© boardworks ltd 2005 1 of 69 ks4 chemistry chemical reactions

© Boardworks Ltd 20051 of 69

KS4 Chemistry

Chemical Reactions


Chemical Reactions

Displacement and precipitation

Neutralization

Introducing chemical reactions

Redox

Thermal decomposition

Summary activities

Contents


What is a chemical reaction?

What is a chemical reaction?

A chemical reaction is a change that takes place when one or more substances (called reactants) form one or more new substances (called products).

reactants productschemical reaction

There are many different types of chemical reactions.


Types of chemical reaction

How many types of chemical reaction can you name?


oxidation &reduction

neutralization

precipitation

reversible

displacement:metals

exothermic &endothermic

thermaldecomposition

displacement:non-metals

Types of chemical reaction

chemical reaction


Exothermic and endothermic reactions

What are exothermic and endothermic reactions?

exothermic reactions give out energy – they get hot

endothermic reactions take in energy – they get cold

ex = out (as in ‘exit’)

en = in (as in ‘entrance’)

Most chemical reactions are exothermic.

thermic = relating to heat


Burning wood on a fire

Burning petrol in a car

Burning gas on a gas hob

Reacting an acid and alkali together

Burning magnesium

Rotting compost

Examples of exothermic reactions

Many exothermic reactions occur in the lab and in everyday life. Can you think of six exothermic reactions?

Exothermic reactions


Irreversible reactions

Most chemical reactions are considered to be irreversible because the products cannot easily be changed back into reactants.

For example, once magnesium has reacted with hydrochloric acid, it is difficult to get the magnesium back.

In equations for irreversible reactions, reactants and products are joined by a ‘one-way’ arrow.

hydrochloricacid

magnesiumchloride

magnesium + hydrogen+


Reversible reactions

Although most chemical reactions are difficult to reverse, there are some reactions that are fully reversible.

One of the best known reversible reactions occurs when copper sulfate crystals are heated.

anhydrous copper sulfate

hydratedcopper sulfate + water

CuSO4.5H2O CuSO4 5H2O+

In equations for reversible reactions, reactants and products are joined by a ‘two-way’ arrow.


Equilibrium reactions

In some reversible reactions, the forward and backward reactions largely occur in the same conditions and at the same rate.

These reaction are said to be in equilibrium – there is no overall change in the amount of products and reactants.

One of the most important equilibrium reactions occurs in the production of ammonia in the Haber process:

hydrogennitrogen + ammonia

N2 (g) 3H2 (g) 2NH3 (g)+

No matter how long the reaction is left, there will always bea mixture of nitrogen, hydrogen and ammonia.


Chemical Reactions


Neutralization


Redox


Summary activities

Contents Contents



Thermal decomposition is a reaction in which a compound is broken down by heat into two or more simpler substances.

Generally, the more reactive a metal, the harder it is to decompose its compounds by heating.

potassium carbonate: is not thermallydecomposed

calcium carbonate: decomposes onstrong heating

silver carbonate: decomposes ongentle heating

increase in reactivity of metal

For example:


easy

hard

medium

easy

medium

How easy will these metal compounds be to decompose: easy, medium or difficult?

Thermal decomposition – easy or hard?

mercury oxide

sodium oxide

iron oxide

silver oxide

zinc oxide

Compound Decomposition

incr

ease

in

rea

ctiv

ity

potassiumsodiumcalcium

magnesiumaluminium

zinciron

coppermercury

silvergold


Thermal decomposition of carbonates

When metal carbonates are heated, they decompose to produce metal oxides and carbon dioxide.

This reaction is performed industrially to make calcium oxide (quicklime) from calcium carbonate (limestone):

Quicklime is used to make concrete and calcium hydroxide (slaked lime).

CaOCaCO3 + CO2

calciumoxide

calciumcarbonate + carbon

dioxide

heat


Most metal oxides are thermally stable – they do not decompose when heated.

