Representing reactions- IGCSE CHEMISTRY WORK SHEETS AND NOTES

Word equationsIf a reaction occurs between magnesium and oxygen, magnesium oxide is produced, here is the word equation for this reaction:

magnesium + oxygen --> magnesium oxide

Some other examples are:

hydrochloric acid + calcium carbonate --> calcium chloride+ carbon dioxide+ water sodium + water --> sodium hydroxide + hydrogen

hydrochloric acid + sodium hydroxide --> sodium chloride + water

Write word equations for the reactions in which the following compounds form from a halogen and another suitable element: hydrogen fluoride, hydrogen chloride, iron III chloride, iron III bromide, sodium chloride, copper chloride.

formulaeThe formula of an element or compound is simply the symbol of each element present and numbers to show how many atoms are present. Carbon dioxide has the formula CO2. This means that it has one carbon atom and two oxygen atoms in each molecule

Use valency to work out the formulae of the following compounds: sodium chloride, potassium bromide, magnesium oxide, calcium sulphide, aluminium nitride, calcium iodide, lithium oxide, aluminium chloride, aluminium sulphide, magnesium nitride.

Calculating relative formula massAdd up the relative atomic mass (found in periodic table) of each atom in the compound.

e.g. Al203 relative atomic masses of Al = 27, O = 16 (found in periodic table).

The formula shows 2 atoms of aluminium and 3 atoms of oxygen so:

formula mass of = (2*27) + (3*16) =54 + 48 = 102

Work out the relative formula masses of the following:

MgO, FeS, O2, H2O, CaBr2, Na2S, CaCO3, NaOH, HCl, (NH4)2SO4.
Relative atomic masses Mg=24, O=16, Fe=56, S=32, Ca=40, Br=80, C=12, Na=23, H=1, Cl=35.5.
Simple balanced equationsIt is possible to write balanced equations for reactions. For example substances such as hydrogen and magnesium combine with oxygen. One method to write them is:Write a word equation first.
Magnesium + oxygen --> magnesium oxide
Write in the formulae of the substances used.
Mg + O2 --> MgO
Balance the equation so that each element has the same number of atoms on each side.
2Mg + O2 --> 2MgO
sodium + oxygen --> sodium oxide
4Na{s} + O2{g} --> 2Na2O{s}
(word equation) hydrogen + oxygen ---> water(formulae)
H2 + O2 -----> H2O(balance)
2H2 + O2 -----> 2H2O


State symbolsThe state symbols are put in a balanced equation to show whether something is a solid, liquid, gas or dissolved in water (aqueous solution). The symbols for these are:
state
Symbol
Solid (s)
Liquid (l)
Gas (g)
Aqueous (aq)
Magnesium + oxygen --> magnesium oxide 2Mg{s} + O2{g} --> 2MgO{s} hydrochloric acid + calcium carbonate --> calcium chloride + carbon dioxide + water2HCl (aq) + CaCO3 ---> CaCl2(aq) + CO2(aq) + H20(l)

Task

Balanced equations and ionic equations

Ionic equations only show ions which change in a reaction and ignore those which do not change.

E.g.word equation
hydrochloric acid + sodium hydroxide --> sodium chloride + water

balanced chemical equation

HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l)

ionic equation

H+(aq) + OH-(aq) ---> H2O(l)

E.g. in the electrolysis of sodium chlorideCl-(aq) --> Cl(g) + e-2Cl(g) --> Cl2(g)

Working out formulae from reacting masseselements reacting magnesium chlorine

symbols of elements Mg Clmasses reacting (from experiment)
2.4g 7.1gmolar mass (look up relative atomic 24g/mol 35.5g/molmass in periodic table)amounts (amount = mass/molar mass) 2.4g/24g/mol 7.1g/35.5g/mol = 0.1mol 0.2molratio of atoms (divide by smallest) 1 : 2formula MgCl2



Task :

Work out formulae of compounds formed when the following react:56g of iron and 32g of sulphur (Fe =56, S =32)2g of hydrogen and 16g of oxygen (H=1, O=16)14g of lithium and 16g of oxygen (Li=7)32g of copper and 8g of oxygen (Cu=64)6.4g of copper and 0.8g of oxygen.

