## Thursday, October 22, 2009

### chemical calculation - IGCSE NOTES / BALANCING EQUATION

Word equations

If 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

1.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.

2. State the name and the number of atoms of each element in the formulae above. Formulae can be worked out from valency.
Valency
atom or ion with that valency

1 hydrogen, group 1 e.g. sodium and potassium, group 7e.g. fluorine and chlorine, ammonium NH4+, hydroxide OH-, nitrate NO3-

2 group 2 e.g. magnesium and calcium, group 6, sulphate SO42-, carbonate CO32-, copper , II Cu2+ iron II Fe2+

3.group 3 e.g. aluminium, group 5, phosphate PO43-, iron III Fe3+

4 group 4 e.g. carbon

name aluminium oxidevalency 3 2symbols Al Oformula Al2O3

Task : 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.

4.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

5. 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 oxideWrite 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 --> 2Mg

Osodium + oxygen --> sodium oxide

4Na{s} + O2{g} --> 2Na2O{s}

(word equation) hydrogen + oxygen ---> water(formulae)

H2 + O2 -----> H2O(balance) 2H2 + O2 -----> 2H2O

6.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 equationHCl(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

7 .Working out formulae from reacting masseselements reacting magnesium chlorinesymbols of elements Mg Clmasses reacting (from experiment)

8.1gmolar 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.
C2.08

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

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?