Reactions of acids and alkalis

Acids

Common acids include:

  • hydrochloric acid, HCl
  • sulfuric acid, H2SO4
  • nitric acid, HNO3

Acids produce hydrogen ions, H+, when they dissolve in water.

Alkalis

Common alkalis include:

  • sodium hydroxide, NaOH
  • potassium hydroxide, KOH
  • calcium hydroxide, CaOH

Alkalis produce hydroxide ions, OH-, when they dissolve in water.

Neutralisation

Alkalis neutralise acids. An indicator is used to identify the point when neutralisation 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 salt 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 acidSulfuric acidNitric acid
Potassium hydroxidePotassium chloridePotassium sulfatePotassium nitrate
Sodium hydroxideSodium chlorideSodium sulfateSodium nitrate
Calcium hydroxideCalcium chlorideCalcium sulfateCalcium 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 formula of a salt you must know the charges of the ions involved.

Common cationsChargeCommon anionsCharge
Hydrogen ion, H++1Group 7 ions, eg Cl--1
Ammonium ion, NH4++1Hydroxide ion, OH--1
Group 1 ions, eg K+, Na++1Nitrate ion, NO3--1
Group 2 ions, eg Mg2+, Ca2++2Oxide ion, O2--2
Copper(II), Cu2++2Carbonate ion, CO32--2
Iron(II), Fe2++2Sulfate ion, SO42--2
Iron(III), Fe3++3

Transition metals 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 equation first write the word equation. Replace the name of each compound with the correct formula.

Mass is conserved during a chemical reaction. No atoms 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)