Calculating reacting masses using equationsYou can work out ratio of the masses of products and reactants by simply multiplying the number of moles shown in the equation by the formula mass of each substance.

Example 1: What mass of magnesium oxide can be made from 12g of magnesium? Relative atomic masses are Mg =24, O = 16.

equation 2Mg(s) + O2(g) --> 2MgO(s)

formula 2*24 1(16*2) 2(24+16)

masses =48 =32 =80 reacting 48g of Mg forms 80g of MgOmasses 1g of Mg forms 80/48 g of MgO 12g of Mg forms 12*80/48 g of MgO = 20g

Example 2: What mass of magnesium oxide can be made from 12g of magnesium?

equation 2Mg(s) + O2(g) --> 2MgO(s)

amounts 2 moles 1 mole 2 moles masses 2*24 1{16*2} 2{24+16} =48g =32g =80g

so 48g Mg forms 80g MgO 1g Mg forms 80/48 g MgO 12g Mg forms 12*80/48 g MgO = 20g Also note that the ratio of amounts of reactants and products in the equation above can be written as:

Amount of Mg/amount of O2 =2/1 Or Amount of O2/amount of MgO = 1/2


Task : problem C2.08 You decide to travel from London to Delhi for a holiday and hire your own Airbus 319 jet. You fly the 2183 miles to Cairo first and make 9.4 tons of carbon dioxide.


After seeing the Pryramids you fly the 1018 miles to Riyadh making 4.5 tons of carbon dioxide. After a brief stop in the Saudi Capital you fly on the 1900 miles to Delhi making 8.1 tons of carbon dioxide.


Finally you return to London travelling 4171 miles and releasing 22 tons of carbon dioxide. The jet burns the fuel kerosine

C15H32 in the reactionC15H32 + 23O2 --> 15CO2 +16H2O(a)

How much carbon dioxide was released into the atmosphere during the trip?(b) What mass of kerosine was burnt?

(c) What effect does the trip have of the environment?

Alkanes- As level work sheet

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

1. One of the terminal steps in the photochemical chlorination of methane is:
A Cl2---------- 2Cl•
B CH3• + CH3• ---CH3 CH3
C CH4 + Cl--- CH3Cl + H+
D CH4 + Cl•-- CH3 + HCl

2. Which one of the following represents a propagation step in the chlorination of methane?
A CH4 + Cl2 -- CH3Cl + HCl
B CH4 + Cl• --- CH3• + HCl
C Cl2 ---- 2Cl•
D CH3• + Cl• --- CH3Cl

3. Which one of the following species is not formed in the photochemical reaction between chlorine and methane?
A H•
B Cl•
C HCl
D CH3CH3

4. Which one of the following is formed in the initiation step in the mechanism of the chlorination of methane?
A chlorine atoms
B hydrogen atoms
C hydrogen chloride
D methyl radicals


5. 10 cm3 of an equimolar mixture of hydrogen and methane were combusted by exploding with 25 cm3 of pure oxygen. All volume measurements were made at 20oC and 1 atmosphere pressure.

The volume of the mixture (in cm3 ) after the explosion was:
A 12.5
B 15
C 17.5
D 20
Section B

Answer all questions

1. Methane reacts with chlorine in the presence of ultraviolet light, the reaction occurring in a series of steps.

(a) The initiation step involves the action of ultraviolet light on chlorine gas.

Write an equation and explain what happens to the chlorine.

equation


explanation

(b) Write two equations for the second step in this mechanism.

1.

2.


(c) Suggest why traces of ethane are found in the product mixture.
2. (a) Kerosene, which contains a mixture of C11 and C12 hydrocarbons, is obtained from crude oil by fractional distillation. Explain the term fractional distillation.

(b) The hydrocarbons from kerosene can be cracked to form other useful products.

(i) Explain the term cracking.

(ii) Using the molecule C11H24, write an equation to illustrate cracking.

(c) Another fraction contains several isomers of C7H16. Draw structures and give systematic names for any two branched chain isomers of C7H16.

