Chemistry AQA - Wrong questions from past papers

Suggest why the oxidation state of chromium is zero in Cr(PF₃)₆ (1 mark)

PF₃ is neural and the complex is neutral

Name the type of bond between N and H in ammonia

Covalent

Complete equation to show formation of one complex that contains Cr in its +3 oxidation state

CrCl₃ + 5H₂O →

CrCl₃ + 5H₂O → Cr[(H₂O)₅Cl]Cl₂

Explain why the enthalpy of hydration of F- ions is more negative than the enthalpy of hydration of Cl- ions

• Fluoride ions are smaller

• Stronger attraction to partially positive H in water

Explain why the 2nd ionisation energy of Ca is greater than 1st ionisation energy

Electron removed is closer to the nucleus

Why does AgCl have covalent character?

Ag+ is highly polarising and distorts electron cloud

The mass of Ba(OH)₂ is dissolved in beaker containing 150cm³ distilled water.

Describe how this solution is used to make 250cm³ of the 0.100 moldm^-3 Ba(OH)₂ solution

• Transfer solution to volumetric flask

• Add washings with distilled water and make up to graduation mark

• Invert many times to ensure solution is homogenous

Before the first titration, the pipette is rinsed with small volume of solution being measured.

State why it is good practice to rinse pipette in this way

So that titration is done with a known concentration of solution

State why the concentration of aluminium sulfate solution cannot be determined by colorimetry

Colourless solution

An excess of aqueous ammonia is added to a solution containing [Al(H₂O)₆]³⁺

Give an ionic equation for the reaction and state one observation

[Al(H₂O)₆]³⁺ + 3NH₃ → Al(H₂O)3(OH)₃ + 3NH₄⁺

Observation - White precipitate

State why a fuel cell does not need to be electrically recharged

Reactants are supplied continuously

Suggest one advantage of using methanol, rather than hydrogen, in a fuel cells for use in cars

Methanol can be stored easily

A student plants to titrate butanoic acid with a solution of ethylamine.

Explain why this titration could not be done using an indicator

• Butanoic acid is a weak acid and ethylamine is a weak base

• So pH change is not sharp

Overall order of reaction definition

The sum of the individual orders

Suggest two ways in which the melting point of crude aspirin would differ from the melting point of pure aspirin

• Greater range of melting points

• Lower melting point

Describe two precautions when heating the mixture of ethanol and crude aspirin

• Heat using water bath

• Heat to temperature below boiling point of ethanol

What is tetramethyl silane used for in NMR?

As the reference/standard

Why is tetramethyl silane used as reference in H NMR?

• Single environment of H so only gives one signal on spectrum

• Signal in area away from other typical H peaks

• As low electronegativity of Si shifts signal right

• Low boiling point so easy to remove

Suggest why an expression for K can be written without the concentration of water

[H₂O] is effectively constant

Suggest why reaction of CH₃CHO + H₂O is slower than reaction of ClCH₂CHO + H₂O

• C in C=O is less partially positive

• Because CH₃ is electron donating

• So higher Ea

Describe how molecules are ionised using electrospray ionisation

1. Sample is dissolved in a volatile solvent

2. Solution is then injected through a capillary tube at a high voltage

3. Each molecule gains an H+ ion

Before adding solution from burette, there was an air bubble below the tap. At the end of titration there was no air bubble.

Explain why this air bubble increases the final burette reading?

Some solution replaces the air bubble

Mr definition

The weighted average masses of atoms in a formula

relative to 1/12 mass of an atom of carbon-12

Relative atomic mass definition

The average mass of 1 atom of an element relative to 1/12 mass of one atom of carbon-12

The mean bond enthalpy definition

the energy required to break 1 mol of bonds in the gaseous state, averaged over a range of compounds containing that bond

Hess’ Law definition

The enthalpy change for a chemical reaction is independent of the route taken and depends only on the initial and final states

Features of dynamic equillibrium

• Concentration of the reactants and products remains constant

• Rate of forward and reverse reaction are equal

State how a buffer solution is made from KOH (aq) and ethanoic acid

Add excess ethanoic acid to KOH (aq)

How does a buffer of ethanoic acid and KOH (aq) resist change in pH when small amount of acid is added?

CH₃COO^- reacts with added H⁺

Describe how the reactions at the electrodes generate an electric current that can power a motor in an external circuit

The electrons released at the negative electrode, pass through an external circuit to the opposite electrode

Suggest an advantage of storing hydrogen as a liquid in a motor of a car?

Takes up less space

An excess of water, followed by an excess of concentrated concentrated ammonia is added to a solution containing [CuCl₄]^2-

Give an observation and an equation

Solution turns deep blue

[CuCl₄]^2- + 4NH₃ + 2H₂O → [Cu(NH₃)₄(H₂O)₂] + 4Cl^-

Explain why [V(H₂O)₆]³⁺ is acidic

• V³⁺ has a high charge density

• V³⁺ weakens the O-H bonds in water in water ligands, causing H+ ions to be released more easily

Suggest why vanadium can be pulled into thin wires

As the layers of positive metal ions can slide over each other

Explain why aqueous solutions containing [CuCl₄]^- ions are yellow

• Visible light is absorbed

• Only yellow light reflected

Why is it better to prepare primary amines from nitriles rather than from haloalkanes?

