9.03.5 Strong and Weak Acids (HT only)
Syllabus
A strong acid is completely ionised in aqueous solution. Examples of strong acids are Hydrochloric, Nitric and Sulphuric acids.
A weak acid is only partially ionised in aqueous solution. Examples of weak acids are Ethanoic, Citric and Carbonic acids.
For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.
As the pH decreases by one unit, the Hydrogen ion concentration of the solution increases by a factor of 10.
Students should be able to:
• use and explain the terms dilute and concentrated (in terms of amount of substance), and weak and strong (in terms of the degree of ionisation) in relation to acids
• describe neutrality and relative acidity in terms of the effect on Hydrogen ion concentration and the numerical value of pH (whole numbers only).
What does this mean?
Strong and Weak Acids
All acids produce H+ ions when they dissolve in water.
But some are better at doing this than others.
When a strong acid dissolves all the acid molecules break up (ionise).
HCl (aq) --> H+ (aq) + Cl- (aq)
You should know that Hydrochloric, Sulphuric and Nitric Acids are all strong (fully ionise).
When a weak acid dissolves only some the acid molecules break up (ionise).
CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)
You should know that Ethanoic Acid (vinegar), Carbonic Acid (sparkling water) and Citric Acid (in lemons, oranges etc)
Dilute and Concentrated.
When you buy squash it is concentrated - lots of flavouring in a small volume of water.
You dilute it (add water) until it tastes right.
This has nothing to do with Strong or Weak - which is all about ionisation.
We could dilute strong acid with a lot of water but it would still be fully ionised so it would still be strong.
We could dissolve lots of Weak acid in a small volume of water but it still won't ionise much so it will still be weak.
Concentration and pH
If we have equal concentrations of a strong acid (Nitric) and a weak acid (Ethanoic) they will not have the same concentration of H+ ions.
The concentration of Hydrogen ions will be much higher in the strong acid, so its pH will be more acidic and therefore lower.
The concentration of Hydrogen ions will be much lower in the weak acid, so its pH will be less acidic and therefore higher (but still less than 7).
Because the concentration of Hydrogen ions in the Strong acid is higher than in the weak acid, it will react faster.
pH and Hydrogen ion concentration
The pH scale doesn't work like other scales you know about.
A difference in concentration of H+ ions of x 10 makes a difference of pH of only 1
pH 3 is more acidic than pH 4 - it must have a higher Hydrogen ion concentration.
It is 2 pH units higher so the Hydrogen ion concentration is 10 x 10 = 100 times higher in pH 3
pH 6 is less acidic than pH 2 - it must have a lower Hydrogen ion concentration.
It is 4 pH units lower so the Hydrogen ion concentration is 10 x 10 x 10 x 10 = 10,000 times lower in pH 6
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Past Paper Questions
2020
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Answer
5.0 Allow 5
2017
