out empirical formula (or molecular formulae) from masses or percentage composition.
Metals today General
Metals are often recognised because they have special physical properties which distinguish them from other materials. Some of these properties are :
1. Metals tend to be more dense than non-metals (Reminder : the density of a substance is the mass of a given volume)
density = mass volume
2. Metals tend to be good conductors of heat 3. Metals tend to be good conductors of electricity
4. Most metals can be beaten into different shapes – they are said to be malleable 5. Many metals are very strong (that is why they are used in bridges and buildings) When choosing a metal for a particular task it is important to take the specific
properties of the metal into account. It is also important to consider the availability
and cost of the metal. The following list shows how the specific properties of a metal can relate to its use.
property use
Melting point All metals are solids at room temperature except mercury which has a very low melting point
Mercury is used in thermometers Density The densities of metals vary
greatly e.g. potassium 0.86g/cm3
to platinum 21.5g/cm3
Aluminium has a density of 2.70g/ cm3
Aluminium is used in aircraft manufacture
Conduction of heat All metals are good conductors of heat
Aluminium, copper and stainless steel are all used for making cooking pots Conduction of
electricity
All metals are excellent conductors of electricity
Copper is used extensively for electrical wiring
Strength Some metals are strong under heavy loads
Steel is used to makes girders and reinforce concrete
Malleability Metals can be bent and flattened into many different shapes
Many everyday objects are made from metals pressed into flat sheets
We live in a ‘throw away’ society. When we have finished drinking our coke or eating our chips we tend to throw the cans and paper into the bin. Some of this waste can be valuable. A lot of metal objects we throw away can be collected and used again. This is called recycling.
Reaction of metals and oxygen
When metals are heated in oxygen, they usually combine to form a metal oxide. A metal oxide is a compound of a metal and oxygen.
metal + oxygen metal oxide
magnesium + oxygen magnesium oxide
2Mg + O2 2MgO
Reaction of metals and water
Metals which lie above hydrogen in the electrochemical series react with water, releasing hydrogen gas and forming the metal hydroxide.
metal + water metal hydroxide + hydrogen
calcium + water calcium hydroxide + hydrogen
Ca + 2H2O Ca(OH)2 + H2
The further up the electrochemical series the more vigorous is the reaction with water.
Reaction of metals with dilute acids
When a metal (which lies above hydrogen in the electrochemical series) is reacted with a dilute acid, hydrogen gas is released and a salt is formed.
metal + acid salt + hydrogen
zinc + hydrochloric acid zinc (II) chloride + hydrogen Zn + 2HCl ZnCl2 + H2
Remember that the salt formed depends on the acid used.
The results of experiments also allow an order of reactivity of metals to be drawn up.
Order of reactivity of metals Potassium
Sodium react with water
Calcium Magnesium
Aluminium react with acid
Zinc
Iron react with oxygen
Tin Lead Copper Mercury Silver Gold
Metals And Their Ores General / Credit
The metals we use in the world today are obtained from rocks in the Earth’s crust. Most of the metals are in these rocks as compounds, usually metal oxides, metal
sulphides or metal carbonates.
Only a small number of metals, such as gold, silver and platinum are found in rock as
uncombined metals. These are the most unreactive metals. These metals were among the first elements to be discovered because they did not have to be extracted from their ores in some way.
Ores are naturally occurring compounds of metals found in rocks. Getting the metal from its ore is not usually a simple process. It normally requires several stages.
1. The ore is crushed into smaller pieces so that the metal compound can be separated from all the other compounds in the rock.
2. If the metal compound is not an oxide, it is usually roasted in air to change it into an oxide.
3. The metal is obtained from the metal oxide. The method depends on the reactivity of the metal.
Obtaining Metals From Their Ores General / Credit
Some of the most unreactive metals are occasionally found as compounds, e.g. silver is sometimes found as its oxide, silver (I) oxide. These metals can be relatively easily obtained from their compounds by heating alone.
2AgO 2Ag + O2
Zinc, tin, iron and copper can all be extracted from their oxides in this way. When these metal oxides are heated with carbon (or carbon monoxide) they break down to give the metal and carbon dioxide.
metal oxide + carbon metal + carbon dioxide (or carbon monoxide)
iron (III) oxide + carbon iron + carbon dioxide 2FeO + C 2 Fe + CO2
The most reactive metals cannot be obtained by heating their metal oxides with carbon. E.g. aluminium oxide has to be broken down using electricity, by a process known as electrolysis.
Reactivity And Reduction Credit
We learned that metal oxides can be converted to metals in three ways. The conversion of a metal oxide to a metal is sometimes called reduction, and the method selected depends on the reactivity of the metal.
The three possible methods of reduction of metal oxides are :
1. Heating alone
2. Heating with carbon (or carbon monoxide) 3. Electrolysis
When metals combine with oxygen there is a release of energy and bonds are formed between metals and the oxygen. To convert the metal oxide back into the metal the same amount of energy must be supplied.
Energy is required to break the bonds between the metal and the oxygen, in the metal oxide. The stronger the bond, the greater the energy needed.
