chemistry basic alevel pp quesion

A sample of rubidium contains the isotopes 85Rb and 87Rb only. The isotope 85Rb has an abundance 2.5 times greater than that of 87Rb Calculate the relative atomic mass of rubidium in this sample. Give your answer to one decimal place. ___________________________________

(85 × 2.5) + 87 × 1 3.5 = 85.6

5.00 g sample of potassium chloride was added to 50.0 g of water initially at 20.0 °C. The mixture was stirred and as the potassium chloride dissolved, the temperature of the solution decreased. (a) Describe the steps you would take to determine an accurate minimum temperature that is not influenced by heat from the surroundings.

(a) Start a clock when KCl is added to water 1 Record the temperature every subsequent minute for about 5 minutes Allow record the temperature at regular time intervals until some time after all the solid has dissolved for M2 1 Plot a graph of temperature vs time 1 Extrapolate back to time of mixing = 0 and determine the temperature

Solid sodium halides react with concentrated sulfuric acid. (i) A sample of solid sodium iodide is reacted with concentrated sulfuric acid. A black solid forms and hydrogen sulfide gas is produced. Write a half-equation for the reaction of sulfuric acid to form hydrogen sulfide. ______________________________________________________________ (1) (ii) Write a half-equation for the formation of the black solid. ______________________________________________________________ (1) (iii) Use your answers to parts (c) (i) and (c) (ii) to write an overall equation for the reaction of sodium iodide with concentrated sulfuric acid.

(c) (i) H2SO4 + 8H+ + 8e(-) → H2S + 4H2O ALLOW SO42- + 10H+ + 8e(-) → H2S + 4H2O ALLOW fractions/multiples IGNORE state symbols 1 (ii) 2I- → I2 + 2e(-) ALLOW fractions/multiples IGNORE state symbols ALLOW 2I - - 2e(-) →I2 1 (iii) H2SO4 + 8H+ + 8I- → H2S + 4H2O + 4I2 ALLOW H2SO4 + 8HI → H2S + 4H2O + 4I2 1 (iv) 'Oxidising agent' box ticked 1 (v) H2SO4 + 2NaF → Na2SO4 + 2HF OR H2SO4 + NaF → NaHSO4 + HF 1 (vi) Fluoride less powerful reducing agent (than iodide) OR Fluoride less easily oxidised than iodide

(d) Water has a known density (of 1.0 g cm-3) 1 Therefore, a volume of 50.0 cm3 could be measured out

(d) Water has a known density (of 1.0 g cm-3) 1 Therefore, a volume of 50.0 cm3 could be measured out

Use the diagram to deduce whether the forward reaction is exothermic or endothermic. Explain your answer.

(ii) Endothermic if wrong no further marks in part (ii) 1 increase T increases yield or moves to right 1 Eqm shifts to reduce T or eqm favours endothermic direction

(ii) × 12

(ii) × 12

Write an equation for the reaction of chlorine with cold water. State a reason why chlorine is added to drinking water, and suggest a disadvantage of treating water in this way.

) Cl2 + H2O ⇌ HCl + HClO OR 2Cl2 + 2H2O ⇌ O2 + 4HCl OR Cl2 + H2O ⇌ 2H+ + Cl− + ClO− 1 Kills bacteria 1 Wasteful as most potable water not used for drinking - used in washing clothes etc OR Some people suffer eye irritation / Some people find the taste unpleasant OR can react with organic compounds to produce harmful substances

(6) (ii) SCl2 reacts with NaF to form SF4 and S2Cl2 and one other product. Write an equation for the reaction

) NaCl (as product in any equation) 1 3 SCI2 + 4 NaF → SF4 + S2Cl2 + 4 NaCl

(2) (d) A 50.0 g sample of water was used in this experiment. Explain how you could measure out this mass of water without using a balance.

) Water has a known density (of 1.0 g cm-3) 1 Therefore, a volume of 50.0 cm3 could be measured out

A nickel catalyst is used in the first stage. Explain why the catalyst is more effective when coated onto an unreactive honeycomb.

