Reactions of acids and alkalis
Acids
Common acidCorrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions. include:
- hydrochloric acid, HCl
- sulfuric acid, H2SO4
- nitric acid, HNO3
Acids produce hydrogen ionElectrically charged particle, formed when an atom or molecule gains or loses electrons., H+, when they dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. in water.
Alkalis
Common alkaliA base which is soluble in water. include:
- sodium hydroxide, NaOH
- potassium hydroxide, KOH
- calcium hydroxide, Ca(OH)2
Alkalis produce hydroxide ions, OH-, when they dissolve in water.
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Neutralisation
Alkalis neutraliseTo be made neutral by removing any acidic or alkaline nature. acids. An indicatorA substance that has different colours, depending upon the pH of the solution it is in. is used to identify the point when neutralisationThe reaction between an acid and a base to form a salt plus water. is reached.
During the reaction, hydrogen ions from the acid react with hydroxide ions from the alkali to form water:
H+(aq) + OH-(aq) \(\rightarrow\) H2O(l)
Salt formation
When an acid reacts with a metal hydroxide, a saltA compound formed by neutralisation of an acid by a base, eg a metal oxide, as the result of hydrogen ions in the acid being replaced by metal ions or other positive ions. Sodium chloride, common salt, is one such compound. and water are produced:
acid + metal hydroxide \(\rightarrow\) salt + water
The name of a salt has two parts:
- the first part comes from the metal in the metal hydroxide
- the second part from the acid used
| Hydrochloric acid | Sulfuric acid | Nitric acid | |
| Potassium hydroxide | Potassium chloride | Potassium sulfate | Potassium nitrate |
| Sodium hydroxide | Sodium chloride | Sodium sulfate | Sodium nitrate |
| Calcium hydroxide | Calcium chloride | Calcium sulfate | Calcium nitrate |
| Potassium hydroxide | |
|---|---|
| Hydrochloric acid | Potassium chloride |
| Sulfuric acid | Potassium sulfate |
| Nitric acid | Potassium nitrate |
| Sodium hydroxide | |
|---|---|
| Hydrochloric acid | Sodium chloride |
| Sulfuric acid | Sodium sulfate |
| Nitric acid | Sodium nitrate |
| Calcium hydroxide | |
|---|---|
| Hydrochloric acid | Calcium chloride |
| Sulfuric acid | Calcium sulfate |
| Nitric acid | Calcium nitrate |
Question
Write a word equation to describe the reaction between hydrochloric acid and sodium hydroxide.
hydrochloric acid + sodium hydroxide \(\rightarrow\) sodium chloride + water
Formulae of salts
To work out the formulaA combination of symbols that indicates the chemical composition of a substance. of a salt you must know the charges of the ions involved.
| Common cations | Charge | Common anions | Charge |
| Hydrogen ion, H+ | +1 | Group 7 ions, eg Cl- | -1 |
| Ammonium ion, NH4+ | +1 | Hydroxide ion, OH- | -1 |
| Group 1 ions, eg K+, Na+ | +1 | Nitrate ion, NO3- | -1 |
| Group 2 ions, eg Mg2+, Ca2+ | +2 | Oxide ion, O2- | -2 |
| Copper(II), Cu2+ | +2 | Carbonate ion, CO32- | -2 |
| Iron(II), Fe2+ | +2 | Sulfate ion, SO42- | -2 |
| Iron(III), Fe3+ | +3 |
| Common cations | Hydrogen ion, H+ |
|---|---|
| Charge | +1 |
| Common anions | Group 7 ions, eg Cl- |
| Charge | -1 |
| Common cations | Ammonium ion, NH4+ |
|---|---|
| Charge | +1 |
| Common anions | Hydroxide ion, OH- |
| Charge | -1 |
| Common cations | Group 1 ions, eg K+, Na+ |
|---|---|
| Charge | +1 |
| Common anions | Nitrate ion, NO3- |
| Charge | -1 |
| Common cations | Group 2 ions, eg Mg2+, Ca2+ |
|---|---|
| Charge | +2 |
| Common anions | Oxide ion, O2- |
| Charge | -2 |
| Common cations | Copper(II), Cu2+ |
|---|---|
| Charge | +2 |
| Common anions | Carbonate ion, CO32- |
| Charge | -2 |
| Common cations | Iron(II), Fe2+ |
|---|---|
| Charge | +2 |
| Common anions | Sulfate ion, SO42- |
| Charge | -2 |
| Common cations | Iron(III), Fe3+ |
|---|---|
| Charge | +3 |
| Common anions | |
| Charge |
transition metalA metal that is located in between group 2 and group 3 (labelled as group 13 on some modern periodic tables) and has brightly coloured compounds. can form ions with different charges, eg iron can form Fe2+ ions and Fe3+ ions. Their charges are shown in the name of the compound, eg iron(II) oxide and iron(III) oxide.
The formula for a compound must produce no overall charge. For example:
- sodium hydroxide is NaOH (one positive charge and one negative charge)
- calcium hydroxide is Ca(OH)2 (two positive charges and two negative charges)
Question
What is the formula of sodium sulfate?
Na2SO4 (two positive charges and two negative charges)
Balanced chemical equations
To write a balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. first write the word equation. Replace the name of each compound with the correct formula.
massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). is conserved during a chemical reaction. No atomThe smallest part of an element that can exist. are created or destroyed, so the numbers of atoms of each element on the left of the equation is the same as on the right. For example:
HCl(aq) + NaOH(aq) \(\rightarrow\) NaCl(aq) + H2O(l)
- sodium chloride is formed from Na+ ions in the alkali and Cl- in the acid
- water is formed from H+ ions in the acid and OH- ions in the alkali
Question
Write a balanced chemical equation for the reaction of dilute sulfuric acid with sodium hydroxide solution.
H2SO4(aq) + 2NaOH(aq) \(\rightarrow\) Na2SO4(aq) + 2H2O(l)