Oxides of the least reactive metals can be thermally decomposed more easily. For example, mercury oxide decomposes when heated strongly:

mercury condenses at the top of the test tube, where it is cooler

Thermal decomposition of metal oxides

2Hg2HgO + O2

oxygen gas escapes

mercurymercury

oxide + oxygenheat


Chemical Reactions


Neutralization


Redox


Summary activities

Contents


The reactivity series

incr

ease

in

rea

ctiv

ity

potassiumsodiumcalcium

magnesiumaluminium

zinciron

coppermercury

silvergold

The reactivity series is a list of metals in order of their reactivity.

The reactivity series can be used to make predictions about the reactivity of metals – for example, how a metal will react with oxygen, water and acids.


Displacement reactions: metals

A metal displacement reaction occurs when a metal is added to a compound of a less reactive metal.

The less reactive metal is displaced from the compound and becomes elemental metal. The more reactive metal forms a new compound.

A metal will always displace another metal that is lower in the reactivity series.

+less

reactive metal

compound of more reactive

metal

compound of less

reactive metal

more reactive

metal+


Displacement reactions – examples

Will silver react with magnesium chloride?

Magnesium is more reactive than copper, so it displaces copper from its compound.

Silver is less reactive than magnesium, so it does not displace magnesium from its compound.

Will magnesium react with copper chloride?

+magnesiumcopper

chloride +magnesium

chloridecopper

+silvermagnesium

chloride no reaction


The Thermit process

Aluminium is more reactive than iron and displaces it from the oxide.

The Thermit process is a displacement reaction between aluminium and iron (III) oxide.

aluminiumoxide

+ ironiron oxidealuminium +

Fe2O3 Al2O3Al + Fe+

magnesiumfuse

aluminium powder (thermite)

iron oxide


The reaction between aluminium and iron oxide is so exothermic that the displaced iron melts.

The reaction is used to weld iron and steel together; for example, railway tracks.

The Thermit process


Is there a displacement reaction?


Displacement reactions: halogens

A halogen displacement reaction occurs when a halogen is added to a metal halide containing a less reactive halogen.

F2 (aq) 2NaCl (aq) 2NaF (aq) Cl2 (aq)++

The less reactive halogen is displaced from the compound and the more reactive halogen bonds with the metal to form a new metal halide.

I

Br

Cl

F

decrease in reactivity

For example:

sodiumchloride

sodiumfluoride

chlorinefluorine + +


Displacement reactions of halogens


Is there a displacement reaction?


What are the symbols for these physical states?

Precipitation reactions

When two aqueous solutions are mixed, they may react to form a product that is insoluble in water. The solid is called a precipitate and the reaction is called a precipitation reaction.

To predict whether a precipitation reaction will occur, information on the solubility of the products is needed.

solid

liquid

gas

aqueous(dissolved in water)

(g)

(l)

(g)

(aq)


Precipitation reactions: sulfur

The precipitation reaction between solutions of sodium thiosulfate and hydrochloric acid is often used to measure rates of reaction.

hydrochloricacid

sodiumchloride

sulfursodium

thiosulfate + + watersulfur

dioxide ++

Na2S2O3

(aq)2HCl(aq)

2NaCl(aq)

S(s)++

SO2

(g)H2O(l)

+ +

Both reactants are colourless.

Sulfur is insoluble and precipitates, turning the solution cloudy.


Precipitation reactions: copper hydroxide

Many metal hydroxides are insoluble and can be formed by precipitation reactions. For example:

ammoniumhydroxide

copper (II)hydroxide

copper (II)sulfate + +

ammoniumsulfate

CuSO4

(aq)2NH4OH

(aq)Cu(OH)2

(s)++

(NH4)2SO4

(aq)

Copper (II) sulfate solution is blue.

Copper (II) hydroxide is insoluble and forms a blue solid at the bottom.


Precipitation reactions: iron hydroxide

Iron (III) hydroxide is another insoluble metal hydroxide that can be formed by a precipitation reaction.

sodiumhydroxide

iron (III)hydroxide

iron (III)chloride + +

sodiumchloride

FeCl3 (aq) 3NaOH (aq) Fe(OH)3 (s) ++ 3NaCl (aq)

Iron (III) chloride solution is yellow.

