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Higher tier

Strong and weak acids

Before starting a discussion on weak and strong acids you should already know that acidity is a property of water. Acids and alkalis are solutions with an excess of H+(aq) and OH-(aq) in water, the state symbol (aq)- short for aqueous or water is telling us that acids and alkalis are all aqueous solutions, this is what the (aq) state symbol is telling you. Acids are substances which dissolve in water to release an excess of hydrogen ions (H+(aq)). You may see the formula for an acid often simplified to HA. Now when an acidic is added to water there are two reactions that can happen; these are:

Reaction 1: HAH+(aq) + A-(aq)
Reaction 2: HAH+(aq) + A-(aq)
(Note the water has been missed out in these equations just to simplify things)

At first glance these two equations might seem similar but in fact they are quite different. The reason for these differences is due to the type of arrow used in the equations; that is → or ⇌.

Strong and weak acids

Students working in the lab

Strong acids which include hydrochloric, sulfuric and nitric acids are acids which fully dissociate or break up in water; that is they are 100% dissociated into ions. This means when they are added to water the molecules that make them up fully break-up or dissociate or ionise into hydrogen ions and an anion (a negatively charged ion) e.g. hydrochloric acid is made by dissolving hydrogen chloride gas in water. Hydrogen chloride gas is very soluble in water and when it dissolves it completely dissociates or breaks up or ionises to form hydrogen ions (H+(aq)) and chloride ions (Cl-(aq)).

HCl(g)H+(aq) + Cl-(aq)
(I have left out the water to simplify the equation)

The hydrogen chloride gas consists of small covalent molecules which break apart and form ions when it dissolves in water. All of the hydrogen chloride molecules dissociate in water, that is the reaction goes to completion. Hydrochloric acid is a strong acid because it is fully dissociated into H+(aq) and Cl-(aq).

Strong alkalis are the same; they are made when a base dissolves and fully dissociates in water to release an excess of hydroxide ions. Metal oxides from group I of the periodic table such as sodium oxide and potassium oxide are strong bases e.g. they dissolve completely in water and dissociate or ionise fully to give a solution of sodium ions and hydroxide ions:

NaOH(s) Na +(aq) + OH-(aq)
(I have left out the water to simplify the equation).

Recall that strong acids have low a pH of 1, 2 or 3 and strong alkalis have a pH of 12, 13 and 14. This is shown on the colour chart for universal indicator below.

Colour chart for universal indicator showing the pH of strong and weak acids and alkalis.

Weak acids are formed when a substance only partly ionises or dissociates when it dissolves in water. This means that when the weak acid molecules are dissolved in water very few of them break apart or dissociate. That is to say the reactions as mentioned earlier are reversible and consist of equilibrium mixtures of reactants and products. This means that the acid is NOT fully ionised. Weak acids include carbonic acid and carboxylic acids such as ethanoic and methanoic acid e.g. when the carboxylic acid ethanoic acid is added to water it form a weak acid. Not many of the ethanoic acid molecules breaks apart or dissociate when it dissolves in water and almost all of the ethanoic acid molecules remain intact with only a very few actually breaking up to form ions, we can show this as:

CH3COOH(l) ⇌ CH3COO-(aq) + H+(aq)

The important point is that there a very few hydrogen ions (H+(aq)); most of the molecules of the ethanoic acid remain intact. The image below summarises the differences between weak and strong acids in terms of their ability to ionise in water.

diagram to show how strong and weak acids dissociate in water

It is a similar story with weak bases. Ammonia for example dissolves readily in water to form a weakly alkaline solution of ammonium hydroxide.

NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
Again this is an equilibrium mixture and does not go to completion as is the case with strong acids and bases.

pH scale

The pH scale as you probably already guessed from the letters p and H has something to do with hydrogen ions. The pH scale is a measure of the concentration of hydrogen ions (H+(aq)) in a solution. The lower the pH the higher the concentration of hydrogen ions present. In fact every time the pH changes by 1 the concentration of hydrogen ions changes by a factor of x10. This means that a solution with a pH of 1 has 10 times more hydrogen ions present than a solution of pH2.

Concentration and pH

It can sometimes be confusing when discussing concentrations of solutions of weak and strong acids and alkalis. For example: The difference of almost nearly 2 pH values means that the concentration of hydrogen ions in the ethanoic acid is almost 100 times less than in the hydrochloric acid but the labels on the bottles are both 0.1 mol/dm3.
For the hydrochloric acid; which is a strong acid that is completely dissociated in water the concentration of 0.1 mol/dm3 refers to the concentration of hydrogen ions. For the ethanoic acid; a weak acid which is only partly dissociated in water the concentration refers to the "undissociated" CH3COOH molecules and NOT to the small number of hydrogen ions present in the solution. So take care with concentrations when you are considering weak and strong acids and alkalis.

Concentrated and dilute orange juice are used to explain the difference between strong and concentrated acids. Another area which often causes confusion is with strong acids and concentrated acids. Some students get these terms mixed up. A hydrochloric acid solution with a concentration of 0.1mol/dm3 is a fairly dilute solution; that is it contains mostly water and only a little acid. A hydrochloric acid solution with a concentration of 5 mol/dm3 is much more concentrated than the 0.1 mol/dm3 solution, that is it contains less water and more acid. However in both solutions the acid is a strong one that is fully dissociated in the water.

Strong and weak refers to how ionised or dissociated the acid molecules are. Concentrated or dilute refers to how much water is present. If the acid is dilute then it contains a large amount of water and as the concentration rises the amount of water present reduces. Ethanoic acid with a concentration of 5 mol/dm3 is a concentrated solution of a weak acid.

Reactions of strong and weak acids

The reactions of weak and strong acids are similar but with a weak acid they are slower and less violent. This is simply because the characteristic reactions of acids are based on the concentrations or number of hydrogen ions (H+(aq)) present and since weak acids have less hydrogen ions present their reactions will be slower. The diagram below shows this by comparing the reactions of hydrochloric (a strong acid) and ethanoic (a weak acid) acids with marble chips.

Comparing the reactions of weak and strong acid with marble chips.

Key Points

Practice questions

Check your understanding - Questions on weak and strong acids

Check your understanding - Aditional questions on weak and strong acids

Check your understanding - Quick quiz on weak and strong acids