Question
One definition of atomic volume is given by the formula:
The table gives the atomic volumes of the first nineteen elements, in the form in which they occur at STP.
(a) Outline why many elements have atomic volumes greater than 10000\(cm^3\) \(mol^{-1}\)
(b) Outline why some of those with larger atomic volumes have values ~11000\(cm^3\) \(mol^{-1}\) and others ~22000\(cm^3\) \(mol^{-1}\)
(c) Suggest why some elements, such as carbon and oxygen, have more than one value for their atomic volume.
(d) Explain why the atomic volumes of elements 11, 12 and 13 show a steady decrease.
(e) Estimate the atomic volume, in \(cm^3\) \(mol^{-1}\), of element 20.
(f) Suggest, giving one reason, whether you could ever know the actual volume of a single atom.
Answer/Explanation
Answer:
(a) gases «and others are solids»
(b) smaller values are diatomic «gases»
OR
larger values are monatomic «gases»
(c) «different» allotropes
(d) Any two of:
increasing «effective» nuclear charge/Z/atomic number/number of protons
increasing number of delocalized/bonding/valence electrons
increasing attractions between positive «metal» ions/cations and delocalized
electrons
OR
stronger metallic bonding
OR
decreasing radii
(e) any estimated value in the range of 20-40 «\(cm^3\) \(mol^{-1}\)».
(f) no AND probability of finding an electron is low
OR
no AND all measurements have uncertainties «even though there will always be
uncertainty as to what the exact value is»
OR
yes AND X-ray diffraction can indicate separation of nuclei «in the element»
OR
yes AND can take a sample of the element, measure its volume and calculate
number of particles
OR
yes AND bond length can be measured by microwave spectroscopy/electron
diffraction/neutron diffraction
Question
This question is about a mug made of a lead alloy.
a. Identify the experiment with the highest rate of lead dissolving.
$\mathrm{b}(\mathrm{i})$ Suggest why the relationship between time and lead concentration for Cola at $16{ }^{\circ} \mathrm{C}$ is not linear.
$\mathrm{b}$ (ii Examine, giving a reason, whether the rate of lead dissolving increases with acidity at $18^{\circ} \mathrm{C}$.
c(i)Lead(II) chloride, $\mathrm{PbCl}_2$, has very low solubility in water.
$$
\mathrm{PbCl}_2(\mathrm{~s}) \rightleftharpoons \mathrm{Pb}^{2+}(\mathrm{aq})+2 \mathrm{Cl}^{-}(\mathrm{aq})
$$
Explain why the presence of chloride ions in beverages affects lead concentrations.
c(ii)A mean daily lead intake of greater than $5.0 \times 10^{-6} \mathrm{~g}$ per kg of body weight results in increased lead levels in the body.
Calculate the volume, in $\mathrm{dm}^3$, of tap water from experiment 8 which would exceed this daily lead intake for an $80.0 \mathrm{~kg}$ man.
▶️Answer/Explanation
Markscheme
a. 6
Note: Accept “orange juice”.
b(i)equilibrium is being established «between lead in solution and in mug»
OR
solution becoming saturated
OR
concentration of lead ions/[ $\left.\mathrm{Pb}^{2+}\right]$ in the solution has increased «over time»
OR
acid concentration has decreased «as reacted with lead»
OR
surface lead has decreased/formed a compound/forms insoluble layer on surface
OR
acid reacts with other metals «because it is an alloy» [
Note: Do not accept “concentration of cola, orange juice, etc… has decreased”
Do not accept a response that only discusses mathematical or proportional relationships.
b(ii)o $A N D$ experiment $7 /$ beer has lowest rate and intermediate acidity/pH
OR
no AND experiment 6/orange juice has fastest rate but lower acidity/higher $\mathrm{pH}$ than lemonade
OR
no $A N D$ experiment 6/orange juice has highest rate and intermediate acidity/pH $[\boldsymbol{V}]$
Note: Accept no AND any comparison, with experimental support, that concludes no pattern/increase with acidity
eg: “rate of $\mathrm{Pb/lead}$ dissolving generally decreases with acidity as tap water has highest rate (after orange juice) while lemonade (lower $\mathrm{pH}$ ) has lower rate”.
c(i)equilibrium shifts to the left/towards reactants $[\boldsymbol{V}]$
lead «compounds/ions» precipitate
OR
concentration of lead «ions»/[ $\left.\mathrm{Pb}^{2+}\right]$ decreases $[\boldsymbol{V}]$
Note: Award [2] for “equilibrium shifts to the left/towards reactants due to common ion effect”.
Accept “lead ions/[ $\left.\mathrm{Pb}^{2+}\right]$ removed from solution” for $\mathrm{M} 2$.
$\mathrm{c}(\mathrm{ii})$ sdaily limit $=5.0 \times 10^{-6} \mathrm{~g} \mathrm{~kg}^{-1} \times 80.0 \mathrm{~kg}=» 4.0 \times 10^{-4}$ «g of lead»
$$
\text { «volume }=\frac{4.0 \times 10^{-4} \mathrm{~g}}{1.5 \times 10^{-2} \mathrm{~g} \mathrm{dm}^{-3}}=» 2.7 \times 10^{-2} / 0.027 « \mathrm{dm}^3 »[\boldsymbol{U}]
$$