Structure (i)
Name
Structure (ii)

(d) Hexane is exploded in the internal combustion engine according to the equation
2C6H14 + 19O2 12CO2 + 14H2O

3.Calculate the molar enthalpy of combustion of C6H14 using the
bond enthalpies given.

kJ mol-1
C-C 346
C-H 413
0-H 453
0=0 490
C=0 740

Alkanes- As level notes edexcel igcse notes

Associated with these alkanes we have alkyl radicals of
general formula CnH2n+1 (symbol R)

e.g. CH3- methyl,
C2H5- ethyl,
C3H7- propyl
This family of alkanes forms an homologous series.

Combustion in a limited and plentiful supply of air.

Combustion

The alkanes are used as fuels and burn in excess air or oxygen producing carbon dioxide and water.
2C2H6 + 7O2 ---- 4CO2 + 6H2O
C3H8 + 5O2 ---- 3CO2 + 4H2O

In general
CnH2n+2 + (3n+1) O2 -nCO2 + (n+1) H2O

In a limited air supply carbon monoxide is produced
2C2H6 + 5O2--- 4CO + 6H2O


Reaction with chlorine or bromine , monohalogenation only (except for methane and chlorine.


Alkanes are generally unreactive. They are saturated and react by substitution.
Halogenation
Chlorine and bromine react with alkanes in the presence of strong sunlight or u.v. light giving a series of products formed by successive replacement of a hydrogen atom by a halogen atom.
R-H + X2 R-X + HX

Mechanism of the photochemical reaction between chlorine and methane viewed as a free radical substitution.

Chlorine reacts explosively with methane in the presence of strong sunlight or u.v. light to give a mixture of products.
CH4 + Cl2 --- CH3Cl + HCl

This is photochemical chlorination.

Mechanism
The mechanism of a reaction is the course believed to be followed by the reactants in combining together and the various stages inv olved in reaching the final products.This is a free radical chain reaction.
Initiation
Cl2 ---2Cl. (a few)

Propogation
Cl. + CH4 CH3. + HCl
CH3. + Cl2 CH3Cl + Cl.
CH3Cl + Cl. CH2Cl. + HCl
CH2Cl. + Cl2 CH2Cl2 + Cl.

Termination
2Cl. Cl2
CH3. + Cl. CH3Cl
2CH3. C2H6


Isomerism in Organic Compounds

Structural isomerism for aliphatic compounds containing up to six carbon atoms, to include branched structures. (Cyclic structures excluded)

Isomerism

Strutural isomerism occurs when 2 or more compounds have the same molecular formula but different structural formula. (ie same number of atoms but bonded together differently).

Draw and name all the structural isomers of C4H10, C5H12 and C6H14.

Environmental problems associated with spillage and combustion of alkane fuels.

Unreactive nature of alkanes towards electrophiles and nucleophiles.

Hydrocarbons- work sheet/ igcse chemistry /As chemistry

1. The systematic name of the following compound

CH3-CH2-C( CH3)Br-CH3

is
A 2-bromo-2-methylbutane
B 3-bromo-3-methylbutane
C 2-bromopentane
D 3-bromopentane

2. 25 cm3 of hydrocarbon was exploded with 100 cm3 of oxygen (an excess). After cooling the volume of gas remaining was 62.5 cm3. On adding concentrated potassium hydroxide and shaking the volume reduced to 12.5 cm3. Which one of the following is the molecular formula of the hydrocarbon?
A CH4
B C2H4
C C2H6
D C3H6

HYDROCARBONS

Petroleum as a source of hydrocarbons by fractional distillation and cracking.

A knowledge of how these processes are carried out, including the names and nature of the fractions obtained, is expected.(Reforming to benzene derivatives is not required in this module.)

Environmental problems associated with spillage and combustion of hydrocarbons, including global warming and ozone depletion.

HYDROCARBONS
(alkanes, alkenes, benzene.)

Sources

1. Petroleum.

Petroleum deposits were formed by the action of pressure and temperature on marine life sediments, mainly from the fatty acid constituents, under the catalytic action of various rocks and acid clays.

