Chemical Change - ReactionsObjectives: At the end of this section

you should be able to:-• Explain the difference between chemical and physical

change.• Use the periodic table to determine valency.• Explain the significance of the law of constant

composition for writing chemical reactions.*** (comeback to)

• Write and balance chemical formulae.• Calculate reacting masses for given reactions.• Explain energy changes in reactions.• Explain Avogadro’s law.• Work out reacting volumes of gases.

Chemical Change - ReactionsObjectives: At the end of this section

you should be able to:-• Distinguish between synthesis and decomposition

reactions• Discuss how energy released by chemical reactions is

essential for life• Describe how metals react with oxygen• Explain the reverse process in which metals are

separated from their oxides• Describe the reactions of non-metals with oxygen• Write balanced equations for reactions involving non-

metals.

Physical and chemical change

Nitrogen separated from air by fractional distillation

Lightning causes nitrogen to react with air to form nitrogen

Physical and chemical change

Nitrogen separated from air by fractional distillation

Lightning causes nitrogen to react with air to form nitrogen

N2 (g) N2 (l)

Gas liquid

FormulaeCH4

CO2

Elements on the LEFT of the periodic table are written FIRST. If elements are above/below each other then the LOWER one is written FIRST.

H

SO

H2SS SO2S

SC

Consider the following analysis of different samples of table salt, what do you notice?

Hint??

Sample no.

Sample mass

Mass of Na

Mass of Cl

1 117g 46g 71g

2 468g 184g 284g

3 59g 23g 36g

Consider the following analysis of different samples of table salt, what do you notice?

Sample no.

Sample mass

Mass of Na

Mass of Cl

1 117g 46g 71g

2 468g 184g 284g

3 59g 23g 36g

Hint?? Ratios??

When elements combine with each other to form compounds, they always combine in an exact ratio and mass with one another.

Consider the following analysis of different samples of table salt:

Sample no.

Sample mass

Mass of Na

Mass of Cl

1 117g 46g 71g

2 468g 184g 284g

3 59g 23g 36g

The Law states:

A certain chemical compound always contains the ……………………. chemically combined in the ………… …………………….. by ……………………..

NaCl CaCl2 KMnO4 K2Cr2O7

Ratios: …….……….. ………. …………….

These ratios ………………. change.

NaCl is always 1:1 and 23g:35,5g – irrespective of whether the NaCl comes from the UK, USA or SA.

The Law states:

A certain chemical compound always contains the same elements chemically combined in the same proportions by mass.

NaCl CaCl2 KMnO4 K2Cr2O7

Ratios: 1:1 1:2 1:1:4 2:2:7

These ratios never change.

NaCl is always 1:1 and 23g:35,5g – irrespective of whether the NaCl comes from the UK, USA or SA.

Molecules

O2

a3CH4

b CO2

c

H2S

CH4 + 2O2 ---> CO2 + 2H2O

Methane + oxygen --> water + carbon dioxide

Conservation of Mass, Atoms & Molecules• Show that mass, atoms are conserved but molecules are not

H2 + O2 H2O

REACTIONSHydrogen and oxygen react to produce water.

H2 + O2 H2O

Ar:

Mr:

Balanced Eqn:

H2 + O2 H2O

LHS:

RHS:

LHS = RHS

MASS is CONSERVED!!

This is true for ALL REACTIONS.

This is the reaction used to power the space shuttle.

REACTIONSHydrogen and oxygen react to produce water. (Synthesis reaction)

H2 + O2 H2O

Ar: H = 1, O = 16

Mr: H2 =2, O2 = 32

H2O = 16+2=18

Balanced Eqn:

2H2 + O2 2H2O

LHS: RHS:

2(2) + 32 = 36 2(2+16) = 36

LHS = RHS

MASS is CONSERVED!!

This is true for ALL REACTIONS.

This is the reaction used to power the space shuttle.

REACTIONS

1. Butane reacts with oxygen 2. Carbon dioxide and water are formed

Butane + Oxygen ---> Carbon dioxide + water

C4H10 + O2 ---> CO2 + H2OWhat mass of carbon dioxide is produced from each

butane molecule?

1 2

REACTIONS

1. Butane reacts with oxygen 2. Carbon dioxide and water are formed

Butane + Oxygen ---> Carbon dioxide + water

2C4H10 + 13O2 ---> 8CO2 + 10H2O

1 2

Reaction equation BALANCED!

Reacting Masses

1. Na + Cl2 --> NaCl

2. C + Cl2 --> CCl4

3. ZnS + O2 --> ZnO + SO2

4. FeS + HCl --> H2S + FeCl2

5. SO2 + H2S --> S + H2O

Calculate the mass of each underlined compound either produced or required. (Balance the reactions first)

Volume - Volume Calculations

H2 + N2 --> NH3

If 3.00 dm3 of nitrogen are reacted to produce ammonia, what volume of hydrogen will be required? (At STP)

Volume - Volume CalculationsH2 + N2 --> NH3

If 3.00 dm3 of nitrogen are reacted to produce ammonia, what volume of hydrogen will be required? (At STP)

1. 3H2 + N2 --> 2NH3

2. n(N2) = v/Mv = 3/22.4 = 0.134mol

3. N2 : H2 1:3 n(H2) = 3(N2)

4. n(H2) = 3(0.13) = 0.401mol

5. v(H2) = n(H2)Mv = 0.401(22.4) = 8.98dm3

Balancing Reactions1. 2Na + Cl2 --> 2NaCl

2NaCl: 2(23+35.5) = 2(58.5)=117g

1. C + 2Cl2 --> CCl4

CCl4: 12 + 4(35.5) = 154g

1. 2ZnS + 3O2 --> 2ZnO + 2SO2

SO2: 2(32 + 2(16)) = 128g

1. FeS + 2HCl --> H2S + FeCl2

FeS: 56 + 32 = 88g

1. SO2 + 2H2S --> 3S + 2H2O

H2S: 2(2 + 32) = 68g

Balancing More Reactions

1. Na + H2O --> NaOH + H2

2. H2 + O2 --> H2O

3. CaCO3 --> CaO + CO2

4. CaCl2 + Na2SO4 --> CaSO4 + NaCl

5. Al(NO3)3 + K2CO3 --> Al2(CO3)3 + KNO3

6. Na3PO4 + MgI2 --> Mg3(PO4)2 + NaI

Balancing More Reactions

1. 2Na + 2H2O --> 2NaOH + H2

2. 2H2 + O2 --> 2H2O

3. CaCO3 --> CaO + CO2

4. CaCl2 + Na2SO4 --> CaSO4 + 2NaCl

5. 2Al(NO3)3 + 3K2CO3 --> Al2(CO3)3 + 6KNO3

6. 2Na3PO4 + 3MgI2 --> Mg3(PO4)2 + 6NaI

BALANCING REACTIONS

1. Butane reacts with oxygen

1 2

Reactants

Substance O C H No.

oxygen 26 13

butane 0 8 20 2

TOTAL 26 8 20

2. Carbon dioxide and water are formed

Products

Substance O C H No.

Carbon dioxide 16 8 0 8

Water 10 0 20 10

TOTAL 26 8 20

butane