4.9 describe experiments to carry out acid-alkali titrations

By doing a titration you are able to find out exactly how much acid is needed to neutralise a certain amount of alkali, exactly how much alkali is needed to neutralise a certain amount of acid.

Method

- Add 25cm³ of alkali to a conical flask (with a pipette and pipette filler)
- Add a few drops of phenolphthalein indicator
- Fill a burette with your acid (NOTE: ensure you have the burette below eye level incase the acid sprays out etc)
- Add the acid to the alkali a bit at a time (using a burette). NOTE: regularly swirl the conical flask to ensure the acid is evenly distributed throughout the alkaline
- Stop adding acid as soon as the solution in the conical flask changes colour (with phenolphthalein, it will go colourless) - this means that the alkali has been neutralised.
- Using the burette, record the amount of acid used to neutralise the alkali.
- Repeat a few times to avoid anomalies etc.



If you are asked to find out the amount in moles of acid needed to neutralise an alkali (or vice versa), just do the same as a mole calculation. For example...

It takes 30cm³ of sulphuric acid of an unknown concentration to neutralise 25cm³ of sodium hydroxide with a concentration of 0.1 moles per dm³. Find the concentration of the sulphuric acid.

- First, work out how many moles of the 'known' substance (sodium hydroxide) you have...

moles = concentration x volume

= 0.1 x (25/1000)

= 0.0025 moles

- Now, write down the equation to work out the ratio of acid : alkali...

2NaOH + H2SO4 ---> Na2SO4 + 2H2O

This means there are two moles of sodium hydroxide to every 1 mole of sulphuric acid.

- This means we need to half the amount of moles of sodium hydroxide to get the amount of moles of sulphuric acid...

0.0025/2 = 0.00125

- Now we know the amount of moles and volume of the acid, so we can work out the concentration...

concentration = moles / volume

= 0.00125 / (30/1000)

= 0.0417 moles per dm³

4.8 describe experiments to prepare insoluble salts using precipitation reactions

You can use a precipitation reaction. To do this just pick two solutions that contains the ions you need and add them together. For example, to make barium sulfate (insoluble) you need to add together one solution containing barium ions and another containing sulphate ions. To do this, you cam mix barium chloride with sulphuric acid...

barium chloride + sulphuric acid ---> barium sulphate + hydrochloric acid

BaCl_2(aq) + H₂SO_4(aq) ---> BaSO_4(s) + 2HCl_(aq)

4.7 describe experiments to prepare soluble salts from acids

You need to pick an insoluble base, and the acid of the salt you want to make (sorry that was a bit confusing, I couldn't really find another way to word it). Basically, if you was to make copper nitrate, mix nitric acid and copper carbonate (remember all carbonates are insoluble except sodium, potassium and ammonium carbonate)...

CuCO_3(s) + 2HNO_3(aq) ---> Cu(NO₃)_2(aq) + CO_2(g) +  H₂O_(l)


All you need to do is add the metal oxide, carbonate or hydroxide (aka the insoluble base) to the acid. The base will react with the acid and dissolve.  

NOTE: This is a neutralisation reaction, you will know when the reaction is over as this is when all the acid has been neutralised (this is when the excess solid will sink to the bottom of the flask and not react anymore)

Next, filter the solution to get rid of the undissolved base and evaporate the water to leave pure salt crystals.

4.6 understand the general rules for predicting the solubility of salts in water

Some salts are soluble whilst others are insoluble, we may get a question where we are given a salt and we have to determine whether is is soluble or insoluble, these are the rules we must learn is order to be able to do this...

- Sodium salts are soluble
- Potassium  salts are soluble
- Ammonium salts are soluble
- All sulphates except barium sulphate and calcium sulphate are soluble
- All chlorides except silver chloride are soluble
- All carbonates except sodium, potassium and ammonium carbonates are insoluble (this is because all sodium, potassium and ammonium salts are soluble remember)

4.5 predict the products of reactions between dilute hydrochloric, nitric and sulphuric acids; and metals, metal oxides and metal carbonates (excluding the reactions between nitric acid and metals)

With metal oxides...

Acid + metal oxide ---> salt + water

Most metal oxides are bases (a base is a substance that can neutralise an acid), this means they will react with acids to form a salt and water. If the acid is hydrochloric acid, the salt will be a metal chloride, if the acid is sulphuric acid, the salt will be a metal sulfate, if the acid is nitric acid, the salt will be a metal nitrate. For example...

hydrochloric acid + copper oxide ---> copper chloride + water
2HCl + CuO ---> CuCl₂ + H₂O


sulphuric acid + zinc oxide ---> zinc sulphate + water
H₂SO₄ + ZnO ---> ZnSO₄ + H₂O


nitric acid + copper oxide ---> copper nitrate + water
2HNO₃ + CuO ---> Cu(NO₃)₂ + H₂O


With metal carbonates...

Acid +  metal carbonate ---> salt + water + carbon dioxide

The same with metal oxides, the salt produced will depend on the acid you use. For example...

hydrochloric acid + sodium carbonate ---> sodium chloride + water + carbon dioxide
2HCl + Na₂CO₃ ---> 2NaCl + H₂O + CO₂


sulphuric acid + calcium carbonate ---> calcium sulphate + water + carbon dioxide
H₂SO₄ + CaCO₃ ---> CaSO₄ + H₂O + CO₂


nitric acid + calcium carbonate ---> calcium nitrate + water + carbon dioxide

2HNO₃ + CaCO₃ ---> Ca(NO₃)₃ + H₂O + CO₂

4.4 define acid as sources of hydrogen ions, H+, and alkalis as sources of hydroxide ions, OH ̄

Basically, if something is acidic it contains H+ (hydrogen ions), if something is alkali it contains OH- (hydroxide ions).

NOTE: Alkalis are just soluble bases (a base is a substance that can neutralise an acid)

4.3 describe the use of universal indicator to measure the approximate pH value of a solution

Universal indicator is just a mixture if dyes. If a solution is strongly acidic, it will go very red, then will go through the colour spectrum (yellow in weakly acidic, green in neutral, blue in weakly alkaline, purple in strongly alkaline) like this...

4.2 understand how the pH scale, from 0-14, can be used to classify solutions as strongly acidic, weakly acidic, neutral, weakly alkaline or strongly alkaline

The pH scale goes from 0-14. The strongest acid has pH0, the weakest acid has a pH6, the strongest alkali has a pH14, the weakest alkali has a pH8 (NOTE: pH7 is neutral, neither acid nor alkali)...



Picture source : bbc bitesize

4.1 describe the use of the indicators litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions

The dye in these indicator changes colour depending on whether the substance you are testing is alkaline or acidic (above or below pH7).

Litmus - red in acidic solutions (under pH7), blue in alkaline solutions(above pH7), purple in neutral solutions (pH7)

Phenolphthalein - colourless in acidic solutions (under pH7), bright pink in alkaline solution (above pH7)

Methyl orange - red in acidic solutions (under pH7), yellow in alkaline solutions (above pH7)