• No further reaction

• Single product formed

• Higher atom economy

Explain the bonding in benzene molecule

• Each C has 3 covalent bonds

• Spare electrons in p orbitals overlap

• Electrons delocalise into a ring

Explain the shape of benzene molecule

• Planar

• 6 carbon ring - 120^o bond angle

• Length of C-C bonds = length of C=C double bonds

Compare the stability of benzene with the hypothetical cyclohexa-1,3,5-triene molecule

• enthalpy of benzene is less exothermic by 152 kJmol^-1

• Benzene is more stable

Nitrobenzene is reduced to form phenylamine.

Explain why an aqueous solution is obtained in this reduction even though phenylamine is insoluble in water

Phenylamine is present as an ionic salt

Explain in terms of bond polarity, why R₂CHBr and KOH react together

• Br is partially negative and C is partially positive so C-Br bond breaks when heated

• OH^- attracted to C⁺ after C-Br bond breaks

Why are haloalkanes attacked by nucleophiles?

• Halogen is more electronegative than carbon

• C is partially positive

• Lone pair of electrons on nucleophile donated to the partially positive carbon

Why may production of a compound from glucose be more sustainable?

• Glucose is a renewable resource

• Biological reaction

• Higher atom economy

How to maximise the yield of product during distillation

• Cool the distillate

• Keep the temperature of the reaction mixture below the boiling points of other components of the mixture

Explain why nonane has a higher boiling point than 2,4-dimethylheptane

• Nonane has stronger van der Waals forces between molecules

• Nonane molecules pack closer together

Cisplatin mechanism of action

Prevents DNA replication in cells by binding to two guanine bases by ligand substitution reaction with DNA in which a bond is formed between the Pt²⁺ ions and a nitrogen atom on a guanine base

Cisplatin risks

• Can lead to adverse effects such as hair loss and skin damage

• Cisplatin damages and prevents DNA replication in healthy cells as well as cancerous cells

Method to produce a pure, crystalline product

• Dissolve crude product in minimum volume of hot solvent

• Filter solution by hot filtration

• Leave to cool and crystalise

• Filter under reduced pressure

• Wash product with ice cold solvent to remove aqueous impurities

• Dry between filter papers or in a warm place

By considering the mechanism of the reaction, explain why CH₃CH₂COCH₃ has no effect on plane polarised light

• Carbonyl group is planar

• The nucleophile can attack above or below the plane with equal probability as part of a nucleophilic addition reaction

• Therefore, equal amounts of each isomer is formed

Suggest why a conical flask may be used in an experiment of acid + sodium hydrogencarbonate instead of a beaker?

Conical flask reduces loss of liquid droplets

Why is chloroethanoic acid a stronger acid than ethanoic acid? (argument with Cl)

• Cl is an electron withdrawing group/has a negative inductive effect

• Weakens O-H bond in chloroethanoic acid

Why is chloroethanoic acid a stronger acid than ethanoic acid (reverse argument with CH3)

• CH₃ is electron donating/has positive inductive effect

• Makes O-H bond stronger

Units of activation energy in Arrhenius equation

Joules

Enthalpy change definition

Heat change at constant pressure

Explain why aqueous solution containing [Fe(H₂O)₆]³⁺ ions has a lower pH than an aqueous solution containing [Fe(H₂O)₆]²⁺ ions

• Fe³⁺ has a greater charge density

• Fe³⁺ is more polarising

• Fe³⁺ weakens O-H bonds in ligands more

Explain why volume of solution added becomes smaller as the end point of the titration is approached

To avoid missing the end point

Electrochemical series definition

List of electrode potentials in order

Suggest why sodium oxide forms a solution with a higher pH than the solution formed from magnesium oxide

• Oxides react with water to form hydroxide ions

• NaOH is more soluble that Mg(OH)₂

Explain why complexes formed from transition metal ions are coloured

• Absorb some wavelengths of visible light

• To excite electrons in d-subshell

• Remaining wavelength of light is reflected to give colour seen

Describe how a calibration graph is produced

• Measure the absorbance of known concentrations

• Plot a graph of absorbance against concentration

• Read value of unknown concentration for the measured absorbance from this graph

Explain how graphical methods can be used to process measured results, to confirm that a reaction is first order

• Plot a graph of concentration against time

• Take tangents to calculate the gradients

• Plot a graph of gradients against concentrations

• Straight line through origin confirms first order

State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change

Pressure in bomb calorimeter is not constant

Setting up an electrochemical cell

• Place piece of metals into the solutions

• Connect the two solutions with a salt bridge

• Connect metals to a high resistance voltmeter

Suggest why aqueous ethanol is a suitable solvent when heating coconut oil with KOH

Can dissolve both the oil and KOH

(petrol) faction definition

A group of compounds with similar boiling points

Carbon neutrality and environmental issues of bioethanol

• Deforestation

• Loss of biodiversity

• 6CO₂ in and 6CO₂ out is not actually carbon neutral as fuel is used in production and distribution

