Acids
An acid is a substance that produces H+ as the only positively charged ions when dissolved in water. In general, non-metalic oxides dissolve in water to produces acids.
Some common acids
- Hydrochloric acid HCl (aq)
- sulphuric acid H2SO4 (aq) (Battery acid)
- Nitric acid HNO3 (aq)
- Ethanoic acid or Acetic acid CH3COOH (aq) (in Vinegar)
- Citric acid (in Lemon juice)
- Methanoic acid (in Ant stings)
- Carbonic acid (in Fizzy drinks)
- Latic acid (in Sour milk)
Properties of acids
- They turn litmus red
- They turn universal indicator red, orange or yellow depending on their strength
- They react with some metals to produce hydrogen gas and a salt.
- They react with carbonates to produce a salt, water and carbon dioxide (fizz or effervesence )
- They have a pH level below 7
- They are corrosive
- They have a sour taste
Bases and Alkalis
A base is a substance that produces OH– ions when dissolved in water. In general, metalic oxides and hydroxides are bases. A soluble base is called an alkali.
Some common bases and alkalis
- Sodium oxide Na2O
- Sodium hydroxide NaOH (caustic soda or soda lime)
- Ammonia solution NH3 (aq)
- Calcium hydroxide Ca(OH)2 (lime water)
Properties of bases and alkalis
- They turn litmus blue
- They turn universal indicator green, blue or purple depending on their strength
- They react with acids to produce water and a salt (neutralisation)
- They are caustic
- They have a pH level above 7
- They have a bitter soapy taste
Neutral substances
A neutral substance is a substance that is neither acidic nor alkaline. Examples are pure water, solutions of many salts e.g. sodium chloride solution.
Properties of neutral substances
- They have no effect on litmus
- They turn universal indicator lime green
- They have a pH level of 7
Indicators
We used indicators to test for the acidity or alkalinity of a substance. Litmus, methyl orange and phenolphthalein are indicators used to test whether a substance is an acid or an alkali. They show us a colour change.
| Indicator | Colour in acid | Colour in alkali |
|---|---|---|
| litmus | red | blue |
| phenolphthalein | colourless | pink |
| methyl orange | red | yellow |
However all these indicators only tell us whether a substance is acid or alkaline. To measure how acidic or alkaline a substance is, we use another indicator known as a universal indicator. This indicator is a mixture of many other indicators. The colour shown by this indicator can be matched against a pH scale.
The pH scale runs from 0 to 14. A substance with a pH of less than 7 is an acid. One with a pH of greater than 7 is alkaline. One with a pH of 7 is said to be neither acid nor alkaline, that is neutral. Water is the most common example of a neutral substance.
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Strong acids | Weak acids | Neutral | Weak bases | Strong bases | ||||||||||
|
|
|
|
| ||||||||||
For more accurate work, a pH meter is used. A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions , indicating its acidity or alkalinity expressed as pH.
781 pH/Ion Meter pH meter manufactured by Metrohm
Reactions of Acids and Bases
Reactions of acids
- With metals:
acid + metal → salt + hydrogen
For example
Magnesium + sulphuric acid → magnesium sulphate + hydrogen
Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (g)So the metal displaces the hydrogen out of the acid, and takes its place. A solution of the salt magnesium sulphate is formed.
- With bases:
acid + base → salt + water
Example for an acid and alkali:
Hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)Example for an acid and insoluble base:
Sulphuric acid + copper(II) oxide → copper(II) sulphate + water
H2SO4 (aq) + CuO (s) → CuSO4 (aq) + H2O (l) - With carbonates:
acid + carbonate → salt + water + carbon dioxide
For example:
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 ( g)
Neutralisation
Neutralisation is a reaction of an acid and a base to produces water and a salt. The reaction of a carbonate with acids is called neutralisation but the reaction of an acid with a metal is not neutralisation.
Acid + base → salt + water
e.g
hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
| Acid | Type of salt | Example |
|---|---|---|
| Carbonic acid | Carbonates | Sodium carbonate (Na2CO3) |
| Ethanoic acid | Ethanoates | Sodium ethanoate (CH3COONa) |
| Hydrochloric acid | Chlorides | Potassium chloride (KCl) |
| Nitric acid | Nitrates | Potassium nitrate (KNO3) |
| Sulphuric acid | Sulphates | Sodium sulphate (Na2SO4) |
Question
- Name the acid. ………[1]
- Describe the pH change of the mixture during the reaction. ……..[1]
- Name one other substance that reacts with this acid to make magnesium sulphate. [1]
- The reaction between magnesium and this acid is exothermic.
State what is meant by the term exothermic. ….[1]
- Sulphuric acid
- The pH increases
- magnesium oxide / magnesium hydroxide / magnesium carbonate
- releases heat energy
solid particles of calcium sulphate stays on the filter paper because they cannot pass through filter paper and the liquid passes through
Titration
Titration is a method of volumetric analysis in which a volume of one reagent (for example an acid) is added to a known volume of another reagent (for example an alkali) slowly from a burette until an end-point is reached. If an acid and alkali are used, then an indicator is used to show that the end-point has been reached.
Titration can be used to prepare salts from a soluble base and an acid. Because in this neutralisation reaction both reactants are in solution, titration is required. Acid is slowly and carefully added to a measured volume of alkali using a burette until the indicator, usually phenolphthalein, changes colour.
An indicator is used to show when the alkali has been neutralised completely by the acid. This is called the end-point. Once you know where the end-point is, you can add the same volume of acid to the measured volume of alkali but this time without the indicator.
Stages in performing titration
Acid-base titration.
Image source
- Add 25 cm3 of sodium hydroxide solution into a conical flask using a pipette.
- Add a few drops of phenolphthalein indicator.
Phenolphthalein is pink in alkaline conditions but colourless in acid. - Clamp a burette to a stand and then add 0.10mol dm−3 solution of hydrochloric acid a the burette using a filter funnel until it is filled up exactly to the zero mark
- Remove the filter funnel.
- Open the burette tap and add the hydrochloric acid little by little to the sodium hydroxide flask, all the while swirling the conical flask.
- Add the acid is until the alkali is completely neutralised. This is shown by the indicator just starting to change colour.
- Record the final reading on the burette at the end-point and perfom further titrations until consistent results are obtained (within 0.1 cm3 of each other).
- Average the results. That is amount of acid and alkali required to neutralise each other.
A student filling a conical flask using a pipette connected to a pipette filler.
A student performing titration.
The hazard symbol for corrosive substances according to directive 67/548/EWG by the European Chemicals Bureau, now known as the Consumer Products Safety
and Quality (CPS&Q) Unit.
Corrosive acids and alkalis, drops of sodium hydroxide
solutions can readily decompose proteins and lipids in living tissues via amide hydrolysis and ester hydrolysis, which consequently cause chemical burns and may induce permanent blindness upon contact with eyes. Solid alkali can also express its corrosive nature if there is water, such as water vapor. Thus, protective equipment, like rubber gloves , safety clothing and eye protection , should always be used
when handling these chemicals or their solutions. The standard first aid measures for alkali or acid spills on the skin is, as for other corrosives, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes.
