2. Context and related work 9
3.6. Generation of the summary
3.6.2. Architecture based on extraction
ChAPter 7: Chemical reactions of metals 137 1 Account for the following uses of gold in terms of its reactivity:
a a wedding ring
b a component of electrical circuits.
2 Explain why more expensive copper pipes are used for carrying hot water rather than cheaper iron pipes.
3 Propose why huge aluminium bars are attached to off-shore gas platforms (constructed of steel).
4 Assess whether copper would be a suitable sacrificial anode for a steel ship.
Zinc is used extensively in galvanised iron production. The galvanised iron is produced by dipping iron into molten zinc. The zinc serves two purposes. First, the zinc forms a protective coating for the iron, excluding oxygen and therefore protecting the iron from rusting. The zinc reacts with the air to form an impervious layer that protects it from corrosion. The second way in which the zinc protects the iron from corrosion is that, in the event that the zinc coating of the iron is damaged, the zinc acts as a sacrificial anode and corrodes in preference to the iron.
The reactivity of zinc also makes it suitable for use in batteries such as dry cells (torch batteries) and button cells (in watches). In these cells the zinc is oxidised and the electrons it loses travel through an external circuit, producing an electric current.
Tin and chromium are used widely to coat a base metal, usually steel, in many applications. This use is due to the low reactivity of these metals together with their shiny appearance. ‘Tin’ cans commonly used as packaging for foods such as soups, baked beans and canned fruit are actually steel cans coated in tin. The bumper bars of many older cars and so-called chrome trimmings are usually steel coated with chromium metal. Both tin and chromium form an impervious oxide layer that protects the underlying metal from corrosion.
Copper, aluminium and titanium are also widely used because of their low reactivity and/or their resistance to corrosion. Copper is used extensively in the plumbing industry because of its ability to resist corrosion. Unlike the cheaper alternative, iron, copper does not react with hot water and is ideal for hot water tanks and pipes. Aluminium, because of its ability to form a protective oxide layer and its light weight, is extensively used in the building industry. Titanium is used in holding tanks and pipes in desalination plants, which convert sea water into fresh water, and factories producing or using acids. The ability of titanium to resist corrosion makes it an ideal metal in these corrosive environments.
* Key points
138 moDUle 2: metals
• Metals vary in their reactivity with oxygen, water and dilute acids.
• Most metals react with oxygen in an oxidation–reduction process to form a metal oxide. This process is also known as corrosion.
• Reactive metals will burn in air.
• Gold, platinum and silver do not react with oxygen.
• Rusting involves the oxidation of iron by oxygen in the air under moist conditions.
• Some metals such as aluminium form an oxide layer that protects the metal beneath from further corrosion.
• Reactive metals react with water to form hydrogen gas and either an oxide or a hydroxide.
• Most metals react with dilute hydrochloric acid and dilute sulfuric acid to produce a salt and hydrogen gas.
• Gold, platinum, silver, mercury and copper do not react with dilute hydrochloric acid or dilute sulfuric acid.
• By comparing the reactivity of metals it is possible to rank them in an activity series.
• Ionisation energy is the amount of energy required to remove the most loosely bound electron from a gaseous atom.
• Metals on the left-hand side of the periodic table tend to have lower ionisation energies than metals in the middle of the periodic table.
• Metals with low ionisation energies are generally more reactive than those with higher ionisation energies.
• The uses of metals are often related to their reactivity as well as their physical properties.
• Oxidation–reduction (redox) reactions involve the transfer of electrons from one species to another.
• Oxidation and reduction occur simultaneously.
• Oxidation can be defined as the loss or donation of electrons.
• Reduction can be defined as the gain or acceptance of electrons.
• An oxidising agent causes the oxidation of another species and is itself reduced.
• A reducing agent causes the reduction of another species and is itself oxidised.
• An oxidation half-equation shows loss of electrons. A reduction half-equation shows gain of electrons.
* ApplicAtion And inVEStiGAtion
ChAPter 7: Chemical reactions of metals 139
• A summary of the reactions covered in this chapter is given in Table 7.5.
Table 7.5
Metal Reaction with oxygen Reaction with water Reaction with dilute acids K
na Ca mg
Burn readily to form
oxides react with cold water to form h2 and hydroxides react with hot water to form h2 and oxide Al
Zn Cr fe Cd Co ni
Burn to form oxides
if finely divided When heated strongly react with steam or superheated steam to form h2 and oxides
react with cold dilute acids to form h2 and salts
sn Pb Cu Ag
react slowly if heated in air or pure oxygen
no reaction
react with warm dilute acids to form h2 and salts
hg Pt Au
no reaction
no reaction
1 Account for the fact that the surface of aluminium appears dull rather than shiny.
2 For each of the following mixtures, predict whether a reaction will take place.
If so, write a balanced equation. If not, write ‘no reaction’.
a lead and water
b silver and dilute hydrochloric acid c powdered zinc and oxygen when heated d iron and dilute sulfuric acid
e copper and dilute hydrochloric acid
140 moDUle 2: metals
3 Given a piece of titanium metal, describe how you could determine its position in the activity series.
4 W, X, Y and Z are metals.
Y reacts with dilute hydrochloric acid and steam but not with hot or cold water.
W does not react with water, steam or dilute hydrochloric acid.
X reacts vigorously with cold water.
Z reacts with hot water.
a Arrange these metals in decreasing order of reactivity, that is, from most reactive to least reactive.
b Identify metals that could be W, X, Y and Z.
5 Gather information from appropriate secondary sources to identify the ionisation energies of the first 20 elements in the periodic table. Plot a graph of these as a function of atomic number and identify any trends.
6 Explain why most metals in the Earth’s crust are found combined with other elements.
7 Construct equations for the reactions that occur when aluminium reacts with:
a dilute hydrochloric acid b dilute sulfuric acid.
8 Define oxidation and reduction in terms of the following:
a oxygen exchange b electron exchange.
9 Identify the oxidising agent in each of the following reactions:
a Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) b 2H2(g) + O2(g) → 2H2O(l)
c Zn(s) + Cl2(g) → ZnCl2(s)
10 Rewrite each of the following as two half-equations and label each as oxidation or reduction.
a 2Zn(s) + O2(g) → 2ZnO(s)
b Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) c SnO2(s) + 2H2(g) → Sn(s) + 2H2O(l)
11 Explain why hot water tanks are made of copper, yet cold water tanks are made of steel (an alloy of iron).
12 Some hot water tanks are made of steel. Account for the practice of suspending a bar of magnesium metal in the water in these tanks.
➲ Investigation
ChAPter 7: Chemical reactions of metals 141 13 Although gold is commonly found and used as a free element, it also forms
compounds. Identify the names of some gold compounds.
14 Iron is galvanised by coating it with a thin layer of zinc. The zinc itself forms a protective layer by reaction with gases in the atmosphere.
a Identify the nature of the protective layer formed by the zinc and the chemical reactions involved in its formation.
b Identify the chemical reactions that occur during the rusting of iron and explain how the zinc interferes in this process if the zinc coating on the iron is damaged.
➲ Investigation
➲ Investigation
Discovery Elements in order Total
period of discovery number
known
Db Rf Sg Bh Hs Mt UunUuuUub Cs Rb Ti In
Figure 8.2 Antoine Lavoisier is considered by many to be the father of modern chemistry.
Despite his great contributions to chemistry, he was guillotined in 1794 during the French Revolution for his role as a tax collector.