Suggest two disadvantages of using a glass beaker on a tripod or gauze instead of a clamped copper calorimeter

• Glass is a worse conductor than copper

• Tripod and gauze are at a fixed point above the flame

Why is benzene more thermodynamically stable that cyclohexa-1,3,5-triene

pi electrons in benzene are delocalised into a ring

Describe how you can distinguish seperate samples of two enantiomers

• Use plane polarised light

• Enantiomers rotate plane polarised light in opposite direactions

Benefits of fewer steps in production methods

Better yield

Benefits of higher atom economy in production method

Less waste

Explain how van der Waals forces arise between molecules

• Random movement of electrons in one molecule creates a temporary dipole

• This induces a dipole in neighbouring molecule

• These temporary dipoles attract

Explain why the first ionisation energy of Group 2 elements decreases down the group

• Number of electron shells increases

• Shielding increases

• Weaker attraction between nucleus and outer electron

Explain why the third ionisation energy of Mg is much higher than the second ionisation energy of Mg

• Electron being removed from a subshell closer to the nucleus

• Less shielding

• Stronger attraction between nucleus and outer electron

Problem with adding too much indicator to conical flask in titration

May react and affect the end point reading

Suggest, in terms of the intermolecular forces for each compound, why CHBr₃ has a higher boiling point than CBr₄

• CBr₄ has van der Waals forces between molecules

• CHBr₃ has van der Waals forces and permanent dipole-dipole forces between molecules

• Intermolecular forces between CHBr₃ molecules are stronger than intermolecular forces between CBr₄ molecules

Explain why CBr₄ is not a polar molecule

• CBr₄ is a completely symmetrical molecule

• Dipoles cancel out

Why does [Fe(H₂O)₆]³⁺ react with sodium carbonate to form CO₂ and [Fe(H₂O)₆]²⁺ forms FeCO₃ instead?

• [Fe(H₂O)₆]³⁺ is more acidic than [Fe(H₂O)₆]²⁺

• As Fe³⁺ has a higher charge density so it is more polarising

Ethanol, ethylamine and

Explain the trend in boiling points of CH₃CH₂OH > CH₃CH₂NH₂ > CH₃OCH₃

Refer to the intermolecular forces in all three compounds in your answer

• Hydrogen bonding in CH₃CH₂OH + CH₃CH₂NH₂ and permanent dipole-dipole forces in CH₃OCH₃

• Hydrogen bonding is the strongest type of intermolecular force

• Hydrogen bonding is stronger in CH₃CH₂OH than in CH₃CH₂NH₂

Alkene → alkane reagent and condition

H₂ with a Ni catalyst

Order of amine base strength

secondary amines > tertiary amines > primary amines > ammonia > aromatic amines

Electrospray ionisation equation

M (g) + H⁺ → MH⁺ (g)

What is formed when NO_x and SO₂ react with sunlight

photochemical smog

Process of removing SO₂ from combustion emissions name

flue gas desulfurisation

Flue gas desulfurisation process

• Gases are passed through

Flue gas desulfurisation (3 equations)

1. CaO (s) + SO₂ (g) → CaSO₃

2. CaCO₃ + SO₂ → CaSO₃ + CO₂

3. CaSO₃ + [O] → CaSO₄

carbon monoxide and nitrogen monoxide reaction in catalytic converter

2NO + 2CO → N₂ + CO₂

catalytic converter catalyst

Platinum coated on a honeycomb structure

Why is platinum coated on a honeycomb structure in catalytic converter?

• Increases surface area

• Reduces cost as less platinum needed

Why can a single Cl free radical cause the decomposition of many ozone molecules?

• Cl free radical is regenerated and can cause a chain reaction in the decomposition of ozone

Suggest one reason why electron pair repulsion theory cannot be used to predict the shape of [CoCl₄]^2- ion

Too many electron in d-subshell

Explain why sodium oxide forms an alkaline solution when it reacts with water

• Sodium oxide contains O^2- ions

• These O^2- ions react with water forming OH^- ions

Explain why methanol is easy to liquefy

Hydrogen bonding is strong enough force to hold methanol molecules together in a liquid

Explain how observations from this reaction show that vanadium exists in at least two different oxidation states

• Two different colours of solution are observed

• Each colour is due to vanadium in a different oxidation state

Explain how a co-ordinate bond is formed in transitional metal complex

Ligand donates an electron pair to the central metal ion

A sample of KCl is added to water initially at 20C.

Describe the steps you would take to determine an accurate minimum temperature that is not influenced by heat from the surrounds

• Start a clock when KCl is added to water

• Record temperature every subsequent minute for about 5 minutes

• Plot graph of temperature against time

• Extrapolate back to of mixing at t = 0 and determine the minimum temperature

NaF + conc. H₂SO₄ products

NaHSO₄ and HF

NaCl + conc. H₂SO₄ products

NaHSO₄ and HCl

KBr + H₂SO₄ products

KHSO₄ and HBr