(a) Define (i) an acid, (ii) a base, according to the Arrhenius theory - Leaving Cert Chemistry - Question 7 - 2011
Question 7
(a) Define (i) an acid, (ii) a base, according to the Arrhenius theory.
(b) Give one example of a common household acid and one example of a common household base.
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Worked Solution & Example Answer:(a) Define (i) an acid, (ii) a base, according to the Arrhenius theory - Leaving Cert Chemistry - Question 7 - 2011
Step 1
Define (i) an acid, (ii) a base, according to the Arrhenius theory.
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Answer
An Arrhenius acid is defined as a substance that produces H+ ions in solution. For example, hydrochloric acid (HCl) dissociates in water to produce H+ ions.
An Arrhenius base is defined as a substance that produces OH− ions in solution. An example of this is sodium hydroxide (NaOH), which dissociates in water to provide OH− ions.
Step 2
Give one example of a common household acid and one example of a common household base.
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A common household acid is vinegar, which primarily contains acetic acid (CH₃COOH). An example of a common household base is baking soda (sodium bicarbonate, NaHCO₃).
Step 3
Explain the term neutralisation. Give one everyday example.
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Neutralisation is the chemical reaction between an acid and a base to produce a salt and water. In this process, the H+ ions from the acid react with the OH− ions from the base to form water (H₂O).
One everyday example of neutralisation is the use of antacids, which are basic substances taken to neutralise excess stomach acid, thereby relieving issues like heartburn.
Step 4
Define pH.
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pH is defined as the negative logarithm of the hydrogen ion concentration in a solution. It is expressed mathematically as:
pH=−extlog10[H+]
This scale ranges typically from 0 to 14, where a pH of 7 is considered neutral.
Step 5
Calculate the pH of (i) 0.1 M hydrochloric acid (HCl), (ii) 0.1 M sulfuric acid (H2SO4).
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Answer
(i) For 0.1 M hydrochloric acid (HCl), which fully dissociates:
pH=−extlog10[0.1]=1
(ii) For 0.1 M sulfuric acid (H₂SO₄), which dissociates as:
ightarrow 2 H^+ + SO_4^{2-}$$
So, the concentration of $H^+$ ions would be $0.2 ext{ M}$. Therefore:
$$pH = - ext{log}_{10} [0.2]
ightarrow 0.7$$
Step 6
Give one limitation of the pH scale.
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Answer
One limitation of the pH scale is that it is not accurate for concentrated solutions or outside the 0 to 14 range. In very strong acidic or basic solutions, the pH scale may not provide an accurate measure of the solution's acidity or basicity.
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