Reactivity of metals

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课程笔记

Metal Oxides and Redox

Metals react with oxygen in the air to produce metal oxides.
Oxidation is the addition of oxygen to a substance; reduction is the removal of oxygen from a substance.
These reactions are called redox reactions because oxidation and reduction occur simultaneously.
Example: copper reacts with oxygen to form black copper oxide: 2Cu + O₂ → 2CuO. Copper has been oxidised.
Example: zinc oxide reacts with carbon: ZnO + C → Zn + CO. Zinc oxide is reduced (loses oxygen) and carbon is oxidised (gains oxygen).

The Reactivity Series

The reactivity series arranges metals in order of their reactivity, from most reactive to least reactive.
Order of reactivity: potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.
Metal atoms form positive ions by losing electrons when they react.
The more reactive a metal, the more easily it loses electrons to form a positive ion.
Non-metals carbon and hydrogen are included in the series because they help predict how metals are extracted.

The reactivity series of metals including hydrogen and carbon

Reactions of Metals with Water

Metals above hydrogen in the reactivity series react with water.
General equation: metal + water → metal hydroxide + hydrogen.
Potassium reacts violently, sodium reacts quickly, lithium and calcium react less strongly.
Magnesium, zinc and iron react very slowly with cold water; copper does not react.
Example: calcium + water → calcium hydroxide + hydrogen: Ca + 2H₂O → Ca(OH)₂ + H₂.

Reactions of Metals with Dilute Acids

Metals above hydrogen in the reactivity series react with dilute acids.
General equation: metal + acid → salt + hydrogen.
The more reactive the metal, the more vigorous the reaction.
Potassium and sodium react violently; magnesium, zinc and iron react less strongly; copper does not react.
Example: magnesium + hydrochloric acid → magnesium chloride + hydrogen: Mg + 2HCl → MgCl₂ + H₂.

Displacement Reactions

A more reactive metal can displace a less reactive metal from its compound.
Displacement can occur between a metal and a metal oxide (by heating) or a metal and an aqueous solution of a metal salt.
Example: zinc + copper oxide → zinc oxide + copper: Zn + CuO → ZnO + Cu. Zinc is the reducing agent.
Example: magnesium + copper sulfate → magnesium sulfate + copper: Mg + CuSO₄ → MgSO₄ + Cu. The blue colour of copper sulfate fades.
The thermite reaction: iron(III) oxide + aluminium → iron + aluminium oxide: Fe₂O₃ + 2Al → 2Fe + Al₂O₃. Aluminium is more reactive than iron.

Displacement of copper by magnesium

Extraction of Metals and Reduction

Most metals are found in ores; a metal ore is a rock containing enough metal to make extraction worthwhile.
Unreactive metals like gold and platinum are found as native metals (uncombined elements).
Metals above carbon in the reactivity series are extracted by electrolysis (e.g., aluminium).
Metals below carbon can be extracted by heating with carbon (reduction), e.g., zinc, iron, copper.
Extraction of a metal from its oxide is a reduction process because oxygen is removed.
Example: zinc oxide + carbon → zinc + carbon dioxide: ZnO + C → Zn + CO.

Oxidation and Reduction in Terms of Electrons

Oxidation is the loss of electrons; reduction is the gain of electrons (OIL RIG).
In a redox reaction, one species is oxidised and another is reduced.
Example: iron + copper sulfate → iron(II) sulfate + copper: Fe + CuSO₄ → FeSO₄ + Cu.
Ionic equation: Fe + Cu²⁺ → Fe²⁺ + Cu. Sulfate ions are spectator ions.
Half equations: Fe → Fe²⁺ + 2e⁻ (oxidation); Cu²⁺ + 2e⁻ → Cu (reduction).

Redox reaction between Fe and Cu²⁺ shown by electron transfer

Electrolysis and Redox

During electrolysis, oxidation and reduction occur at the electrodes.
At the cathode (negative electrode), positive ions gain electrons (reduction).
At the anode (positive electrode), negative ions lose electrons (oxidation).
Example at cathode: Pb²⁺ + 2e⁻ → Pb (reduction).
Example at anode: 2Br⁻ → Br₂ + 2e⁻ (oxidation).

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练习题

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1.In terms of oxygen, what name is given to the loss of oxygen from a substance?
Easy
- AOxidation
- BReduction
- CNeutralisation
- DDisplacement
2.In terms of oxygen, what name is given to the gain of oxygen by a substance?
Easy
- AReduction
- BOxidation
- CElectrolysis
- DDisplacement
3.The reaction of nickel(II) oxide with carbon is a redox reaction. Which statement explains whether carbon is oxidised or reduced?
Medium
- ACarbon is oxidised because it gains oxygen
- BCarbon is reduced because it gains oxygen
- CCarbon is oxidised because it loses oxygen
- DCarbon is reduced because it loses oxygen
4.Which two non-metals commonly appear in the reactivity series? (select all that apply)
Medium
- ACarbon
- BHydrogen
- COxygen
- DNitrogen
- EChlorine
5.Place these metals in order from most reactive to least reactive.
Medium
- gold
- iron
- lithium
- magnesium
6.Sodium is above copper in the reactivity series. What type of reaction is: 2Na + CuO → Cu + Na₂O?
Medium
- AReduction
- BOxidation
- CNeutralisation
- DDisplacement
7.Which metals can give a coloured flame when they react with cold water?
Medium
- AAluminium and zinc
- BCopper and zinc
- CPotassium and sodium
- DMagnesium and iron
8.Which statement about displacement reactions is correct?
Easy
- AA less reactive metal displaces a more reactive metal from its compounds
- BA more reactive metal displaces a less reactive metal from its compounds
- CDisplacement reactions require heating
- DDisplacement reactions are easier to see when they are in solution

闪卡

In terms of oxygen, what is oxidation?

