Chemistry Flashcards
What describes the polarity of the P—Cl bond and the polarity of the
phosphorus(III) chloride molecule?
P-Cl bond: polar
Molecule: polar
Phosphorus has one naturally occurring isotope with mass number 31.
Chlorine exists as two isotopes with mass numbers 35 and 37.
Give the formulae and mass/charge ratio of the ions responsible for the
molecular ion peaks in the mass spectrum of phosphorus(III) chloride, PCl3.
(2)
- P(35Cl)3+ –> 136
- P(35Cl)2 37Cl+ —> 138
- P(37Cl)2 35Cl+ —> 140
- P(37Cl)3+ —-> 142
Potassium halides react with concentrated sulfuric acid to form
potassium hydrogensulfate and the different products shown in the table.
Potassium halide Products
potassium chloride hydrogen chloride
potassium bromide hydrogen bromide, bromine and sulfur dioxide
potassium iodide hydrogen iodide, iodine, hydrogen sulfide and sulfur
By referring to any changes in oxidation numbers when these halides react with
concentrated sulfuric acid, explain which halide is the strongest reducing agent.
(3)
- I- is strongest reducing agent because it reduces sulfur from +6 to -2 (in H2S)
- but Br- reduces S from -2 to +4
- Cl- does not reduce S because no change in oxidation number
Explain why the first ionisation energy of magnesium is higher than that of sodium.
(3)
- Magnesium has a greater nuclear charge than sodium
- but shielding stays the same
- but attraction between valence electrons and nucleus of magnesium is greater, so harder to remove an electron
The table shows the enthalpy changes of reaction when 1 mol of different acids
are neutralised by sodium hydroxide solution, at 298 K.
hydrochloric acid, HCl –58
nitric acid, HNO3 –58
sulfuric acid, H2SO4 –115
ethanoic acid, CH3COOH –56
Comment on the relative enthalpy changes of reaction, using the data from the
table and including any relevant equations.
(6)
- HCl and HNO3 have same value as both are strong acids that completely dissociate in water, and are monoprotic.
- equation: HCl —> H+ + Cl-
HNO3—> H+ + NO3- - H2SO4 is highest as it is a diprotic acid, and fully dissociates.
- so roughly double of HCl and HNO3
- ethanoic acid however is a weak acid, so partially dissociates in water.
- Enth change of neutralisation also needs some energy to dissociate acid, so is less exothermic than value for strong acid.
- equation: CH3COOH —> H+ + CH3COO-
Glycolic acid has an acid dissociation constant of 1.5 × 10–4 mol dm–3 compared
with a value of 1.7 × 10–5 mol dm–3 for ethanoic acid.
(i) Give a possible explanation as to why the value of Ka for glycolic acid is
approximately ten times larger than that of ethanoic acid.
(2)
- glycolic acid has extra O which pulls electrons
- therefore, H+ ions is more easily lost.
Give a reason why the partial pressure of carbon is not included in the expression.
- because carbon is a solid, and its pressure stays constant.
Carbon monoxide reacts with steam.
CO(g) + H2O(g) U CO2(g) + H2(g)
At 1100 K, Kc = 1.00
In an experiment, 1 mol of carbon monoxide was mixed with 1 mol of steam,
2 mol of carbon dioxide and 2 mol of hydrogen.
Deduce, with reasons, the direction in which the reaction will shift to reach equilibrium.
(3)
since Kc is 1, there must be equal concs of reactants and products in eq.
- However, the concentration of products needs to decrease, and concentration of reactants needs to increase.
- therefore, the eq will shift to the left.
Ethanol can be made in industry by the reaction of ethene with steam, using
a phosphoric(V) acid catalyst.
CH2=CH2(g) + H2O(g) ⇌ CH3CH2OH(g) ΔrH = −45 kJ mol
−1
The reaction is carried out at 300°C and 60 atm. An initial yield of 5 % is achieved
when the ethene and steam first pass through the reactor.
(a) Explain the chemical reasons for the conditions used and why such a
low initial yield is acceptable in the industrial process.
(6)
- catalyst reduces activation energy needed for the reaction to take place.
- Higher temp increases rate.
- However, high temp will reduce yield as reaction will move in the backward direction to counteract the change, (in endothermic direction)
- so 300 degrees is compromise between rate and yield
- low yield acceptable as unused reactant can be recycled through the reactor again.
A student investigated the role of the catalyst in this reaction.
Procedure
Step 1 Heat a sample of KClO3 , in a test tube, with a known mass of insoluble
catalyst until the mass remains constant.
Step 2 Mix the contents of the test tube with water.
Step 3 Filter the mixture and rinse with deionised water.
Step 4 Dry the remaining solid.
Step 5 Measure the mass of the dry solid.
Explain how each of the steps in this procedure is needed to show that the
catalyst is not used up in this reaction.
(4)
- heat to constant mass so all of the KClO3 decomposes
- solid product will dissolve in water.
- rinsing ensures any soluble impurities are removed.
- drying the remaining solid ensures that no water contributes to the mass of the final product.
- mass should be the same as the catalyst’s mass at the start.
In an aqueous solution of sulfuric acid, the following equilibria exist.
H2SO4(aq) + H2O(l) U H3O+
(aq) + HSO4
−
(aq) Ka very large
HSO4
−
(aq) + H2O(l) U H3O+
(aq) + SO4
2−(aq) Ka = 0.012 mol dm−3
Explain, in terms of these equilibria, why the concentration of hydrogen ions
in a 0.10 mol dm−3 solution of sulfuric acid is not 0.20 mol dm−3.
No calculation is required.
(2)
- the first eq, H2SO4 ionisises completely
- but in the second one, the concentration of H30+ is very small, whereas the H30+ in the 1st one is in higher conc.
This question is about the forces between molecules and ions.
*(a) Some data for three small molecules are shown.
Molecule Mr
Boiling temperature
/ °C
Fluorine 38.0 −188
Hydrogen chloride 36.5 −85
Methanol 32.0 65
Explain the large variation in boiling temperatures, given the small range in
Mr values.
Detailed descriptions of the forces involved are not required.
(6)
- Fluorine has only London Forces
- HCl forms permanent dipole-dipole interactions as well
- Methanol can form Hydrogen bonds as it contains hydrogen and Oxygen, which have a large electronegativity difference
- More energy required to overcome hydrogen bonds, as they are the strongest intermolecular force
- less energy required to overcome Londons as they are the weakest intermolecular force.
Suggest why a pure sample of magnesium carbonate will not be produced from
the decomposition of magnesium oxalate even if the sample is heated for longer.
(1)
- because the magnesium carbonate may begin to decompose before the oxalate has finished decomposing.
Give the reason for the ratio of peak heights at m / z values of 102, 100 and 98.
(2)
- Cl exists in 2 isotopes- 35Cl and 37Cl that are in ratio of 3:1
- Therefore, two chlorine atoms would give ratio of 9:6:1
The standard enthalpy change of combustion, ΔcH , of octane is –5470kJmol–1.
Give two reasons why this value, measured under standard conditions, is
different from the value obtained using bond enthalpy data.
(2)
- bond enthalpies are averaged across a large number of compounds
- Octane is a liquid, whereas bond enthalpies only take into account gaseous substances
