Reactions of Acids

Acids are substances which release hydrogen ions in solution. Their readiness to release hydrogen ions determines the strength of the acid and where it falls on the pH scale - stronger acids have a lower pH. Try to memorise the equations for acid reactions for your exam!

Reactions of acids and metals

Metals react with acids to form a salt and hydrogen. For example, when sodium reacts with hydrochloric acid, sodium chloride and hydrogen gas are formed. You can test for the presence of hydrogen gas by placing a lit splint near the test tube. Hydrogen gas will cause the splint to burn with a squeaky pop.

The reaction between acids and metals are examples of redox reactions. Have a look at the equation above. Magnesium is converted into a positive magnesium ion (remember that a salt is an ionic compound, made up of positive metal ions and negative non-metal ions). To form a positive ion, magnesium has lost its two outer electrons so it has been oxidised. Hydrogen has been reduced because it has been converted from a positive hydrogen ion (HCl is made up of H⁺ and Cl^-) to a neutral hydrogen molecule. It has gained electrons to be converted from H⁺ to H₂.

Neutralisation reactions

Acids are neutralised by alkalis / bases to produce a salt and water. A base is simply a substance which can react with an alkali to neutralise it. An alkali is a type of base which is soluble in water. You can recognise alkalis because they will contain hydroxide ions (e.g. sodium hydroxide, potassium hydroxide etc). All alkalis are bases but not all bases are alkalis.

Acids can also be neutralised by metal carbonates to form a salt, water and carbon dioxide. You can test for the presence of carbon dioxide by bubbling it through limewater and see whether the solution turns cloudy.

The particular salt produced in a reaction between a base or a carbonate depends on the acid that you use.

Hydrochloric acid produces chloride salts
Sulfuric acid produces sulfate salts
Nitric acid produces nitrate salts

This gives us the second part of the salt’s name. The first part comes from the metal that we’ve reacted. For example, if I react lithium with hydrochloric acid, lithium chloride will be formed. If I react magnesium with sulfuric acid, I’ll get magnesium sulfate.

Soluble salts

Soluble salts can be prepared by reacting an acid with an insoluble base (usually a metal oxide or metal hydroxide). The method is as follows:

Heat the acid in a water bath to speed up the rate of reaction.
Once the acid is warm, add the insoluble base. Add the solid base bit by bit, until there is some unreacted solid left over (it is in excess).
The acid and base will have reacted to produce a soluble salt and water. Filter to remove the excess unreacted base to leave a solution containing only the salt and water.
Evaporate off the water by gently heating the solution. Allow to cool and the soluble salt will crystallise.
Filter the solid salt, wash and dry.

pH scale

The pH scale goes from 0 to 14. A very strong acid will have a pH of 0-1 whereas a very strong alkali will have a pH of 13-14. Pure water which is a neutral substance will have a pH of 7.

Universal indicator can be used as a more precise measurement of pH. It gives a range of different colours depending on the pH of a solution (shown above) which can be compared to a colour chart to give a more accurate pH value.

Acids are a source of hydrogen ions (H+) and alkalis are a source of hydroxide ions (OH-). A base is something which will become an alkali when added to water. For example, magnesium oxide is a base because it will dissolve in water to form magnesium hydroxide (Mg(OH)₂) which is an alkali because it is a source of hydroxide ions.

Alkalis can neutralise acids (or vice versa) - hydrogen ions combine with hydroxide ions to form water, forming a neutral solution. The equation for the reaction is:

Titrations (triple science)

Titrations are used to find out the concentration of an acid or an alkali. Let’s say we have an acid of an unknown concentration, we can react it with an alkali of known concentration and record the volume needed for neutralisation to calculate its concentration. The method for a titration is described below:

Using a pipette, add a measured volume of alkali into a conical flask along with a few drops of indicator.
Fill a burette with acid and record the starting volume.
Turn the tap on the burette to slowly add acid to the alkali until the indicator changes colour (neutralisation has occurred at this point).
Record the volume of acid added to the alkali.
Repeat the titration until you get concordant titres - results that are very similar to each other so that you know you have reliable data.
Titrations also work the other way round, where alkali is added to the acid.

Worked example: titration calculation

A student titrated 60 cm³ of hydrochloric acid to 1 mol dm^-3 solution of sodium hydroxide, which had a volume of 30 cm³. Calculate the concentration of the hydrochloric acid.

The first thing we need to do is calculate the moles of the substance with a known concentration. In this case, it's the sodium hydroxide. To calculate moles, we need to convert cm³ to dm³ by dividing by 1000. Then we can multiply concentration and volume to find out the moles.
30/1000 = 0.03 dm³ x 1 mol dm^-3 = 0.03 mol
The next thing is to write an equation for the reaction. Here, the equation is: HCl + NaOH --> NaCl + H2O
From the equation we can see that there is a 1 : 1 ratio between the acid and alkali, so if we have 0.03 mol of NaOH then we also have 0.03 mol of HCl.
Now that we know the moles and volume of HCl, we can work out its concentration. 0.03 / (60/1000) = 0.5 mol dm³.

Strong and weak acids (triple science)

Strong acids, like hydrochloric acid, completely ionise in aqueous solutions. This means that every single HCl molecule breaks apart into hydrogen ions and chloride ions. This is what gives it such a low pH, since it releases hydrogen ions more readily than weaker acids. Nitric acid and sulfuric acid are also examples of strong acids.

A weak acid, such as ethanoic acid, only partially ionises in aqueous solutions. This means that some ethanoic acid molecules will release a hydrogen ion but others will remain completely intact. We represent partial ionisation of weak acids using a double-headed arrow. Other weak acids include citric acid and carbonic acid. You find these acids in various food items – ethanoic acid is an ingredient in vinegar, citric acid is found in citrus fruits and carbonic acid is found in fizzy drinks. The fact that these acids only partially ionise means that they are safe to consume. Start gulping hydrochloric acid and you’re in trouble.

The stronger the acid (i.e. the more readily it releases hydrogen ions), the lower the pH. This is why hydrochloric, sulfuric and nitric acids have a very low pH (around 1-2) whereas ethanoic and citric acids have a higher pH (around 3-6). As the pH decreases by 1 unit, the hydrogen ion concentration of the solution increases by a factor of 10. For example, a solution with a pH of 4 has 10x as many hydrogen ions compared to a solution with a pH of 5. A pH difference of 2 means that there is 100x difference in hydrogen ion concentration and a pH difference of 3 means a 1000x difference in hydrogen ion concentration, and so on.

Did you know…

In ancient Egypt. a mixture of acacia bark and honey was inserted into the vagina as a contraception. This actually worked - the acacia ferments into lactic acid, which creates a low pH and kills any sperm. Women were even buried with the mixture to prevent becoming pregnant in the afterlife. Later, in the 17th and 18th centuries, women inserted half a squeezed lemon into their cervix as a contraceptive. The citric acid worked in the same way, killing any sperm and preventing the woman from conceiving.

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