Properties of Acids
- Acids have pH values of below 7, have a sour taste (when edible) and are corrosive
- In acidic conditions blue litmus paper turns red and methyl orange indicator turns red
- Acids are substances that can neutralise a base, forming a salt and water
- When acids react, they will lose electrons to form positively charged hydrogen ions (H+)
- The presence of H+ ions is what makes a solution acidic
Example: Hydrochloric Acid
HCl (aq) → H+ (aq) + Cl- (aq)
Typical reactions of acids
Acids and metals
- Only metals above hydrogen in the reactivity series will react with dilute acids.
- When acids react with metals they form a salt and hydrogen gas:
Acid + Metal → Salt + Hydrogen
Examples of reaction between acids and metals:
Acids with bases (alkalis)
- Metal oxides and metal hydroxides can act as bases
- When they react with acid, a neutralisation reaction occurs
- In all acid-base neutralisation reactions, salt and water are produced
Acid + Base → Salt + Water
Examples of reaction between acids and bases:
Acids with metal carbonates
- Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water:
Acid + Metal Carbonate → Salt + Carbon Dioxide + Water
Examples of reaction between acids and carbonates:
Properties of Bases
- Bases have pH values of above 7
- A base which is water-soluble is referred to as an alkali
- In basic (alkaline) conditions red litmus paper turns blue and methyl orange indicator turns yellow
- Bases are substances which can neutralise an acid, forming a salt and water
- Bases are usually oxides or hydroxides of metals
- When alkalis react, they gain electrons to form negative hydroxide ions (OH–)
- The presence of the OH– ions is what makes the aqueous solution an alkali
Example: Sodium Hydroxide
NaOH (s) → Na+ (aq) + OH– (aq)
Typical reactions of bases
Bases and acids
- When they react with an acid,a neutralisation reaction occurs
- Acids and bases react together in a neutralisation reaction and produce a salt and water:
Acid + Base → Salt + Water
Examples of reaction between bases and acids:
Alkalis and ammonium salts
- Ammonium salts undergo decomposition when warmed with an alkali
- Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali
- A salt, water and ammonia are produced
Example:
NH4Cl + NaOH →NaCl + H2O + NH3
- This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH4+)
- Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper
- The litmus paper will turn from red to blue if ammonia is present
Neutrality, Relative Acidity & Alkalinity
The pH scale
- The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is
- It goes from 1 – 14 (extremely acidic substances can have values of below 1)
- All acids have pH values of below 7, all alkalis have pH values of above 7
- The lower the pH then the more acidic the solution is
- The higher the pH then the more alkaline the solution is
- A solution with a pH of 7, such as water, is described as being neutral
The pH scale showing acidity, neutrality and alkalinity
Universal indicator
- Universal indicator is a mixture of different indicators which is used to measure the pH
- A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours
The pH scale with the Universal Indicator colours which can be used to determine the pH of a solution
The importance of pH and soil acidity
- Soil pH is analysed to indicate the acidity or alkalinity of soil
- Most plants favour a pH value of between 5 and 8
- Changes in soil which cause a pH to be outside this range adversely affect plant processes resulting in reduced growth and crop yield
- Soils may become acid from acid rain, overuse of fertilisers which contain ammonium salts or by the excessive breakdown of organic matter by bacteria
- Crushed or powdered limestone (calcium carbonate) or lime (calcium oxide) or slaked lime (calcium hydroxide) is added to neutralise the excess acidity in the soil
- The addition process must be carefully monitored though, as if added in excess, further damage could be done if the pH goes too high
Extended Only
Proton Transfer, Weak & Strong Acids & Bases
Proton transfer
- The earlier definition of an acid and a base can be extended
- In terms of proton transfer, we can further define each substance in how they interact with protons
Acids
- Acids are proton donors as they ionize in solution producing protons, H+ ions
- These H+ ions make the aqueous solution acidic
Bases (Alkalis)
- Bases (alkalis) are proton acceptors as they ionize in solution producing OH– ions which can accept protons
- These OH– ions make the aqueous solution alkaline
Diagram showing the role of acids and bases in the transfer of protons
Strong acids and bases
- Acids and alkalis can be either strong or weak, depending on how many ions they produce when dissolved in water
- Strong acids and bases ionize completely in water, producing solutions of very low pH for an acid or very high pH for a base
- Strong acids include HCl and H2SO4 and strong bases include the Group I hydroxides
Weak acids and bases
- Weak acids and bases partially ionize in water and produce pH values which are closer to the middle of the pH scale
- Weak acids include organic acids such as ethanoic acid, CH3COOH and weak bases include aqueous ammonia
- For both weak acids and bases, there is usually an equilibrium set-up between the molecules and their ions once they have been added to water
- Example of a weak acid: propanoic acid
CH3CH2COOH ⇌ H+ + CH3CH2COO-
- Example for a weak base: aqueous ammonia
NH3 + H2O ⇌ NH4+ + OH-
- In both cases the equilibrium lies to the left, indicating a high concentration of intact acid / base molecules, with a low concentration of ions in solution
Effect of concentration on strong and weak acids and alkalis
- A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution
- It does not necessarily mean that the acid or base is strong though, as it may be made from a weak acid or base which does not dissociate completely
- For example a dilute solution of HCl will be more acidic than a concentrated solution of ethanoic acid, since most of the HCl molecules dissociate but very few of the CH3COOH do
