Exam Qs

Question 1

Explain, in terms of structure and bonding, why nickel has a high melting point (2)

Answer 1

• Contains positive ions AND delocalised electrons
• so there is strong attraction between them/ strong metallic bonds

Question 2

Explain why nickel is ductile

Answer 2

Layers can slide over one another

Question 3

Explain why isotopes of the same element have the same chemical properties

Answer 3

Chemical properties depend on the number of electrons

Question 4

State which property of atoms causes a bond to be polar

Answer 4

Electronegativity

Question 5

Explain in terms of molecular forces why HF had a higher boiling point than HCl

Answer 5

HF - hydrogen bonding
HCl - stronger VDWs and dipole dipole
BUT hydrogen bonding is the strongest

Question 6

How is a co-ordinate bond formed

Answer 6

Electron pair donated

Question 7

State why the standard enthalpy of formation of hydrogen gas is 0 (1)

Answer 7

it is an elements (DON’T refer to standard state)

Question 8

Define standard enthalpy of combustion (3)

Answer 8

enthalpy change when 1 mol of a substance is completely burned in oxygen at standard conditions

Question 10

Give one reason why bond enthalpy that you calculated is different from the mean bond enthalpy in the data book

Answer 10

Data book value is derived from a number of different compounds

Question 11

Suggest how the students method and the analysis of the results could be improved in order to determine a more accurate value for the enthalpy of reaction (6)

Answer 11

1. Insulate the beaker or use a polystyrene cup or a lid -> to reduce heat loss
2. Record the temperature for a suitable time before adding the metal
   -> establish an accurate initial temperature
3. Extrapolate the cooling back to the point of addition -> to establish a (theoretical) maximum temperature

Question 12

What is electronegativity

Answer 12

The power of an atom to attract a pair of electrons in a covalent bond

Question 13

Explain how permanent dipole-dipole forces arise between HCl molecules

Answer 13

1. Difference in electronegativity leads to bond polarity
2. And there is an attraction between delta +ve on one molecule and delta -ve on another

Question 14

Suggest one reason, other than incomplete combustion or heat transfer to the atmosphere, why the student’s value for the enthalpy of combustion of methanol is different from that in a Data Book.

Answer 14

• heating of calorimeter not taken into account
• evaporation of alcohol

Question 15

The student said correctly that using a thermometer with an overall uncertainty forthe rise in temperature of ±0.5 °C was adequate for this experiment.
Explain why this thermometer was adequate for this experiment.

Answer 15

Because temperature change is much bigger than uncertainty

Question 16

Carbon monoxide is produced when incomplete combustion takes place in
engines. Nitrogen monoxide is another pollutant produced in car engines.

Write an equation to show how these pollutants react together in a catalytic
converter.

Answer 16

2CO + 2NO -> 2CO2 + N2

Question 17

Platinum, palladium and rhodium are metals used inside catalytic converters.
A very thin layer of the metals is used on a honeycomb ceramic support.

Explain why a thin layer is used in this way. (2)

Answer 17

1. To reduce amount of metals needed
2. Provide large surface area

Question 18

Write two equations to show how chlorine radicals react with ozone molecules in the upper atmosphere.

Answer 18

1. Cl• + O3 -> ClO• + O2
2. ClO• + O3 -> Cl• + O2

Question 19

Give one reason why bonds in molecules such as carbon dioxide and
1,1,1-trifluoroethane absorb infrared radiation.

Answer 19

Bonds vibrate/bend OR bonds are polar

Question 20

Explain how sodium chloride and magnesium oxide differ in their bonding and physical properties. (6 marks)

Answer 20

1.Both sodium chloride and magnesium oxide have giant ionic lattice structures with strong electrostatic forces of attraction between oppositely charged ions.

2. However, in sodium chloride the ions are Na⁺ and Cl⁻, while in magnesium oxide the ions are Mg²⁺ and O²⁻.
3. The charges in MgO are higher, so the electrostatic forces are stronger. Therefore, magnesium oxide has a higher melting point than sodium chloride.
4. Both conduct electricity when molten because ions are free to move. Both are soluble in water, but MgO is less soluble due to the stronger ionic bonding.

