Reactivity: What Are the Mechanisms of Chemical Change? — Free Chemistry SL Practice Questions

Glycine ($\text{NH}_2\text{COOH}$) is the simplest amino acid. In aqueous solution, glycine exists a zwitterion: $^+\text{NH}_3\text{CH}_2\text{COO}^-$. A chemist titrates $25.0\,\text{cm}^3$ of a $0.200\,\text{mol}\,\text{dm}^{-3}$ solution of glycine hydrochloride ($^+\text{NH}_3\text{CH}_2\text{COOH}\cdot\text{Cl}^-$, the fully protonated form) with $0.200\,\text{mol}\,\text{dm}^{-3}$ NaOH. $$pK_{a1}\ (\text{carboxyl group, } {-}\text{COOH}) = 2.34$$ $$pK_{a2}\ (\text{ammonium group, } {-}^+\text{NH}_3) = 9.60$$ $$K_w = 1.0 \times 10^{-14}\ \text{at } 298\,\text{K}$$

A student investigates the acid-base properties of two unknown weak acids, HA and HB, using a pH meter and a burette. The student prepares $25.0\,\text{cm}^3$ samples of each acid at a concentration of $0.100\,\text{mol dm}^{-3}$ and titrates each separately with $0.100\,\text{mol dm}^{-3}$ sodium hydroxide (NaOH) solution. The following data are recorded: - For acid HA: the pH at the half-neutralisation point (when $12.5\,\text{cm}^3$ of NaOH has been added) is $4.76$. - For acid HB: the pH at the half-neutralisation point (when $12.5\,\text{cm}^3$ of NaOH has been added) is $3.35$. - The total volume of NaOH required to reach the equivalence point for both acids is $25.0\,\text{cm}^3$.

A student performs a conductometric titration of $25.0\,\text{cm}^3$ of $0.100\,\text{mol}\,\text{dm}^{-3}$ weak monoprotic acid HX with $0.100\,\text{mol}\,\text{dm}^{-3}$ sodium hydroxide (NaOH). Conductivity is measured after each addition of NaOH. The student observes that conductivity decreases initially, reaches a minimum, then increases sharply.

A chemist studies the neutralisation of two bases with hydrochloric acid (HCl, $0.100\,\text{mol dm}^{-3}$).

Vanadium(V) oxide ($\text{V}_2\text{O}_5$) is used as a catalyst in the Contact Process for the manufacture of sulfuric acid. In one step of the catalytic cycle, $\text{V}_2\text{O}_5$ is reduced to vanadium(IV) oxide ($\text{VO}_2$) by sulfur dioxide ($\text{SO}_2$), producing sulfate ions ($\text{SO}_4^{2-}$). In a subsequent step, $\text{VO}_2$ is re-oxidised back to $\text{V}_2\text{O}_5$ by oxygen gas. Standard electrode potentials at $298\,\text{K}$: $$\text{SO}_4^{2-}(\text{aq}) + 4\text{H}^+(\text{aq}) + 2e^- \rightleftharpoons \text{SO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l}) \quad E^\circ = +0.17\,\text{V}$$ $$\text{V}_2\text{O}_5(\text{s}) + 2\text{H}^+(\text{aq}) + 2e^- \rightleftharpoons \text{V}_2\text{O}_4(\text{s}) + \text{H}_2\text{O}(\text{l}) \quad E^\circ = +1.00\,\text{V}$$ The Contact Process operates at approximately $450\,°\text{C}$.

A student adds $5.00\,\text{g}$ of zinc powder to $100.0\,\text{cm}^3$ of $0.500\,\text{mol dm}^{-3}$ copper(II) sulfate solution. $$\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}$$ The student observes the blue colour of the solution fading and a brown solid forming.

