Chem PPQ got wrong

Give a chemical explanation for why a carton of milk expands on freezing

The hydrogen bonds in ice hold H₂O molecules further apart (than in water)

Give a chemical explanation for why potassium is placed immediately after argon in the periodic table

Potassium atoms have 1 more proton than argon

Give a chemical explanation for why the reaction of ethane with chlorine under UV radiation is a poor method for preparing a high yield of chloroethane

Further substitution occurs

Give a chemical explanation for the concentration of water being 56moldm^-3

1dm³ water has a mass of 1000g

n(H₂O) in 1dm³ = 1000/18 approx 56mol

Give a chemical explanation why the C-C bonds in benzene are all the same length

The pi bonds in benzene are delocalised

Give a chemical explanation for why the rate of hydrolysis of 1-bromobutane is faster than that of 1-chloro-butane

The C-Br bond is weaker (than the C-Cl)

Within the permafrost in Arctic regions of the Earth, large amounts of methane are trapped within ice as ‘methane hydrate’, CH4•xH2O. Methane makes up about 13.4% of the mass of ‘methane hydrate’. Scientists are concerned that global warming will melt the permafrost, releasing large quantities of methane into the atmosphere. (a) The H–O–H bond angle in ice is about 109° but about 105° in gaseous H2O. Explain why there is this difference. (3)

• Ice has hydrogen bonds/bonding

• H2O(g) has 2 bonded pairs and 2 lone pairs

• lone pairs repel more than bonded pairs

Within the permafrost in Arctic regions of the Earth, large amounts of methane are trapped within ice as ‘methane hydrate’, CH4•xH2O. Methane makes up about 13.4% of the mass of ‘methane hydrate’. Scientists are concerned that global warming will melt the permafrost, releasing large quantities of methane into the atmosphere.Determine the formula of ‘methane hydrate’, CH4•xH2O. In the formula, show the value of x to two decimal places

Within the permafrost in Arctic regions of the Earth, large amounts of methane are trapped within ice as ‘methane hydrate’, CH4•xH2O. Methane makes up about 13.4% of the mass of ‘methane hydrate’. Scientists are concerned that global warming will melt the permafrost, releasing large quantities of methane into the atmosphere.

Calculate the volume of methane, in dm3, that would be released from the melting of each 1.00 kg of ‘methane hydrate’ at 101 kPa and 0 °C. Give your answer to three significant figures. (4)

Explain why a CF4 molecule has polar bonds but does not have an overall dipole.

• F atom is more electronegative than C atom OR F is very/the most electronegative

• No overall dipole - CF4 is symmetrical OR tetrahedral OR dipoles cancel OR dipoles act in opposite directions

Explain why a small proportion of molecules in water have a relative molecular mass of 20.

What mass of carbon dioxide (in g) is formed by the complete combustion of 42.0 m3 (measured at RTP) of propane?

Ignore (i) and (ii)

The student was not sure that all the oxygen had been removed from the lead oxide.

Suggest two modifications that the student could make to their method to be confident

that all the oxygen had been removed. Explain your reasoning.

State 2 limitations of using radical substitution in organic synthesis

• further substitutions

• produces different termination products

• more than one termination step

• mixture of products are formed

• substitution at different positions along the chain

D

What does the curly arrows show in a reaction mechnism?

The movement of an electron pair

What is meant by heterolytic fission?

heterolytics

one bonded atom receives both/2 electrons.

fision

breaking of a covalent bond

What is meant by structural isomers?

Compounds with the same molecular formula but different structural formula

just b (4)

Why is this a minor product?

Because the intermediate/carbocation in the formation of compound B is less stable than the intermediate in the formation of compound A

MAC reacts with water in the presence of AgNO3(aq) and ethanol. Draw the structure of the organic product of this reaction. State what you would observe in this reaction and identify the compound responsible for the observation.

Formation of white precipitate/solid/suspension and precipitate is silver chloride AgCl

Suggest how HBr molecule can act as an electrophile

(The H atom of HBr) accepts a pair of electrons

(4)

Why are silicon, carbon, oxygen and chlorine all classified as p-block elements?

Their highest energy electrons are in a p-orbital/p sub’shell

How could a student who has reacted magnesium with phosphoric acid obtain a sample of magnesium phosphate (hint: looking for how to get pure sample not how to get product)

Separation of solid

• filter to obtain solid/precipitate

• requires realisation that solid is filtered off

• solid may be stated within ‘removal of water’

Removal of water

• dry (solid)

• OR evaporate (water/solution/liquid)

An alcohol can be prepared by hydrolysing the haloalkane C2H5CHBrCH3 with aq sodium hydroxide

outline the mechanism for this reaction

show curly arrows and relevant dipoles

C

D

State the similarities and differences between the atomic structure of isotopes of the same element

Similarities: same number of protons AND electrons

Differences: Different number of neutrons

How can you decrease % uncertainty when using a mass balance?

• Use balance weighing to 3/more decimal places

• use a larger mass/amount

What is meant by the term alkali (1)

releases OH- ions in aqueous solution

What is meant by the term standard solution? (1)

A solution of a known concentration

What is an acid

releases H+ ions

What is a base (1)

A substance which readily accepts H+ ions