IGCSE CHEMISTRY LESSON 2

Section 1 Principles of Chemistry a)States of matter b)Atoms c)Atomic structure d)Relative formula mass e)Chemical formulae and chemical equations f)Ionic compounds g)Covalent substances h)Metallic crystals i)Electrolysis

Lesson 2 a)Relative formula mass b)Chemical formulae and Chemical equations 1.15 calculate relative formula masses (Mr) from relative atomic masses (Ar) 1.16 understand the use of the term mole to represent the amount of substance 1.17 carry out mole calculations using relative atomic mass (Ar) and relative formula mass (Mr).

What is RELATIVE ATOMIC MASS? By definition: “mass of a particular atom compared to the mass of an atom of hydrogen” Relative atomic mass is also the same as the mass number – it’s that simple!

Relative Atomic Mass and Atomic Number Na Relative Atomic Mass = total number of protons and neutrons ATOMIC NUMBER = number of protons (also electrons) Relative atomic mass is always bigger than the atomic number

Look up the Mass Numbers of the following elements: OxygenCarbon IronLead IodineHelium NitrogenCopper CalciumTin

Look up the Mass Numbers of the following elements: Oxygen 16 Carbon 12 Iron 56 Lead 207 Iodine 127 Helium 4 Nitrogen 14 Copper 63.5 Calcium 40 Tin 119

When atoms join together to form a compound (eg. Magnesium oxide) we can calculate the RELATIVE FORMULA MASS:

Magnesium oxide MgO = 40

Chemical formulae MgO

Chemical formulae MgO 1 Mg, 1 O

Chemical formulae MgO 1 Mg, 1 O H 2 SO 4

Chemical formulae MgO 1 Mg, 1 O H 2 SO 4 2H, 1 S, 4 O

Chemical formulae MgO 1 Mg, 1 O H 2 SO 4 2H, 1 S, 4 O Ca(OH) 2

Chemical formulae MgO 1 Mg, 1 O H 2 SO 4 2H, 1 S, 4 O Ca(OH) 2 1 Ca, 2 O, 2 H

Chemical formulae MgO 1 Mg, 1 O H 2 SO 4 2H, 1 S, 4 O Ca(OH) 2 1 Ca, 2 O, 2 H 2Na 2 CO 3

Chemical formulae MgO 1 Mg, 1 O H 2 SO 4 2H, 1 S, 4 O Ca(OH) 2 1 Ca, 2 O, 2 H 2Na 2 CO 3 4 Na, 2 C, 6 O

When atoms join together to form a compound (eg. Magnesium oxide) we can calculate the RELATIVE FORMULA MASS: Magnesium oxide MgO = 40

Now calculate the Relative Formula Mass for the following compounds:

CompoundFormulaRelative formula mass Calcium oxide CaO Sulphuric acid H 2 SO 4 Sodium nitrate NaNO 3 Aluminium hydroxide Al(OH) 3 Lead carbonate PbCO 3

Now calculate the Relative Formula Mass for the following compounds: CompoundFormulaRelative formula mass Calcium oxide CaO = 56 Sulphuric acid H 2 SO = 98 Sodium nitrate NaNO = 85 Aluminium hydroxide Al(OH) = 78 Lead carbonate PbCO = 267

Be aware! Relative Atomic Mass A r

Be aware! Relative Atomic Mass A r Relative Formula Mass M r

The mole

You may think that the mole is a very blind animal that lives underground

The mole The mole is also a measure widely used in chemistry

The mole One mole is the relative formula mass (M r ) of a compound, or relative atomic mass (A r ) of an element, expressed in grams

The mole For example, the A r of carbon is 12.

The mole For example, the A r of carbon is 12. So the mass of one mole of carbon is 12g.

The mole For example, the A r of carbon is 12. So the mass of one mole of carbon is 12g. How simple is that?

The mole What’s the mass of one mole of lead?

The mole What’s the mass of one mole of lead? A r of lead (Pb) = 207 So that mass of one mole of lead is 207g

The mole Don’t panic! Let’s get a little more clever now and calculate the mass of moles from the relative formula mass (M r )

The mole For example, the M r of sodium hydroxide: NaOH = = 40

The mole For example, the M r of sodium hydroxide: NaOH = = 40 So the mass of one mole of sodium hydroxide is 40g

The mole What’s the mass of one mole of calcium carbonate?

The mole What’s the mass of one mole of calcium carbonate? M r of CaCO 3 = = 100

The mole What’s the mass of one mole of calcium carbonate? M r of CaCO 3 = = 100 So one mole of CaCO 3 = 100g This isn’t rocket science!

