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

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Question 1

Multiple choiceEasy1 mark

Four monoprotic acids are prepared as aqueous solutions at the same temperature and with the same initial concentration. Which sample contains the weakest acid?

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Answer: B. At equal initial concentration and temperature, the weakest acid ionizes to the smallest extent. Sample S has the smallest ionized fraction, 5%, so it is the weakest.

A: Q is partially ionized, but choosing it ignores the still smaller ionized fraction for S.
C: P has the greatest ionization, 100%; this reverses the relationship and identifies the strongest acid.
D: R has a smaller ionized fraction than Q, but its 20% exceeds S’s 5%.

Remember: compare the extent of ionization, not the initial concentration.

Question 2

Multiple choiceMedium1 mark

A voltaic cell operates by the reaction $\mathrm{Mg(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2(g)}$ Initially, $0.0040\ \mathrm{mol}$ Mg and $0.0060\ \mathrm{mol}$ HCl are available. What amount of $\mathrm{H_2}$ forms, and what is the sign of the electrode where it forms?

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Answer: D. The equation requires $2\ \mathrm{mol}$ HCl per $1\ \mathrm{mol}$ H₂, so HCl limits the reaction: $n(\mathrm{H_2})=0.0060/2=0.0030\ \mathrm{mol}$ H⁺ is reduced at the hydrogen electrode, making it the cathode; the cathode is positive in a voltaic cell.
A: gives the correct amount but assigns the hydrogen electrode the negative sign.
B: uses the amount of Mg as though it all reacts, despite insufficient HCl.
C: uses the Mg amount without checking the limiting reagent and also assigns the cathode a negative sign.
Remember: in a voltaic cell, reduction occurs at the positive cathode.

Question 3

Multiple choiceMedium1 mark

Four oxygen-containing species are compared: OH, OH⁻, H₂O and H₃O⁺. Which analysis correctly identifies a radical and predicts its reactivity from its electron arrangement?

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Answer: C. Neutral OH has 6 + 1 = 7 valence electrons. Its Lewis representation has one unpaired electron, so it is a radical and is expected to be highly reactive.

A: omits the extra electron from the negative charge; OH⁻ has 8 valence electrons, all paired. B: counts H₂O correctly but mistakes paired lone-pair electrons for an unpaired electron. D: fails to subtract one electron for the positive charge; H₃O⁺ has 8 valence electrons, all paired.

Remember: add an electron for a negative charge and subtract one for a positive charge.

Question 4

Multiple choiceMedium1 mark

The displayed equation shows a reaction between hydrogen chloride and hydroxide ions. Which species donates both electrons to the new bond, and which atom receives the donated pair?

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Answer: D. A nucleophile supplies both electrons in the new bond. The figure shows an O—H bond in water and the loss of the H—Cl bond; therefore O in OH⁻ supplies the pair to H in HCl.

A: Reverses the donor role: HCl provides the H atom, but does not supply the pair that forms the new bond.
B: Identifies OH⁻ as the donor but mistakes the Cl atom, which leaves as chloride, for the atom receiving the pair.
C: Treats the negatively charged chloride product as the nucleophile; it is formed after the O—H bond is made.

Remember: identify a nucleophile by the source of the bonding pair, not merely by its charge.

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