2 (iii) M1 Increase in / more / large(r) / big(ger) surface area / surface sites Mark independently For M1 accept "an increase in surface" M2 increase in / more successful / productive / effective collisions (in a given time) (on the surface of the catalyst / with the nickel)

Sodium bromide reacts with concentrated sulfuric acid in a different way from sodium chloride. Write an equation for this reaction of sodium bromide and explain why bromide ions react differently from chloride ions

2NaBr + 2H2SO4 Na2SO4 + Br2 + SO2 + 2H2O Allow ionic equation 2Br- + 2H2SO4 Br2 + SO42- + SO2 + 2H2O 1 Br- ions are bigger than Cl- ions 1 Therefore Br- ions more easily oxidised / lose an electron more easily (than Cl- ions)

Strontium metal can be extracted by heating strontium oxide with aluminium metal. In this reaction, strontium vapour and solid aluminium oxide are formed. Write an equation for the reaction and state the role of the aluminium in the process. Explain why strontium forms a vapour but aluminium oxide is formed as a solid. Equation

3SrO + 2Al ⟶ Al2O3 + 3Sr 1 Al acts as a reducing agent 1 Sr is collected as a vapour because 1 Al2O3 is an ionic lattice and so has strong ionic attractions 1 Than Sr which is a metallic structure with (relatively) weaker bonding

In terms of atomic structure, explain why the van der Waals' forces in liquid argon are very weak.

Argon particles are single atoms with electrons closer to nucleus 1 Cannot easily be polarised (or electron cloud not easily distorted)

Write an equation, including state symbols, to show the reaction taking place when the standard enthalpy of combustion for ethanol is measured.

C2H5OH(I) + 3O2(g) ⟶ 2CO2(g) + 3H2O(I)

Write an ionic equation, with state symbols, to show the reaction of calcium with an excess of water.

Ca(g) Ca+(g) + e-

Write an ionic equation, with state symbols, to show the reaction of calcium with an excess of water.

Ca(s)+ 2H2O(l) Ca2+(aq) + 2OH-(aq) + H2(g)

State and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium. Trend ______________________________________________________________ Explanation

Decrease If answer to 'trend' is not 'decrease', then chemical error = 0 / 3 1 Ions get bigger / more (energy) shells Allow atoms instead of ions 1 Weaker attraction of ion to lost electron

Explain, in terms of electron transfer, the meaning of the term oxidising agent

Electron acceptor / species that accepts electrons / species that gains electrons NOT electron pair NOT just 'gain of electrons'

Van der Waals' forces exist between all molecules. Explain how these forces arise.

Electron movement in first molecule / temporary dipole Allow description 1 Induces a dipole in another molecule Allow description 1 (Induced-temporary) attraction or δ+ attracts δ- in different/adjacent molecules

Silver cyanide can be precipitated from sodium cyanide solution by adding an excess of silver nitrate solution. Describe how you would obtain a pure dry sample of silver cyanide from this mixture.

Filter Must be in this order 1 Wash (the residue) with water 1 Dry by pressing between filter paper or in air

Suggest one reason why the reaction of sodium fluoride with concentrated sulfuric acid is different from the reaction with sodium iodide.

Fluoride less powerful reducing agent (than iodide) OR Fluoride less easily oxidised than iodide

(a) State and explain the general trend in first ionisation energy across Period 3.

General increase If not increase then CE 1 Greater nuclear charge / more protons 1 Same shielding / electrons added to same shell Allow similar 1 Stronger attraction (from nucleus) for outer electron(s) Allow electron in outer shell

(d) A solution of sodium chlorate(l) was added to a colourless solution of potassium iodide. Suggest what is observed. Explain the reaction that leads to this observation. ________________________________________

Goes brown (or shades of brown) Allow black ppt/solid but NOT black solution or purple 1 Due to iodine or I3− Correct ½ equation scores M2 and M3 1 Because I− oxidised

Use the diagram to deduce whether the forward reaction involves an increase or a decrease in the number of moles of gas. Explain your answer.

