Strong and Weak Acids 1 of 21 Boardworks Ltd 2016
Strong and Weak Acids 2 of 21 Boardworks Ltd 2016
What are acids? 3 of 21 Boardworks Ltd 2016 All acids contain a positive hydrogen ion (H + ) bound to a negative ion (A - ). The general formula of an acid is HA, where H represents the hydrogen ion and A represents the accompanying negative ion. When in solution, the acid ionises. This means that it forms its charged ions, which are released into solution:
Strong acids 4 of 21 Boardworks Ltd 2016 Acids can be strong or weak depending on how much they ionise in solution. Strong acids ionise fully in solution, so that all of their H + ions are released into the mixture. Hydrochloric acid is a strong acid as it fully ionises in solution: Other strong acids include nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ).
Weak acids 5 of 21 Boardworks Ltd 2016 Weak acids do not fully ionise in solution. This is because some of their H + ions stay attached to the acid molecule, (HA). The ionisation of a weak acid is a reversible reaction: Ethanoic acid (vinegar) is a typical weak acid as it does not fully ionise in solution: Other weak acids include citric acid in lemons and carbonic acid (H 2 CO 3 ).
Identifying strong and weak acids 6 of 21 Boardworks Ltd 2016
Strong or weak? 7 of 21 Boardworks Ltd 2016
What does the ph scale show? 8 of 21 Boardworks Ltd 2016 The ph scale runs from 0 to 14. The numbers 0 to 6 represent acidic conditions and 8 to 14 represent alkaline conditions. A ph value 7 is neutral. What are the ph values of some everyday items? 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 increasing acidity increasing alkalinity
ph of strong and weak acids 9 of 21 Boardworks Ltd 2016 ph is a measure of the concentration of H + ions in solution. A lower ph means more H + ions are present in solution. As strong acids ionise fully in solution, they contain many H + ions per molecule of acid, producing a lower ph. Weak acids do not fully ionise. They therefore contain fewer H + ions per molecule of acid and: have higher ph values are worse conductors of electricity react less quickly. higher H + concentration lower H + concentration
Acidity and acid strength 10 of 21 Boardworks Ltd 2016 The acidity of an aqueous solution depends on the number of H + ions present. It is a measure of how acidic the solution is, and indicates the strength of an acid. Acid strength is measured in ph. It is the concentration of H + ions in solution. The higher the concentration of H + ions, the lower the ph, and the stronger the acid. The ph scale also measures alkalinity. As the concentration of OH ions increases, the concentration of H + decreases. This results in an increase in ph.
Neutrality 11 of 21 Boardworks Ltd 2016 A solution is neutral at ph 7 when the concentration of H + ions equals the concentration of OH - ions When an acid reacts with a base, a neutralisation reaction occurs: As the base (OH - ) is added, the number of free hydrogen ions in solution decreases, causing an increase in ph. This means the neutrality of a solution is determined by the concentration of hydrogen ions present.
Diluting acids 12 of 21 Boardworks Ltd 2016 Strong acids have a relatively high concentration of H + ions, and therefore a low ph. When a strong acid is diluted, the concentration of H + falls and the ph rises. This decreases the acid strength. ph H + concentration (mol/dm 3 ) 1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 decreasing H + concentration As the concentration of H + ions decreases by a factor of 10, the ph increases by one unit.
Diluting acids 13 of 21 Boardworks Ltd 2016
Strong and Weak Acids 14 of 21 Boardworks Ltd 2016
Reactivity of strong and weak acids 15 of 21 Boardworks Ltd 2016 A solution of ethanoic acid reacts more slowly than hydrochloric acid of the same concentration. Hydrochloric acid is fully dissociated, so all the hydrogen ions are available to react. Ethanoic acid is partially dissociated, so there are fewer hydrogen ions to react. The lower concentration of H + ions in ethanoic acid leads to a lower collision frequency between reactant molecules, and so a slower reaction.
Acids and carbonates 16 of 21 Boardworks Ltd 2016 When a carbonate is mixed with an acid, the products are a salt, carbon dioxide and water. What would you expect to observe in this reaction? For example, the reaction of nitric acid (a strong acid) and copper carbonate: What difference might you see in the reaction of ethanoic acid and copper carbonate?
Weak acids and carbonates 17 of 21 Boardworks Ltd 2016 As ethanoic acid is a weak acid, only moderate fizzing would be expected. This is because ethanoic acid has fewer H + ions in solution than a strong acid of the same concentration. This means there are fewer collisions occurring with the carbonate and so a slower rate of reaction. What would happen if the concentration of ethanoic acid was increased?
Reacting with metals 18 of 21 Boardworks Ltd 2016
Strong and Weak Acids 19 of 21 Boardworks Ltd 2016
Glossary 20 of 21 Boardworks Ltd 2016
Multiple-choice quiz 21 of 21 Boardworks Ltd 2016