Steady as you go! Important equation

Steady as you go! Important equation Number of = mass used moles M r

Steady as you go! Important equation Number of = mass used moles M r Eg how many moles are there in 20g of sodium hydroxide?

Steady as you go! Important equation Number of = mass used moles M r Eg how many moles are there in 20g of sodium hydroxide? Number of = 20 moles 40 = 0.5

Steady as you go! Important equation Number of = mass used moles M r Eg how many moles are there in 20g of sodium hydroxide? Number of = 20 moles 40 = 0.5 We could even re-arrange the equation

Number of = mass used moles M r Mass used = number of x M r moles

Number of = mass used moles M r Mass used = number of x M r moles Eg. What’s the mass of 2 moles of calcium carbonate?

Number of = mass used moles M r Mass used = number of x M r moles Eg. What’s the mass of 2 moles of calcium carbonate? Mass used = 2 x 100 = 200g

The mole

Lesson 2 a)Relative formula mass b)Chemical formulae and Chemical equations 1.18 write word equations and balanced chemical equations to represent the reactions studied in this specification 1.19 use the state symbols (s), (l), (g) and (aq) in chemical equations to represent solids, liquids, gases and aqueous solutions respectively 1.20 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation 1.21 calculate empirical and molecular formulae from experimental data 1.22 calculate reacting masses using experimental data and chemical equations 1.23 carry out mole calculations using volumes and molar concentrations.

Lesson 2 a)Relative formula mass b)Chemical formulae and Chemical equations 1.18 write word equations and balanced chemical equations to represent the reactions studied in this specification 1.19 use the state symbols (s), (l), (g) and (aq) in chemical equations to represent solids, liquids, gases and aqueous solutions respectively 1.20 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation 1.21 calculate empirical and molecular formulae from experimental data 1.22 calculate reacting masses using experimental data and chemical equations 1.23 carry out mole calculations using volumes and molar concentrations.

Using chemical symbols

Each element is represented by a different symbol …… eg Fe for iron, Na for sodium, O for oxygen All of these symbols are in the PERIODIC TABLE

Using chemical symbols Each element is represented by a different symbol …… eg Fe for iron, Na for sodium, O for oxygen All of these symbols are in the PERIODIC TABLE We can use these symbols to show molecules of compounds, and they can show us the ratio of the different elements which combine to form compounds

Using chemical symbols Each element is represented by a different symbol …… eg Fe for iron, Na for sodium, O for oxygen All of these symbols are in the PERIODIC TABLE We can use these symbols to show molecules of compounds, and they can show us the ratio of the different elements which combine to form compounds Eg. H 2 O CO 2 NH 3

Using chemical symbols Let’s recap on what we covered earlier.

Using chemical symbols Eg. Sodium hydroxide NaOH 1 atom Na: 1 atom oxygen: 1 atom hydrogen

Using chemical symbols Eg. Sodium hydroxide NaOH 1 atom Na: 1 atom oxygen: 1 atom hydrogen Eg. Calcium carbonate CaCO 3 1 atom Calcium, 1 atom carbon, 3 atoms oxygen

Using chemical symbols Eg. Magnesium nitrate Mg(NO 3 ) 2 1 atom Magnesium, 2 atoms nitrogen, 6 atoms oxygen

Using chemical symbols Eg. Magnesium nitrate Mg(NO 3 ) 2 1 atom Magnesium, 2 atoms nitrogen, 6 atoms oxygen Eg. Aluminium sulphate 3Al 2 (SO 4 ) 3 6 atoms aluminium, 9 atoms sulphur, 36 atoms oxygen

Equations

REACTANTS PRODUCTS

Equations REACTANTS PRODUCTS Sodium + water sodium + hydrogen hydroxide

Equations REACTANTS PRODUCTS Sodium + water sodium + hydrogen hydroxide Na + H 2 O NaOH + H 2

Equations REACTANTS PRODUCTS Sodium + water sodium + hydrogen hydroxide Na + H 2 O NaOH + H 2 2Na + 2H 2 O 2NaOH + H 2

Equations REACTANTS PRODUCTS Sodium + water sodium + hydrogen hydroxide Na + H 2 O NaOH + H 2 2Na + 2H 2 O 2NaOH + H 2 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g)

Equations You need to know your state symbols (s)= solid (l) = liquid (aq) = aqueous solution (g) = gas

Equations You also need to be able to: write word equations when required recall formulae of simple covalent compounds write down formulae for simple ionic compounds interpret chemical formulae of molecules interpret symbol equations including state symbols

Simple covalent formulae Water H 2 O Carbon dioxide CO 2 Ammonia NH 3 Hydrogen H 2 Oxygen O 2 Nitrogen N 2 Sulphur dioxide SO 2 Methane CH 4

Simple Ionic Formulae Sodium chloride NaCl Calcium chloride CaCl 2 Magnesium oxide MgO Hydrochloric acid HCl Sulphuric acid H 2 SO 4 Nitric acid HNO 3 Sodium hydroxide NaOH Potassium hydroxide KOH Calcium hydroxide Ca(OH) 2 Calcium carbonate CaCO 3 Aluminium oxide Al 2 O 3 Iron oxide Fe 2 O 3

Writing balanced equations Writing balanced chemical equations can be good / bad for your health!

