- Describe the characteristic properties of acids as in reactions with metals, bases and carbonates.
- Describe the characteristic properties of bases in reactions with acids and with ammonium salts.
- Describe the reaction between hydrogen ions and hydroxide ions to produce water, H+ + OH– → H2O, as neutralisation.
- Classify oxides as acidic, basic, amphoteric or neutral based on metallic/non-metallic character.
Recall that an acid is a substance that dissociates in water to produce hydrogen (H+) ions whereas an base is a substance which can neutralize an acid, but may or may not be soluble in water. An alkali is a form of base which is soluble in water and dissociates to release the hydroxide ion (OH-). The H+ and OH– ions not only give an acid or alkali its physical property, it is also responsible for their chemical properties.
The most simple chemical reaction involving an acid and base is the neutralisation reaction. Neutralisation can be simply represented as such:
Acid + Base (soluble/non-soluble) ==> Salt + Water
Specifically, for a neutralisation between an acid and alkali, the H+ ions in the acid solution reacts with the OH– in the alkali, or vice versa, to produce H2O molecules. In other words, the H+ ions is neutralised by the OH– ions (or vice versa).
When the number of H+ ions is equal to the number of OH– ions, complete neutralisation can occur and the final solution is neutral, with a pH of 7. Otherwise, if there is a higher number of H+ or OH– ions to begin with, there will be a higher number of H+ or OH– at the end of the neutralisation, resulting in a pH smaller or larger than 7, respectively.
Examples of neutralisation reaction:
NaOH (aq) + HCl (aq) ==> NaCl (aq) + H2O (l)
CuO (s) + HCl (aq) ==> CuCl2 (s) + H2O (l)
Ca(OH)2 (s) + 2HNO3 (aq) ==> Ca(NO3)2 (aq) + 2H2O (l)
Acid can also react with reactive metals, such as magnesium and zinc to make a salt and hydrogen.
Acid + Metal ==> Salt + Hydrogen
The hydrogen produced caused bubbling in the acid solution during the reaction. It can be detected using a lighted splint, which causes the hydrogen ignites with a ‘pop’ sound.
Examples of acid and metal reaction:
Ca (s) + 2HNO3 (aq) ==> Ca(NO3)2 (aq) + H2 (g)
Mg (s) + 2HCl (aq) ==> MgCl2 (s) + H2 (g)
When an acid reacts with carbonates, a salt, water and carbon dioxide are produced.
Acid + Carbonate ==> Salt + Water + Carbon Dioxide
The carbon dioxide produced caused bubbling in the acid solution during the reaction and can be detected using limewater. Limewater turns milky white when carbon dioxide is bubbled through it.
Examples of acid and carbonate reaction:
Na2CO3(s) + HCl (aq) ==> 2NaCl (aq) + CO2 (g) + H2O (l)
ZnCO3 (s) + 2HNO3 (aq) ==> Zn(NO3)2 (aq) + H2O (l) + CO2 (g)
When a ammonium salt is added to an alkali, a salt, water and ammonia gas is produced.
Alkali + Ammonium Salt ==> Salt + Water + Ammonia Gas
Ammonia has a characteristic sharp, choking smell and turns damp red litmus paper turn blue. It also forms a white smoke of ammonium chloride when hydrogen chloride gas, from concentrated hydrochloric acid, is held near it.
Examples of alkali and ammonium salt reaction:
NH4Cl (s) + NaOH (aq)→ + NaCl (aq)+ H2O (l)+ NH3 (g)
The name of salt produced in a chemical reaction can be predicted from the identity of the reagents used. The first part of the name is ‘ammonium’ if the base used is ammonia. Otherwise, it is the name of the metal in the base/carbonate (or of the metal used). The second part of the name comes from the acid used. An example of the naming of salt created from the neutralisation reaction is shown below.
Oxides are chemical compounds with one or more oxygen atoms combined with another element.
In general, an oxide that combines with water to give an acid or a base are known as an acidic or basic oxide, respectively. The acidic and basic oxide often exist in the anhydride form, which is that they are a solid which that assimilate H2O to form either an acid or a base. On the other hand, an oxide which combines with water to give a solution which can act both as an acid or base is known as an amphoteric oxide. An oxide which shows neither basic nor acid properties is known as a neutral oxide.
How will I be able to determine the specific characteristic of an oxide if I am not able to conduct an experiment on the oxide?
The characteristic of an oxide can also be determined by looking at the other element, not the oxygen, that is found within the oxide compound.
Acid oxides are the oxides of non-metals (Periodic table Groups 14-17). When added into water, acid oxides form a acidic solution.
These acid anhydrides combine with bases to produce salt and water, similar to how an acid will react with a base. For example:
Basic oxides on the other hand, are oxides of metals (from Group I or II of Periodic table). If soluble, they react with water to produce hydroxides.
These metallic oxides or basic anhydrides react with acid to form a salt and water.
Amphoteric oxides are metallic oxides, which show both basic and acidic properties. A common example of amphoteric oxide is aluminium oxide.