Silver is not very reactive. It will not hold onto oxygen strongly. Heat is all that is needed to release the oxygen.
Metals such as copper and iron are more reactive. Heat alone is not enough to make them release their oxygen. Carbon, or carbon monoxide, is required to help ‘pull’ the oxygen away.
The most reactive metals hold onto the oxygen so strongly that neither carbon, nor carbon monoxide is able to pull the oxygen away. A supply of electricity is required to obtain these metals.
Metal oxides are ionic compounds. To change the metal oxides into metals, the ionic bonds must be broken. As part of this change the metal ions gain electrons to form metal atoms.
Mg2+ + 2e Mg
Alloys General
Although there are only about 70 naturally occurring metal elements, there are thousands of different metallic substances in common use.
Often the properties of a pure metal make it unsuitable for the purposes we wish to use it. For example, it may rust easily or it is quite brittle.
The properties of metals can be altered by adding small amounts of other elements, to make new substances called alloys. Alloys are mixtures of metals or of metals with non-metals.
Examples of common alloys include :
Alloy
Main Metal
Other
elements
Uses
mild steel iron carbon girders, car bodies stainless steel iron chromium, nickel cutlery, sink tops 18 carat gold gold silver, copper jewellery
brass copper zinc ornaments, taps,
door handles
bronze copper tin gears, bearings,
statues, propellers
solder lead tin electrical
connections The most important alloys are those based on steel. The iron needed to make steel is extracted by heating its ores with carbons.
This is carried out industrially by using the blast furnace.
The blast furnace is a very tall steel tower, lined on the inside with fireproof bricks.
Iron ore is loaded from the top along with coke (almost pure carbon) and limestone.
Hot air is blown from the base of the furnace.
The coke initially reacts with the oxygen from the air, forming carbon dioxide. This reaction heats the furnace.
carbon + oxygen carbon dioxide
C + O2 CO2
As the carbon dioxide moves through more coke it reacts forming carbon monoxide.
carbon dioxide + carbon carbon monoxide
CO2 + C 2CO
The carbon monoxide reduces the iron (III) oxide in the ore to iron.
iron (III) oxide + carbon monoxide iron + carbon dioxide
Molten iron and slag (limestone combined with impurities from the ore) are collected at the base of the furnace.
The molten iron is taken to an oxygen furnace. There oxygen is passed through the molten iron and reacts with the unreacted carbon to give carbon dioxide. Other elements are then added to make different steels.
Empirical Formulae And Percentage Composition Credit
We know that the formula for a compound gives us information about the ratio of atoms found in the compound.
For example, Fe2O3 contains 2 atoms of iron and 3 atoms of oxygen. The ratio of atoms
(Fe : O) is 2:3.
The gram formula mass of a compound can easily be found if the relative atomic mass of each element is known.
For example the gram formula mass of Na2SO4 is (2 x 23) + (1 x 32) + (4 x 16) = 142g
This quantity of a substance is also called 1 mole of the substance. One mole of any compound has a mass equal to the gram formula mass.
The formula of a compound not only gives us the ratio of atoms found in the compound but also the ratio of moles. By calculating the mole ration we can work out the simplest (or empirical) formula for a compound.
The following example shows how to calculate an empirical formula.
In an experiment a chemist found that a substance was made of 23g of sodium and 8g of oxygen. Find the simplest (or empirical) formula of the substance.
Step 1 Write down the symbols of the elements present Na O
Step 2 Write down the mass (or percentage) of each
element present 23 8
Step 3 Divide each by its RAM 23/23 8/16 =1 =0.5
Step 4 Divide each answer by the smallest 1/0.5 0.5/0.5 =2 =1
Therefore Empirical formula = Na2O
In the above example, the formula Na2O is the correct formula for sodium oxide. This
does not always happen when working out empirical formula.
Using the same method to work out the empirical formula for ethane (C2H6) you would get the answer CH3, because that is the simplest formula for the compound.
The only way to know if you have the correct formula is to know the formula mass. Example : The simplest formula of a compound was calculated to be CH3. The formula
mass of the compound is known to be 30. What is the correct formula for this compound?
The formula mass of CH3 is 12 + (3 x 1) = 15
Since the formula mass is 30, there must be twice as many atoms present. The correct formula must be C2H6.
Percentage Composition
Sometimes a chemist is not given the mass of each element in the compound, but the results are given as percentages. This is called percentage composition for the compound and the figures can be used in exactly the same way to calculate the empirical formula.
It is also important that you can calculate the percentage composition of a compound from its formula.
Example
What is the percentage composition of sodium carbonate?
Step 1 Write the correct formula for the compound Na2CO3
Step 2 Work out the formula mass (2 x 23) + 12 + (3 x 16) = 106
Step 3 Write down the mass of each element in the formula Na = 46 C = 12 O = 48
Step 4 Calculate the percentage of each element Na = 46/106 x 100 = 43.4% dividing each mass by the formula mass and C = 12/106 x 100 = 11.3%