Writing balanced equations The basic rule (golden rule #1) Hydrogen + oxygen  water H 2 (g) O 2 (g) H 2 O

Writing balanced equations The basic rule (golden rule #1) Hydrogen + oxygen  water H 2 (g) O 2 (g) H 2 O LEARNING POINT!! Gases like hydrogen (H 2 ) and oxygen (O 2 ) are diatomic - they like to go around in pairs

Writing balanced equations The basic rule (golden rule #1) Hydrogen + oxygen  water H 2 (g) O 2 (g) H 2 O 2H 2O 2H 1O THERE MUST BE EQUAL NUMBERS OF ATOMS OF EACH ELEMENT ON BOTH SIDES OF THE EQUATION

Writing balanced equations The basic rule (golden rule #1) Hydrogen + oxygen  water H 2 (g) O 2 (g) H 2 O 2H 2O 2H 1O THERE MUST BE EQUAL NUMBERS OF ATOMS OF EACH ELEMENT ON BOTH SIDES OF THE EQUATION x

Writing balanced equations The basic rule (golden rule #1) Hydrogen + oxygen  water 2H 2 (g) O 2 (g) 2H 2 O 4H 2O 4H 2O THERE MUST BE EQUAL NUMBERS OF ATOMS OF EACH ELEMENT ON BOTH SIDES OF THE EQUATION

Writing balanced equations The basic rule (golden rule #1) Hydrogen + oxygen  water 2H 2 (g) O 2 (g) 2H 2 O 4H 2O 4H 2O THERE MUST BE EQUAL NUMBERS OF ATOMS OF EACH ELEMENT ON BOTH SIDES OF THE EQUATION

Writing balanced equations FOUR important steps: 1 Write a word equation for the reaction 234

Writing balanced equations FOUR important steps: 1 Write a word equation for the reaction 234 Put in the formulae for the elements or compounds

Writing balanced equations FOUR important steps: 1 Write a word equation for the reaction 23 Balance the equation – put numbers in front of the reactants and products where necessary 4 Put in the formulae for the elements or compounds

Writing balanced equations FOUR important steps: 1 Write a word equation for the reaction 23 Balance the equation – put numbers in front of the reactants and products where necessary 4 Put in the formulae for the elements or compounds Complete the balanced symbol equation using state symbols

Writing balanced equations Example 1: 1234 Magnesium + Oxygen  Magnesium oxide

Writing balanced equations Example 1: 1234 Magnesium + Oxygen  Magnesium oxide Mg + O 2  MgO

Writing balanced equations Example 1: 1234 Magnesium + Oxygen  Magnesium oxide Mg + O 2  MgO LEARNING POINT!! Gases like hydrogen (H 2 ) and oxygen (O 2 ) are diatomic - they like to go around in pairs

Writing balanced equations Example 1: 1234 Magnesium + Oxygen  Magnesium oxide Mg + O 2  MgO

Writing balanced equations Example 1: 1234 Magnesium + Oxygen  Magnesium oxide Mg + O 2  MgO Mg + O 2  2MgO

Writing balanced equations Example 1: 1234 Magnesium + Oxygen  Magnesium oxide Mg + O 2  MgO Mg + O 2  2MgO Mg(s) + O 2 (g)  2MgO(s)

Writing balanced equations Example 2: 1234 nitrogen + hydrogen  ammonia

Writing balanced equations Example 2: 1234 nitrogen + hydrogen  ammonia N 2 + H 2  NH

Writing balanced equations Example 2: 1234 nitrogen + hydrogen  ammonia N 2 + H 2  NH N 2 + 3H 2  2NH

Writing balanced equations Example 2: 1234 nitrogen + hydrogen  ammonia N 2 + H 2  NH N 2 + 3H 2  2NH N 2 (g) + 3H 2 (g)  2NH 3 (g)

End of Lesson 2 In this lesson we have covered: Relative Formula Mass Chemical Formulae Chemical Equations