A student investigates the redox reaction between acidified potassium manganate(VII) and iron(II) sulfate in aqueous solution. A solution of $\text{FeSO}_4$ of unknown concentration is titrated against a standard $0.0200\,\text{mol dm}^{-3}$ solution of $\text{KMnO}_4$ under acidic conditions. $$\text{MnO}_4^-(aq) + 5\text{Fe}^{2+}(aq) + 8\text{H}^+(aq) \rightarrow \text{Mn}^{2+}(aq) + 5\text{Fe}^{3+}(aq) + 4\text{H}_2\text{O}(l)$$ The student records the following titration data. In each trial, $25.00\,\text{cm}^3$ of $\text{FeSO}_4$ solution is measured by pipette into the conical flask. Trial — Burette reading / $\text{cm}^3$ 1 — 22.40 | 22.10 | 3 — 22.20 4 — 22.15

A student constructs a voltaic cell with a zinc electrode in $1.00\ \text{mol dm}^{-3}$ $\text{ZnSO}_4\text{(aq)}$ and a copper electrode in $1.00\ \text{mol dm}^{-3}$ $\text{CuSO}_4\text{(aq)}$, connected by a salt bridge containing $\text{KNO}_3\text{(aq)}$. $$E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\ \text{V} \qquad E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\ \text{V}$$ $$M(\text{Zn}) = 65.4\ \text{g mol}^{-1} \qquad F = 96\,500\ \text{C mol}^{-1}$$ After operating for some time, the mass of the zinc electrode decreases by $0.327\ \text{g}$, and the blue colour of the $\text{CuSO}_4$ solution is observed to fade near the copper electrode.

Hydrogen peroxide, $\text{H}_2\text{O}_2$, can act as both an oxidizing agent and a reducing agent. In acidic solution, hydrogen peroxide reacts with iodide ions, $\text{I}^-$, according to the following half-equations: Half-equation 1: $\text{H}_2\text{O}_2\text{(aq)} + 2\text{H}^+\text{(aq)} + 2e^- \rightarrow 2\text{H}_2\text{O}\text{(l)}$ Half-equation 2: $2\text{I}^-\text{(aq)} \rightarrow \text{I}_2\text{(aq)} + 2e^-$

Hydrogen peroxide, $\text{H}_2\text{O}_2$, is used as a disinfectant and bleaching agent. Its structural formula is H–O–O–H, containing one O–O bond and two O–H bonds. Bond enthalpies: $\text{O–O} = +146\,\text{kJ mol}^{-1}$; $\text{O–H} = +463\,\text{kJ mol}^{-1}$

Silicon dioxide ($\text{SiO}_2$) and carbon dioxide ($\text{CO}_2$) are both oxides of Group 14 elements. Silicon dioxide exists a giant covalent lattice in which each silicon atom is bonded to four oxygen atoms. Carbon dioxide exists as discrete molecules. Selected bond

Ozone ($\text{O}_3$) in the stratosphere absorbs harmful ultraviolet radiation. The bond between the central oxygen atom and one of the terminal oxygen atoms in ozone is intermediate in length between a typical $\text{O}{-}\text{O}$ single bond ($148\,\text{pm}$) and a typical $\text{O}{=}\text{O}$ double bond ($121\,\text{pm}$). The observed $\text{O}{-}\text{O}$ bond length in ozone is $128\,\text{pm}$.

Methane ($\text{CH}_4$) and ammonia ($\text{NH}_3$) are both covalent molecules whose central atoms are from Period 2.

Dinitrogen ($\text{N}_2$) and dioxygen ($\text{O}_2$) are both diatomic molecules found in air. $\text{N}_2$ has a triple bond ($\text{N}{\equiv}\text{N}$) while $\text{O}_2$ has a double bond ($\text{O}{=}\text{O}$).

Ammonia ($\text{NH}_3$) can act as a Lewis base and form a coordinate bond with boron trifluoride ($\text{BF}_3$), producing the adduct $\text{H}_3\text{N} \!\cdot\! \text{BF}_3$.

In the complex ion hexaaquairon(III), $[\text{Fe}(\text{H}_2\text{O})_6]^{3+}$, each water molecule is bonded to the central iron ion via a coordinate covalent bond.

The hexaaquairon(III) ion, $[\text{Fe}(\text{H}_2\text{O})_6]^{3+}$, undergoes stepwise ligand substitution with thiocyanate ions, $\text{NCS}^-$, in acidic solution.

Carbon monoxide (CO) is a toxic gas that binds to the iron(II) ion in haemoglobin via a coordinate bond, forming a more stable complex than oxygen does.

Aluminium chloride ($\text{AlCl}_3$) exists a dimer, $\text{Al}_2\text{Cl}_6$, in the gas phase. Each aluminium atom forms coordinate bonds with chlorine atoms from the other $\text{AlCl}_3$ unit.