Increase (if wrong no further marks in part (i) 1 higher P gives lower yield or moves to left 1 Eqm shifts to reduce P or eqm favours side with fewer moles 1

A colourless solution contains a mixture of sodium chloride and sodium bromide. Using aqueous silver nitrate and any other reagents of your choice, develop a procedure to prepare a pure sample of silver bromide from this mixture. Explain each step in the procedure and illustrate your explanations with equations, where appropriate.

Indicative chemistry content Stage 1: formation of precipitates • Add silver nitrate • to form precipitates of AgCl and AgBr • AgNO3 + NaCl → AgCl + NaNO3 • AgNO3 + NaBr → AgBr + NaNO3 Stage 2: selective dissolving of AgCl • Add excess of dilute ammonia to the mixture of precipitates • the silver chloride precipitate dissolves • AgCl + 2NH3 → Ag(NH3)2+ + Cl− Stage 3: separation and purification of AgBr • Filter off the remaining silver bromide precipitate • Wash to remove soluble compounds • Dry to remove water

(iv) Explain why nickel is ductile (can be stretched into wires).

Layers / planes / sheets of atoms or ions can slide over one another

Define the term standard enthalpy of combustion.

M1 The enthalpy change / heat change at constant pressure when 1 mol of a compound / substance / element M2 is burned / combusts / reacts completely in oxygen OR burned / combusted / reacted in excess oxygen M3 with (all) reactants and products / (all) substances in standard / specified states OR (all) reactants and products / (all) substances in normal states under standard conditions / 100 kPa / 1 bar and specified T / 298 K

(2) (d) Pure magnesium reacts completely with an excess of dilute sulfuric acid. The reaction of pure calcium with an excess of dilute sulfuric acid is very rapid initially. This reaction slows down and stops before all of the calcium has reacted. Use your knowledge of the solubilities of Group 2 sulfates to explain why these reactions of magnesium and calcium with dilute sulfuric acid are so different.

Magnesium sulfate is soluble and calcium sulfate is insoluble / slightly soluble / magnesium sulfate is more soluble / calcium sulfate is less soluble / correct trend in solubility (M1) Any order. M1 requires a comparison of the two solubilities. Calcium sulfate coats the surface of the calcium (M2) Coating prevents further contact with / reaction by the acid (M3)

A data book value for the H-H bond enthalpy is 436 kJ mol−1. Suggest one reason why this value is different from your answer to part (a).

Mean bond enthalpies are not the same as the actual bond enthalpies in CO2 (and / or methanol and / or water)

(e) The Ne atom and the Mg2+ ion have the same number of electrons. Give two reasons why the first ionisation energy of neon is lower than the third ionisation energy of magnesium.

Mg2+ ion smaller than Ne atom / Mg2+ e- closer to nucleus (Not 'atomic' radius fo Mg2+) 1 Mg2+ has more protons than Ne / higher nuclear charge or e- is removed from a charged Mg2+ion / neutral neon atom (accept converse arguments)

Outline how the TOF mass spectrometer is able to separate these two species to give two peaks.

Positive ions are accelerated by an electric field 1 To a constant kinetic energy 1 The positive ions with m / z of 104 have the same kinetic energy as those with m / z of 118 and move faster 1 Therefore, ions with m / z of 104 arrive at the detector first 1

Magnesium ribbon reacts with hot water. Heated magnesium ribbon reacts with steam. State two differences between these reactions

Slower with hot water or faster with steam The hot water produces Mg(OH)2 / the hydroxide OR steam produces MgO / the oxide (Slow) bubbling with hot water OR bright white light / flame / white solid with steam

Predict whether an atom of 88Sr will have an atomic radius that is larger than, smaller than or the same as the atomic radius of 87Rb. Explain your answer. Atomic radius of 88Sr compared to 87Rb ___________________________________ Explanation

Smaller Chemical error if not smaller, CE = 0/3 If blank mark on. 1 Bigger nuclear charge / more protons in Sr Not bigger nucleus. 1 Similar/same shielding

a) Bromine (Br2), strontium chloride (SrCl2) and iodine monochloride (ICl) all have similar Mr values. Suggest, with reasons, the order of melting points for these three substances.

