Synoptic Exam Questions (AQA AS Chemistry): Exam Questions

Butane (C₄H₁₀) is a saturated hydrocarbon used as a fuel in camping stoves.

Butane is a member of the alkane homologous series. State two characteristics of a homologous series.

Write a balanced equation for the complete combustion of butane gas.

A mass of 5.25 g of butane gas is burned completely in oxygen. The carbon dioxide gas produced is collected at a temperature of 50 °C and a pressure of 105 kPa.

Calculate the volume of carbon dioxide gas produced, in dm³.
(The ideal gas constant R = 8.31 J K^-1 mol^-1)

Butane exists as structural isomers. The isomer, 2-methylpropane, is also used as a fuel.

Draw the skeletal formula of 2-methylpropane.

The enthalpy of combustion of 2-methylpropane is -2869 kJ mol^-1.

Use the mean bond enthalpies in the table to calculate the enthalpy of combustion for butane (C₄H₁₀) and suggest a reason, based on bonds, why the values for the two isomers are different.

A chemist is investigating reactions of 3-bromopropan-1-ol, shown below.

In the first experiment, 3-bromopropan-1-ol is heated with an excess of aqueous sodium hydroxide. The C–Br bond is broken and a new organic product, propane-1,3-diol, is formed.

Outline the mechanism for this reaction.

Include curly arrows, relevant lone pairs and dipoles.

The chemist wants to confirm that the bromide ion has been produced.

Describe a chemical test that could be carried out on the resulting solution to confirm the presence of bromide ions.

Include the reagents used and the expected observation.

The reaction in part (a) is repeated using 3-chloropropan-1-ol.

The rate of reaction is observed to be significantly slower.

Explain this difference in rate.

Explain why propane-1,3-diol has a significantly higher boiling point (214 °C) than 3-bromopropan-1-ol (162 °C).

In a second experiment, 3-bromopropan-1-ol is heated with hot, ethanolic potassium hydroxide. This causes an elimination reaction to occur.

Draw the displayed formula of the organic product of this elimination reaction.

Geraniol is a primary alcohol found in rose oil. Its structure is shown below.

Geraniol is heated under reflux with an excess of acidified potassium dichromate(VI). A new product, Z, is formed.

Name the functional group in product Z that is formed from the alcohol group in geraniol.

State the expected colour change during this reaction.

The chemist wants to confirm that the reaction has occurred. The product Z is separated from the reaction mixture.

Describe a chemical test, other than using Tollens' reagent or Fehling's solution, that would give a positive result with product Z but a negative result with the starting material, geraniol.

The chemist performs another test by shaking a sample of geraniol with bromine water.

State the expected observation and name the type of reaction occurring.

In the reaction in part (a), the dichromate(VI) ion, Cr₂O₇^2-, is converted to the Cr³⁺ ion.

Deduce the oxidation state of chromium in the Cr₂O₇^2- ion and write a balanced half-equation for the conversion of Cr₂O₇^2- in acidic solution to Cr³⁺ ions.

Another naturally occurring alcohol is linalool, shown below, which is a structural isomer of geraniol.

Explain why linalool would not react with acidified potassium dichromate(VI).

Strontium is a Group 2 element that reacts with nitrogen gas at high temperatures to form an ionic compound, strontium nitride.

Strontium has a higher first ionisation energy than barium but a lower first ionisation energy than calcium.

Explain why strontium has a higher first ionisation energy than barium.

The nitride ion is formed when a nitrogen atom gains three electrons.

Write the full electron configuration for a nitride ion (N^3-).

A 3.55 g sample of pure strontium nitride is added to 200 cm³ of deionised water. It reacts completely according to the equation below. The resulting solution is transferred to a 250 cm³ volumetric flask and made up to the mark with deionised water.

Sr₃N₂ (s) + 6H₂O (l) → 3Sr(OH)₂ (aq) + 2NH₃ (aq)

A 25.0 cm³ sample of this strontium hydroxide solution is then titrated with 0.150 mol dm^-3 hydrochloric acid.

Calculate the volume of hydrochloric acid, in cm³, required to reach the end-point.

State and explain the shape of the ammonia molecule, NH₃.