Do you know Acids from Alkalis?
Acids & Alkalis Quiz
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What are Bases?
Bases are substances that can neutralise acids, typically forming a salt. These materials are usually metal oxides or carbonates, such as magnesium oxide. Alkalis are bases that are soluble in water.
Bases are commonly used in cleaning materials like detergents but are also used in some industrial chemical processes and in manufacturing. Acidic solutions are also readily found around the home in substances such as vinegar, though some of the main uses are in industrial and chemical processes. For example, sulfuric acid is used in the manufacturing of fertilisers as well as in more mundane settings such as car batteries.
Formulas for acids and alkailis
Remembering some basic formulas and examples of acids, alkalis and polyatomic ions will help you to understand the equations and make everything seem a bit less daunting. Polyatomic ions are ions that are made up of multiple atoms and they often appear in acids and alkalis. The most important ones to remember for GCSE are carbonate (\(\mathrm{CO_3^{2-}}\)), sulfate (\(\mathrm{SO_4^{2-}}\)), sulfite (\(\mathrm{SO_3^{2-}}\)), nitrate (\(\mathrm{NO_3^{-}}\)), ammonium (\(\mathrm{NH_4^{+}}\)) and hydroxide (\(\mathrm{OH^{-}}\)). These ions are especially useful to remember because they can explain what is happening in neutralisation reactions between acids and bases. Just take a look at the sulfate ion (\(\mathrm{SO_4^{2-}}\)) — it is not a coincidence that sulfuric acid has the formula \(\mathrm{H_2SO_4}\). Sulfuric acid is formed by two positive hydrogen ions which have lost their electrons coming together with the sulfate ion.
Another example of why the polyatomic ions are useful to remember is the formulas for metal hydroxides which are bases. These have formulas that include the hydroxide ion. For instance, \(\mathrm{NaOH}\) is the formula for sodium hydroxide and if you understand that it was formed when a sodium ion came together with a hydroxide ion, you can understand the following reaction:
\(\mathrm{NaOH_{(aq)} \rightarrow Na^{+}_{(aq)} + OH^{-}_{(aq)}}\)
Overall, remembering the formulas for the basic polyatomic ions as well as for some common acids and alkalis can really help you along.
What are Strong and Weak Acids?
Strong acids completely ionise or dissociate in water, whereas weak acids partially ionise in water. Hydrochloric acid is a commonly used strong acid which completely dissociates in water as shown by the equation below:
\(\mathrm{HCl_{(aq)} \rightarrow {H^{+}}_{(aq)} + {Cl^{-}}_{(aq)}}\)
The equation demonstrates that hydrochloric acid breaks down (dissociates/ionises) into its component ions in the presence of water. An example of a weak acid is carbonic acid and, as shown by the equation below, it only partially dissociates in water:
\(\mathrm{H_{2}CO_{3(aq)} \rightleftharpoons 2{H^{+}}_{(aq)} + {CO_{3}^{2-}}_{(aq)}}\)
The equation shows a partial dissociation of carbonic acid to its ions. This is demonstrated by the reversible reaction symbol, which tells us the reaction is continuously going back and forth. Note the symbols after each compound — these are state symbols which represent the state of matter that the compound is currently in. Regardless of the specific acid or alkali involved, the overall ionic equation for a neutralisation reaction is:
\(\mathrm{{H^{+}}_{(aq)} + {OH^{-}}_{(aq)} \rightarrow H_{2}O_{(l)}}\)
This shows that a hydrogen ion from the acid reacts with a hydroxide ion from the base to form water. The spectator ions — those that do not change during the reaction — are omitted to give this simplified ionic equation.
What is the difference between pH and strength?
Acidity and alkalinity are measured by the pH scale which is logarithmic, so each step up or down the scale means a difference of 10x. Acidic solutions have a pH of less than 7, neutral solutions have a pH of 7 and alkaline solutions have a pH of more than 7.
The concentration of an acid or alkaline solution is related to the number of hydrogen or hydroxide ions per unit volume, this means that even weak acids such as citric acid can be the same pH as strong acids such as hydrochloric acid.
Strength is a measure of how much acids or alkalis ionise or dissociate in water.
What are Strong and Weak Alkalis?
Strong alkalis, just like strong acids, completely ionise or dissociate in water, whereas weak alkalis partially ionise in water. Check below to see the formula for sodium hydroxide which is a strong alkali.
Ammonia is an example of a weak alkali, as shown by the equation below:
ammonia + water ⇌ ammonium + hydroxide
\(\mathrm{NH_{3(g)} + H_{2}O_{(l)} \rightleftharpoons {NH_{4}^{+}}_{(aq)} + {OH^{-}}_{(aq)}}\)
Is sodium hydroxide a strong or weak alkali?
Sodium hydroxide is an example of a strong alkali since it completely ionises in water as shown by the below equation.
\(\mathrm{NaOH_{(aq)} \rightarrow {Na^{+}}_{(aq)} + {OH^{-}}_{(aq)}}\)
If you want to learn more about acids and alkalis and test your knowledge, check out our quizzes below.