Chemistry unit 3

Things to help with unit 3.

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Titrations

• used to check samples of solutions for purity
• used to measure the volume of one solution that exactly reacts with another

Steps of titrations

1. If sample is solid, it is weighed and then dissolved to form a known volume of solution (normally 100cm cubed)

2. a pipette is used to measure accurately a volume of this solution (e.g. 10cm cubed). A safety pipette filler is used to draw solution into pipette and is then emptied into conical flask.

3.A few drops of indicator are added to conical flask (will change colour when titration complete)

4.A second chemical is placed in the burette. This is often an acid or alkali and must be of a known concentration

5.Solution from burette is let into conical flask 1 drop at a time, stirring in between. The volume needed is known when the solution changed colour.

6.Results can be used to find amount of synthesised chemical in sample and therefore its purity.

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Titrations and calculations

• A balanced symbol equation can be the starting point to find out the quantity of reactants used and the quantity of products made.

Step 1: write down the balanced symbol equation

Sulfuric acid is neutralised by sodium hydroxide to form sodium sulphate and water:

H2SO4 + 2NaOH → Na2SO4 + 2H2O

Step 2: work out the relative formula mass

Can be worked out using relative atomic mass from periodic table.

H2SO4  = 98, NaOH = 40, Na2SO4 = 142, H2O = 18

Step 3: work out masses of reactants and products

H2SO4 + 2NaOH → Na2SO4 + 2H2O

98 + (2x40) = 142 + (2x18)     →       98 + 80 = 142 + 36

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Titrations and Calculations cont.

Questions

What mass of sulphuric acid reacts with sodium hydroxide to produce 10g of sodium sulphate?

142g of Na2SO4 is made from 98g of H2SO4

10g of Na2SO4 is made from 10 x (98 ÷ 142) = 6.9g of H2SO4

What mass of sodium sulphate is made when 5.0 tonnes of sodium hydroxide is reacted with sulphuric acid?

80 tonnes of NaOH is used to make 142 tonnes of Na2SO4

5.0 tonnes of NaOH is used to make 5 x (142 ÷ 80) = 8.875 tonnes of Na2SO4

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Solubility

• Many gases and most ionic compounds = soluble in water
• many covalent substances = insoluble in water (little attraction between their molecules and water molecules)
• solubility - the amount of a solute you can dissolve in a solvent
• measured in - grams of solute per 100 grams of solvent at a particular temp.
• normally, solubility increases as temperature increases
• saturated solution - when as much solute as possible has been dissolved
• as a hot saturated solution cools down, some will crystallise out of the solution
• solubility curve - shows the amount of a solute which dissolves to produce a saturated solution at any given temperature
• can use solubility curves to predict how much solute will form when we cool a hot solution down

solubility of gases

• affected by - temperature and pressure
• as temperature increases, solubilty decreases
• as pressure increases, solubility increases
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Flame Tests

• To identify metals in groups 1 and 2

method

• put the compound to be tested in a platinum wire loop (that has been dipped in concentrated hydrochloric acid)
• hold the loop in the roaring blue flame of the bunsen burner
• Use the colour of the flame to identify the metal element in the compound

Element                        Flame Colour

• lithium                 bright red
• sodium                golden yellow
• potassium           lilac
• calcium               brick red
• barium                green
• copper                blue-green

(p.s. to learn them, helps if you sing them to the tune of Eliza Doolittle's rollerblades ;))

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Using sodium hydroxide solution

•  to help us with the identification
• sodium hydroxide + aluminium ions / calcium ions / magnesium ions = white precipitates
• If you add more sodium hydroxide, precipitate formed of aluminium ions dissolves (but not calcium or magnesium)
• Ca and Mg ions distinguished by flame test; calcium = brick red, magnesium = no colour
• sodium hydroxide + some metal ions = coloured precipitates
• NaOH + copper ions = light blue precipitate
• NaOH + iron(II) ions = dirty green
• NaOH + iron(III) ions = reddish-brown
• Sodium hydroxide + ammonium ions = ammonia + water
• When solution is warmed, ammonia gas is given off which can be tested using damp red litmus paper
• red litmus paper turns blue because ammonia is alkaline gas
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Carbonates

• a carbonate + a dilute acid (e.g. sodium hydroxide) = carbon dioxide (and it fizzes)
• Remember! - Carbon dioxide turns limewater cloudy

Copper Carbonate

• Is a green substance
• Heat it to give black copper oxide and carbon dioxide

Zinc Carbonate

• a white substance
• Heat it to give white zinc oxide (BUT when it is still hot it is a lemon-yellow) and carbon dioxide
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Halides, Sulphates and Nitrates

The Halides test

• Add nitric acid and silver nitrate solution to the unknown solution
• Colour of the precipitate tells us which of the halide ions is present
• chloride ions - white precipitate
• bromide ions - cream precipitate
• iodide ion - pale yellow precipitate

Sulphates

• Sulphate ions produce a white precipitate when we add hydrochloric acid followed by barium chloride solution
• precipitate = barium sulphate (insoluble salt)

Nitrates

• Use the test for ammonia (add sodium hydroxide to the unknown solution)
• If no ammonia detected, add aluminium powder
• This reduces nitrate ions to ammonium ions which then reacts with the sodium hydroxide
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Testing for organic substances

• organic substance = substances which are based on the element, carbon
• unsaturated hydrocarbons have a carbon-carbon double bond
• unsaturated hydrocarbon (colourless) + bromine water (orange-yellow) = products (colourless)

combustion analysis

• Can find the empirical formula of an organic compound by burning it and measuring the amounts of product formed.
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Thank you - really didn't understand titrations and this was really helpful :)

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