SrCl2> ICl > Br> If wrong can award 1 for one in the correct 'position' 2 SrCl2 strong ionic bonds / (strong electrostatic attraction between opposite ions) 1 Lattice so many strong bonds to overcome 1 ICl has dipole-dipole between molecules - weaker than ionic bonds 1 Br2 has van der Waals forces between molecules - much weaker

State the structure of, and bonding in, silicon dioxide. Other than a high melting point, give two physical properties of silicon dioxide that are characteristic of its structure and bonding.

Structure: Macromolecular / giant molecule / giant covalent Mark independently 1 Bonding: Covalent / giant covalent 1 Physical Properties: Any two from: Hard/ Brittle / not malleable Insoluble Non conductor

In terms of structure and bonding, explain why sulfur has a higher melting point than phosphorus.

Sulfur molecules (S8) are larger than phosphorus (P4) 1 Therefore van der Waals' forces between molecules are stronger 1 Therefore more energy needed to loosen forces between molecules

Suggest two reasons why the reaction was faster at first with calcium and sulfuric acid than with magnesium and hydrochloric acid. Reaso

Sulfuric acid has 2 × the H+ ion concentration Larger concentration of H+ is sufficient. 1 Calcium is more reactive (than magnesium)

The value of Kc for this reaction is 1 × 10-5 at 1500 K. Explain the significance of this value for an industrial chemist interested in manufacturing nitrogen monoxide by the direct combination of the elements.

The product yield is very small (1) Yield does not justify cost of producing high temp (1)

State why the curve starts at the origin

There are no molecules / particles with zero energy OR All of the molecules / particles are moving / have some energy

State why the volume of the flask need not be known when calculating a value for Kc.

V cancels in Kc expression or no moles same on top and bottom of expression or total moles reactants = moles products, i.e. total no of moles does not change

Describe what you would observe when, in the absence of air, magnesium is heated strongly with water vapour at temperatures above 373 K. Write an equation for the reaction that occurs.

White powder / solid / ash / smoke Ignore ppt / fumes 1 Bright / white light / flame Allow glows white / glows bright 1 Mg + H2O → MgO + H2

The elements phosphorus, sulfur, chlorine and argon are in the p block of the Periodic Table. (a) State why these elements are classified as p block elements.

a) Outer electrons are in p orbitals

Explain why magnesium has a higher melting point than sodium.

b) Mg2+ / magnesium ion has higher charge than Na+ Allow Mg2+ ions smaller / greater charge density than Na+ ions Allow Mg atoms smaller than Na (atoms) Allow magnesium has more delocalised electrons Must be a comparison Ignore reference to nuclear charge 1 Attracts delocalised / free / sea of electrons more strongly / metal-metal bonding stronger / metallic bonding stronger

What is the number of atoms in 0.0100 mol of NH3? (The Avogadro constant L = 6.022 × 1023mol-1) A 6.02 × 1025 B 1.20 × 1023 C 1.81 × 1022 D 2.41 × 1022

d

Suggest two reasons why the value obtained by the student in part (c) is different from the value calculated in part (e)

• heat loss or not all heat transferred to the apparatus or heat absorbed by the apparatus or (specific) heat capacity of the apparatus not considered • incomplete combustion / not completely burned / reaction is not complete • The idea that the water may end up in the gaseous state (rather than liquid) • reactants and / or products may not be in standard states. • MBE data refers to gaseous species but the enthalpy of combustion refers to liquids in their standard states / liquid propanone and liquid water in standard states • MBE do not refer to specific compounds OR MBE values vary with different compounds / molecules OR are average / mean values taken from a range of compounds / molecules