Iron (III) hydroxide is insoluble and forms a deep brown solid at the bottom.


Precipitation and solubility

To help work out whether a precipitate will form in a reaction, there are some general rules about solubility.

All compounds of sodium, potassium and ammonium.

All nitrates.

Most chlorides, except silver and lead chlorides.

Most sulfates, except lead, barium and calcium sulfates.

Most carbonates and hydroxides, except those of sodium, potassium and ammonium.

Soluble

Insoluble


There are three steps to working out whether a precipitate will be formed in a reaction:

1. Write down the names of the reactants.

3. Are the products soluble or insoluble?

Predicting precipitation reactions

sodium chloride & lead nitrate

Lead chloride is insoluble and will form a precipitate.

2. Swap over the non-metal.

sodium nitrate & lead chloride

Example

leadnitrate

sodiumnitrate

sodiumchloride + +

leadchloride

2NaCl (aq) Pb(NO3)2 (aq) 2NaNO3 (aq) ++ PbCl2 (s)


Will a precipitate be formed when sodium nitrate and magnesium sulfate react?

Will there be a precipitation reaction? (1)



sodium nitrate &magnesium sulfate

Both products are soluble so no precipitate will form.


sodium sulfate & magnesium nitrate

magnesiumsulfate

sodiumsulfate

sodiumnitrate + +

magnesiumnitrate

2NaNO3 (aq) MgSO4 (aq) Na2SO3 (aq) ++ Mg(NO3)2 (aq)


Will there be a precipitation reaction? (2)

Will a precipitate be formed when sodium sulfate and barium nitrate react?



sodium sulfate &barium nitrate

Barium sulfate is insoluble and will form a precipitate.


sodium nitrate & barium sulfate

bariumnitrate

sodiumnitrate

sodiumsulfate + +

bariumsulfate

Na2SO4 (aq) Ba(NO3)2 (aq) 2NaNO3 (aq) ++ BaSO4 (s)


Precipitation: true or false?


Chemical Reactions


Neutralization


Redox


Summary activities

Contents


Acids

What are acids? They are substances that:

Have a pH below 7.

Turn litmus red.

Turn universal indicator yellow, orange or red.

Form solutions containing H+ ions. The more H+ ions in the solution, the stronger the acid.

weakacid

strongacid

1 2 3 4 5 6 7 8 9 10 11 12 13 14

neutral

Universal indicator pH scale


HNO3

Common acids

What are some common acids?

sulfuric acid

hydrochloric acid

nitric acid

ethanoic acid (vinegar)

H2SO4

HCl

CH3COOH

strong

strong

strong

weak

Acid Formula Strength

1 2 3 4 5 6 7 8 9 10 11 12 13 14


Bases

What are bases? They are substances that:

Have a pH above 7.

Turn litmus blue.

Turn universal indicator dark green, blue or purple.

Are capable of neutralizing acids.

weak base

strongbase

1 2 3 4 5 6 7 8 9 10 11 12 13 14

neutral

Universal indicator pH scale


basespH > 7

More about bases

Bases are usually oxides, hydroxides or carbonates of metals. Ammonia is a base that doesn’t contain a metal.

Some bases are soluble in water – they are called alkalis.

Most carbonates and hydroxides are insoluble in water, except those of sodium, potassium and ammonium.

alkalissolublebases

All alkaline solutions contain OH– ions. The more OH– ions in the solution, the stronger the alkali.


Common bases

Ca(OH)2

What are some common bases?

sodium hydroxide *

potassium hydroxide *

calcium hydroxide *

ammonia *

NaOH

KOH

NH3

strong

strong

strong

weak

1 2 3 4 5 6 7 8 9 10 11 12 13 14

calcium carbonate CaCO3 weak

Base Formula Strength

* = the base is also an alkali


Neutralization reactions

A neutralization reaction occurs when an acid reacts with a base or alkali to produce a neutral solution of salt and water.

alkali saltacid + water+


The first part of the salt comes from the first part of the base, for example:

The second part of the salt comes from the acid:

Naming salts

The salt formed in a neutralization reaction takes its name from both the base and the acid.

magnesium oxide magnesium salt ammonium hydroxide ammonium salt

sulfuric acidsulfate

hydrochloric acid chloride

nitric acid nitrate


sulfuric acid potassium sulfatepotassium hydroxide

calcium chloride

magnesium nitrate

copper sulfate

aluminium nitrate

What is the name of the salt?

