1. 1
   A hospital's anaesthesia team relies on cylinders of medical oxygen gas during surgical procedures. Oxygen gas is made of O2 molecules, and the biomedical engineering staff are being trained on why oxygen gas behaves as it does at room temperature and under pressure in the cylinders.

   (a)

   A hospital's anaesthesia team uses cylinders of oxygen gas to support patients during surgery. Oxygen exists as O2 molecules, formed from two non-metal atoms bonded together. What type of bonding holds the two oxygen atoms together in an O2 molecule?

   [1 mark]

   • AIonic bonding
   • BMetallic bonding
   • CCovalent bonding
   • DNo bonding, the atoms are simply next to each other

   (b)

   The training material explains that oxygen gas has a very low boiling point, which is why it must be stored as a compressed gas or as a cold liquid in some hospital supply systems. What is the main reason simple molecular substances like oxygen have low boiling points?

   [1 mark]

   • AThe covalent bonds between the two oxygen atoms are weak and easily broken during boiling
   • BThere are only weak intermolecular forces between separate O2 molecules, which require little energy to overcome
   • COxygen molecules contain delocalised electrons that prevent boiling
   • DOxygen forms a giant lattice structure like an ionic compound

   (c)

   Explain, in terms of forces, why oxygen gas has a low boiling point and does not conduct electricity, even though the bond between the two oxygen atoms in each molecule is strong.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A beverage company carbonates its soft drinks using carbon dioxide gas, which dissolves in the drink under pressure and is released as bubbles when the bottle is opened. The company's food scientists are explaining to new staff how carbon dioxide molecules are held together and why the gas behaves as it does.

   (a)

   A beverage company uses carbon dioxide gas to carbonate its fizzy drinks. Carbon dioxide has the formula CO2, with a carbon atom sharing electrons with two oxygen atoms. What is the correct description of covalent bonding as shown in this molecule?

   [1 mark]

   • AAtoms share one or more pairs of electrons
   • BElectrons are completely transferred from carbon to oxygen
   • CIons with opposite charges attract each other
   • DElectrons move freely between many atoms in a lattice

   (b)

   The food scientists note that carbon dioxide is a gas at room temperature, escaping quickly as bubbles once a bottle is opened. Which statement best explains this in terms of intermolecular forces compared to covalent bonds?

   [1 mark]

   • AThe covalent bonds between carbon and oxygen atoms are broken easily at room temperature
   • BCarbon dioxide molecules are held together by metallic bonding
   • CCarbon dioxide has no bonds at all holding its atoms together
   • DThe weak intermolecular forces between separate CO2 molecules are overcome easily at room temperature, but the strong covalent bonds within each molecule remain intact

   (c)

   Describe how a dot and cross diagram would represent the covalent bonds in a molecule of carbon dioxide, CO2, in which carbon forms two double bonds, one to each oxygen atom.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A cleaning products manufacturer produces household ammonia solution by dissolving ammonia gas in water. The company's chemists are documenting the molecular structure of ammonia (NH3) for a technical data sheet explaining the compound's properties.

   (a)

   A cleaning products manufacturer uses ammonia solution, made by dissolving ammonia gas (NH3) in water, in some of its products. Describe the covalent bonding in a molecule of ammonia, stating how many covalent bonds nitrogen forms and why.

   [3 marks]

   (b)

   The technical data sheet also needs to explain why ammonia gas has a much higher boiling point (-33 degrees Celsius) than methane (-162 degrees Celsius), even though both are simple molecular substances of a similar size, and why ammonia does not conduct electricity as a pure liquid. Give a full explanation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An automotive engineer is comparing the fuels and gases used in a vehicle, from methane gas used in some fuel systems to the much larger hydrocarbon molecules found in diesel fuel, to understand why these substances differ so much in their physical state and boiling point at room temperature.

   (a)

   An automotive engineer is investigating why different fuels and gases used in a vehicle's engine and fuel system, including small molecules such as methane and larger molecules such as those in diesel fuel, have very different boiling points and physical states at room temperature. Explain, with reference to covalent bonding and intermolecular forces, why simple molecular substances generally have low melting and boiling points, and why melting and boiling points tend to increase as molecule size increases.

   [6 marks]

   (b)

   The engineer also wants to understand why none of these fuels, whether gaseous or liquid, conduct electricity, unlike the metal components of the fuel system and the ionic compounds used in the vehicle's battery. Explain why simple molecular covalent substances do not conduct electricity in any state, and contrast this with the electrical behaviour of ionic compounds and metals, referring to the particles responsible for carrying charge in each case.

   [6 marks]

   Total for question 4: 12 marks

End of questions