Crude petroleum is a complex mixture of gaseous, liquid and solid hydrocarbons such as alkanes , cycloalkanes, aromatics (benzene) and some alkenes. Also present are some compounds of oxygen, nitrogen and sulphur.

It has no uses in its raw form so to provide useful products its components must be partly separated and if necessary modified. The fundamental process of refining is primary distillation.

Refining

The function of an oil refinery is to manufacture from crude oil those quantities of the oil products required by consumers. This is carried out by the use of various physical and chemical processes.

Refinery gas

( 1 - 2 per cent of crude oil) contains hydrocarbons that are gases at normal temperatures. It includes the alkanes with one to four carbon atoms in their molecules, with methane as a major component. The main use of refinery gas is as a gaseous fuel.

Gasoline

( 15 - 30 per cent) has is a complex liquid mixture of hydrocarbons containing mainly C5 - C10 compounds whose boiling points range from 40oCto 180oC. The major use of gasoline is as a fuel in internal combustion engines. A considerable part proportion of this fraction is used to produce chemicals by cracking.

Kerosene

( 10 - 15 per cent) consists mainly of C4 to C 12 hydrocarbons, with boiling point from 162 to 250 degrees C. It is used as a fuel in jet engines and for domestic heating. It can be cracked to produce extra gasoline.

The diesel oil or gas oil

(15 - 20 per cent) containing C 13 - C25 compounds, boils between to 120 - 350 degrees C. It is used in diesel engines where the fuel is ignited by compression instead of by a spark. And also for industrial heating purposes. It can also be cracked to produce extra gasoline.

Residue

(40 - 50 per cent) boils above 350 degrees C and is a highly complex mixture of non-volatile hydrocarbons. Most of it is used as fuel oil in large furnaces such as those in power stations or big ships. Proportion of it is used to make lubricating oils and waxes. Both these contain C 26 - C 40 carbons

Distillation

Crude oil is fractionally distilled to give 4 main fractions.
C1-C4
refinery gases
methane, ethane, propane, butane
C8-C16
light distillates
petrol, aviation fuel, kerosene, benzene
C17-C20
middle distillates
heating oil, diesel, feedstock for cracking
C21+
residue
paraffin wax, lubricating oil, petroleum jelly, bitumen

2. Coal Tar

When coal is carbonised (burnt in the absence of air) one of the products is a viscous black liquid called coal tar. This is distilled into five main fractions. The light oil fraction (boil pt. up to 170oC) is a source of benzene.

Cracking as a source of alkenes and shorter chain alkanes.

Cracking

In this process larger molecules are broken down into smaller ones, either by high temperature and pressure (thermal cracking) or by a catalyst (catalytic cracking).

Thermal Cracking

When alkanes are heated to high temperatures their molecules vibrate strongly enough to break and form smaller molecules. One of these molecules is usually an alkane. Reducing chain length generally results in unsaturation. Such reactions are known as cracking

e.g. C8H18 C5H12 + CH3CH=CH2

octane pentane propene
Thermal cracking is generally used for cracking residues to middle distillates.

Catalytic Cracking

By using a catalyst, cracking can be made to occur at fairly low temperature. This is known as catalytic cracking.

Catalytic cracking is the most important source of petrol and raw materials for the chemical industry. Heavier fractions can be cracked to produce extra gasoline. Cracking tends to produce branched-chain rather than straight-chain alkanes, so the gasoline produced this way has a high octane rating. Processes similar to cracking can be used to convert low-grade gasoline to high grade fuel.

The catalysts are usually natural clays and synthetic alumina/ silica mixtures (Al2O3/SiO2).

Isomerisation

This involves breaking up straight chainalkanes and reassembling them as branched chain isomers.
Both of these processes are important in the production of unleaded gasoline.



Catalytic Reforming

Reforming involves converting straight chain alkanes into ring molecules such as arenes and cycloalkanes.

Benzene C6H6 and other aromatic compounds can be made by passing petrol vapour over a heated platinum catalyst.
500oC/15 atm
C6H14 -,--- C6H6 + 4H2
hexane Pt catalyst benzene

The u.s.a. obtains about half its benzene in this way.