Base Acid Salt

calcium hydroxide

magnesium oxide

copper oxide

aluminium hydroxide

hydrochloric acid

nitric acid

nitric acid

sulfuric acid

What are the names of the salts formed from these bases and acids?


Neutralization reactions: hydroxides

When a hydroxide is mixed with an acid, OH– ions react with H+ ions from the acid to form water:

OH– H2OH+ +

hydrochloricacid

potassiumchloride

potassiumhydroxide + + water

KOH (aq) HCl (aq) KCl (aq) ++ H2O (aq)

For example:

Ca(OH)2 (aq) H2SO4 (aq) CaSO4(aq) ++ 2H2O (aq)

sulfuricacid

calciumsulfate

calciumhydroxide + + water


Neutralization reactions: oxides

When an oxide is mixed with an acid, O2– ions react with H+ ions from the acid to form water:

O2– H2O2H+ +

hydrochloricacid

calciumchloride

calciumoxide + + water

CaO (aq) 2HCl (aq) CaCl2 (aq) ++ H2O (aq)

For example:

sulfuricacid

coppersulfate

copperoxide + + water

CuO (aq) H2SO4 (aq) CuSO4 (aq) ++ H2O (aq)


Neutralization reactions: carbonates

When a carbonate is mixed with an acid, CO32– ions react

with H+ ions from the acid to form water and carbon dioxide:

CO32– H2O2H+ + + CO2

nitricacid

calciumnitrate

calciumcarbonate + + water +

carbondioxide

CaCO3 (aq)

2HNO3

(aq)

Ca(NO3)2

(aq)++ H2O

(aq)+ CO2

(g)

For example:


Complete the neutralization reaction


Obtaining insoluble salts

How can insoluble salts be obtained?

2. The mixture of products is filtered, trapping the salt.

1. The acid and alkali are mixed, and the salt forms by precipitation.

3. The salt is rinsed in distilled water, then left to dry.

For example, barium sulfate can be obtained by mixing barium chloride with sulfuric acid.

sulfuricacid

bariumsulfate

bariumhydroxide + + water

BaOH2 (aq) H2SO4 (aq) BaSO4 (s) ++ 2H2O (aq)


Obtaining soluble salts from bases

How can soluble salts be obtained following a reaction between an acid and a base (insoluble)?

2. More copper oxide is added until no more will dissolve. This means that all the acid has been used up.

1. Copper oxide is added to sulfuric acid. When it is heated, the copper oxide dissolves, forming a blue solution.

3. The mixture is filtered to remove the excess copper oxide.

For example, obtaining copper sulfate from copper oxide and sulfuric acid:

4. The filtrate is heated to evaporate some of the water. When it cools, copper sulfate crystals will form.


Obtaining soluble salts from alkalis (1)

How can soluble salts be obtained following a reaction between an acid and an alkali (soluble)?

There is no obvious sign when the reaction is complete, so an indicator must be used to show when the solution is neutral.

This process is called titration.

hydrochloricacid

sodiumchloride

sodiumhydroxide + + water

NaOH (aq) HCl (aq) NaCl (aq) ++ H2O (aq)

For example, the reaction between sodium hydroxide and hydrochloric acid produces sodium chloride, which is soluble.


Obtaining soluble salts from alkalis (2)

3. When all the alkali has reacted with the acid, the indicator turns colourless. The amount of acid used is noted.

4. The experiment is repeated, but without adding the indicator, as this makes the salt impure.

To run the titration:

5. The salt solution is heated to evaporate the water. Crystals of sodium chloride will remain.

1. 25 cm3 of sodium hydroxide is added to a flask. Two drops of the indicator phenolphthalein are added. This turns pink.

