IB Chemistry HL Reactivity 3: What are the mechanisms of chemical change? questions

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Question 1

Multiple choiceMedium1 mark

An aqueous solution of sodium hydrogencarbonate is at 298 K. For $H_2CO_3$ $K_{\mathrm a1}=4.3\times10^{-7}$ and $K_{\mathrm a2}=4.7\times10^{-11}$ $K_{\mathrm w}=1.0\times10^{-14}$ Which option pairs the predominant hydrolysis of $HCO_3^-$ with the expected pH relative to 7?

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Answer: D. $HCO_3^-$ can act as an acid or a base. Its base strength is $K_{\mathrm b}=K_{\mathrm w}/K_{\mathrm a1}=2.3\times10^{-8}$ which is greater than its acid strength, $K_{\mathrm a2}=4.7\times10^{-11}$ The predominant hydrolysis produces $OH^-$ so the solution is basic.

A: Treats hydrogencarbonate as an acid and predicts the acidic effect of producing $H_3O^+$ although this hydrolysis is weaker.
B: Identifies the predominant hydrolysis but reverses the pH effect of producing $OH^-$
C: Predicts a basic solution but pairs it with acid hydrolysis, which produces $H_3O^+$

Remember: for an amphiprotic ion, compare its acid strength with its base strength.

Question 2

Multiple choiceMedium1 mark

A piece of magnesium is added to dilute hydrochloric acid. Which equation represents the reaction?

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Answer: B. Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas. Magnesium chloride has the formula MgCl₂; balancing the atoms gives Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
A: identifies the products but leaves chlorine unbalanced by using one HCl.
C: balances the atoms but gives magnesium the wrong charge in the salt formula, writing MgCl instead of MgCl₂.
D: balances the atoms but represents the hydrogen as individual atoms rather than diatomic hydrogen, H₂.
Remember: hydrogen released in these reactions is H₂(g).

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