Oxidation is the gain of oxygen by an element or compound.

In terms of oxygen, what is reduction?

Reduction is the loss of oxygen from an element or compound.

Why is the reaction 2Cu + O₂ → 2CuO an oxidation reaction?

Copper gains oxygen to form copper oxide, so copper has been oxidised.

In the reaction ZnO + C → Zn + CO, which substance is reduced and why?

Zinc oxide is reduced because it loses oxygen.

In the reaction ZnO + C → Zn + CO, which substance is oxidised and why?

Carbon is oxidised because it gains oxygen to form carbon monoxide.

What is a redox reaction?

A reaction in which oxidation and reduction happen at the same time.

How are metals arranged in the reactivity series?

Metals are placed in decreasing order of reactivity, from most reactive at the top to least reactive at the bottom.

Why do metal atoms form positive ions when they react?

Metal atoms lose electrons to form positive ions; the more easily a metal loses electrons, the more reactive it is.

Which two non-metals are commonly included in the reactivity series?

Carbon and hydrogen.

Why is carbon included in the reactivity series?

Carbon is a cheap reducing agent that can remove oxygen from metal oxide ores, so its position shows whether a metal oxide can be reduced by carbon.

Why is hydrogen included in the reactivity series?

Whether a metal reacts with water or dilute acid depends on its position relative to hydrogen.

Give the general word equation for a metal reacting with cold water.

metal + water → metal hydroxide + hydrogen

Give the general word equation for a metal reacting with a dilute acid.

metal + acid → metal salt + hydrogen

Which metals react with cold water to form a metal hydroxide and hydrogen?

Metals above hydrogen in the reactivity series, such as potassium, sodium, lithium and calcium.

Which metals will NOT react with dilute acids?

Metals below hydrogen in the reactivity series, such as copper.

Describe the reaction of potassium and sodium with water.

Potassium reacts violently and sodium reacts quickly, both producing a metal hydroxide and hydrogen.

Describe the reaction of magnesium, zinc and iron with cold water.

Their reactions with cold water are very slow; iron shows slow rusting.

Why does aluminium not react with water even though it is high in the reactivity series?

It has a protective oxide layer that prevents reaction with water and makes its reaction with dilute acids quite slow.

What is a displacement reaction?

A reaction in which a more reactive metal displaces a less reactive metal from its compound.

What is the rule for whether a displacement reaction occurs?

It occurs when the solid metal is more reactive than the metal in the compound.

In the reaction Zn + CuO → ZnO + Cu, which metal is the reducing agent?

Zinc, because it is more reactive than copper and removes oxygen from copper(II) oxide.

What is the thermite reaction and what is it used for?

Iron(III) oxide reacts with aluminium to form iron and aluminium oxide; it is highly exothermic and the molten iron is used for welding rail tracks.

Write the equation for the thermite reaction.

Fe₂O₃ + 2Al → 2Fe + Al₂O₃

In Mg + CuSO₄ → MgSO₄ + Cu, what colour change is seen?

The blue colour of the copper sulfate solution fades as colourless magnesium sulfate forms, and copper coats the magnesium.

What is a metal ore?

A rock that contains enough of a metal to make it worthwhile extracting.

What is a native metal? Give examples.

A metal found uncombined in the Earth's crust because it is unreactive; examples include gold and platinum.

Why are unreactive metals such as gold found as the metal itself?

They do not easily react with other substances because of their chemical stability, so they need no chemical extraction.

Which metals are extracted by electrolysis and why?

Metals above carbon in the reactivity series, such as potassium, sodium, calcium, magnesium and aluminium, because they are too reactive to be reduced by carbon.

Which metals are extracted by heating with carbon and why?

Metals below carbon, such as zinc, iron and copper, because carbon is a cheap reducing agent that removes the oxygen from their oxides.

Give the general word equation for extracting a metal oxide with carbon.

metal oxide + carbon → metal + carbon dioxide

Why is electrolysis an expensive extraction method?

It requires large amounts of electricity.

In terms of electrons, what is oxidation?

Oxidation is the loss of electrons.

In terms of electrons, what is reduction?

Reduction is the gain of electrons.

What does the mnemonic OIL RIG stand for?

Oxidation Is Loss, Reduction Is Gain (of electrons).

Write the half equation for iron being oxidised in Fe + CuSO₄ → FeSO₄ + Cu.

Fe → Fe²⁺ + 2e⁻

Write the half equation for copper ions being reduced in Fe + CuSO₄ → FeSO₄ + Cu.

Cu²⁺ + 2e⁻ → Cu

In Fe + CuSO₄ → FeSO₄ + Cu, which species is oxidised and which is reduced?

Iron is oxidised (loses electrons) and copper ions are reduced (gain electrons).

What are spectator ions?

Ions that appear unchanged on both sides of an ionic equation and take no part in the reaction, so they can be removed.

In electrolysis, where does reduction occur?

At the cathode (negative electrode), where positive ions gain electrons.

In electrolysis, where does oxidation occur?

At the anode (positive electrode), where negative ions lose electrons.

Give the half equation for reduction at the cathode during electrolysis of lead compounds.

Pb²⁺ + 2e⁻ → Pb

Give the half equation for oxidation at the anode during electrolysis of bromide compounds.

2Br⁻ → Br₂ + 2e⁻

In V³⁺ + Fe²⁺ → V⁴⁺ + Fe, which change is oxidation?

V³⁺ to V⁴⁺ is oxidation because vanadium loses an electron.

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