2. Hydrochloric acid is added to the flask, a little at a time, from a burette.


Matching reactants and salts


Neutralization: true or false?


Chemical Reactions


Neutralization


Redox


Summary activities

Contents


Oxidation = adding oxygen to a substance.

What is redox?

What does redox mean?

reduction and oxidation

So far, these terms have been used to describe the gain and loss of oxygen.

Reduction = removing oxygen from a substance.

E.g. production of iron oxide during rusting

E.g. extracting iron from ironoxide in a blast furnace.


Oxidation and ions

When a metal burns, it is oxidized to form a metal oxide. What happens to the metal atoms during the reaction?

For example, when magnesium burns to form magnesium oxide, each magnesium atom loses 2 electrons and becomes a magnesium ion with a +2 charge.

Metal atoms lose electrons to form positive ions. The oxygen atoms accept these electrons and form negative ions.

O2-Mg Mg2+

2 electrons

O


Oxidation and electron loss

What has happened to magnesium when it reacts with oxygen?

It has been oxidized.

It has lost electrons by changing from Mg to Mg2+.

Magnesium can lose electrons to substances other than oxygen, e.g. when it reacts with chlorine or sulfur. These reactions both involve Mg Mg2+, so they are also oxidation.

Oxidation is the loss of electrons.

O2-Mg Mg2+

2 electrons

O


Reduction and ions

When iron is extracted from iron oxide (iron ore), the oxygen is removed and the iron is said to be reduced.

When the oxygen is removed, 3 electrons are transferred back to each iron ion, which become atoms.

O

O

O

Fe

Fe

2 electrons from each ion

O2-

O2-

O2-

Fe3+

Fe3+


Reduction and electron gain

What has happened to iron when oxygen is removed?

It has been reduced.

It has gained electrons by changing from Fe3+ to Fe.

Reduction is the gain of electrons.

O

O

O

Fe

Fe

2 electrons from each ion

O2-

O2-

O2-

Fe3+

Fe3+


Electrons and OILRIG

An easy way to remember what happens to electrons during oxidation and reduction is to think…

O

I

L

R

I

G

xidation

s

oss

eduction

s

ain

…OILRIG

…of electrons

…of electrons


The overall reaction is reduction and oxidation = redox.

magnesium loses electrons:

Redox reactions

When a substance is oxidized, it loses electrons. Another substance must gain these electrons and become reduced.

For example, when magnesium burns:

Oxidation and reduction always take place together.

oxygen gains electrons:

Mg Mg2+ = oxidation

O O2– = reduction


oxidized

reduced

Oxidized or reduced?

For each reaction, decide which product is oxidized and which product is reduced.

reduced

reducedoxidized

oxidized

calcium + oxygen calcium oxide

zinc oxide hydrogen+ + zinc water

aluminium iron oxide +iron aluminium oxide+


Oxidized or reduced?


Chemical Reactions


Neutralization


Redox


Summary activities

Contents


Glossary (part 1)

displacement – A type of reaction in which a metal or halogen replaces a less reactive metal or halogen in a compound.

equilibrium – A type of reaction in which products are broken down at the same rate as reactants are combining.

endothermic – A type of reaction that takes in energy.

exothermic – A type of reaction that gives out energy.

neutralization – A type of reaction in which an acid and base react to form a salt and water.

oxidation – A type of reaction involving the loss of electrons.


Glossary (part 2)

precipitation – A type of reaction in which two aqueous solutions react to form an insoluble product.

reaction – A change that takes place when one or more substances form one or more new substances.

redox – A type of reaction in which oxidation and reduction take place together.

reduction – A type of reaction involving the gain of electrons.

decomposition – A type of reaction in which a substance is broken down into two or more simpler substances.


Anagrams


Identify the reactions


Multiple-choice quiz

© boardworks ltd 2005 1 of 69 ks4 chemistry chemical reactions

Documents

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