Question 21

Describe the bonding and structure in diamond and graphite and explain their physical properties. (6 marks)

Answer 21

1. Diamond and graphite are giant covalent (macromolecular) structures.
2. In diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement.
3. In graphite, each carbon atom is bonded to three others in hexagonal layers with delocalised electrons between the layers.
4. Diamond is hard, has a high melting point, and does not conduct electricity because it has no free electrons.
5. Graphite conducts electricity due to delocalised electrons, is soft because the layers can slide over each other, and also has a high melting point due to strong covalent bonds within the layers.

Question 22

Explain how hydrogen bonding arises and how it affects the boiling points of water and hydrogen sulfide. (6 marks)

Answer 22

1. Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (such as N, O, or F), creating a large dipole.
2. The δ+ hydrogen is attracted to a lone pair on a δ– atom of another molecule.
3. In water, hydrogen is bonded to oxygen, which is very electronegative and has lone pairs, so hydrogen bonds form between water molecules.
4. Hydrogen sulfide does not form hydrogen bonds because sulfur is less electronegative. Therefore, water has stronger intermolecular forces than H₂S and a much higher boiling point.

Question 23

Compare and explain the bonding and structure of silicon dioxide and carbon dioxide. (6 marks)

Answer 23

1. Silicon dioxide has a giant covalent structure where each silicon atom is covalently bonded to four oxygen atoms.
2. Carbon dioxide is a simple molecular compound with double covalent bonds between carbon and oxygen atoms and weak van der Waals forces between molecules.
3. SiO₂ has a high melting point due to many strong covalent bonds that require a lot of energy to break.
4. CO₂ has a low melting point because only weak intermolecular forces need to be overcome.

Question 24

Describe the bonding in a metal and explain why metals are malleable and conduct electricity. (6 marks)

Answer 24

1. Metallic bonding consists of a lattice of positive metal ions surrounded by a sea of delocalised electrons. These electrons are free to move throughout the structure.
2. Metals conduct electricity because the delocalised electrons can carry charge.
3. Metals are malleable because the layers of positive ions can slide over each other while the delocalised electrons maintain the metallic bond, allowing the metal to change shape without breaking.

Question 25

Compare the intermolecular forces and boiling points of an aldehyde and a carboxylic acid with similar Mr values. (6 marks)

Answer 25

1. Carboxylic acids have both a C=O group and an –OH group, which allow them to form hydrogen bonds between molecules. 2. Aldehydes contain a polar C=O group, so they experience permanent dipole-dipole interactions, but not hydrogen bonding. 3. Hydrogen bonding is stronger than dipole-dipole forces, so more energy is required to break the intermolecular forces in carboxylic acids. 4. As a result, carboxylic acids have higher boiling points than aldehydes of similar molecular mass.

Question 26

Explain the differences in solubility in water between hexane, hexanal, and hexanoic acid. (6 marks)

Answer 26

1. Hexanoic acid contains an –OH and a C=O group and can form hydrogen bonds with water. This makes it highly soluble in water, especially for small-chain acids. 2. Hexanal contains a polar C=O group and can form dipole-dipole interactions with water, so it is moderately soluble. Hexane is non-polar and cannot form hydrogen bonds or dipole interactions with water, so it is insoluble. 3. Solubility in water depends on the ability to form hydrogen bonds with water molecules.

Question 27

Describe cycloalkanes boiling points

Answer 27

1. Cycloalkanes generally have higher boiling points than their corresponding straight-chain alkanes 2. This is due to the increased surface area and stronger VDWs forces that result from the ring structure of cycloalkanes, allowing them to pack more closely together and experience stronger intermolecular attractions.

Question 28

State the meaning of the term stereoisomers and explain how they arise

Answer 28

1. Molecules with the same structural formula but a different spacial arrangement of atoms 2. Explain how it arises by discussing that C=C cannot rotate, 3. Explains how it arises by discussing that each C in C=C has two different groups

Question 29

Suggest one reason other than the use of mean bond enthalpies why a blue for the enthalpy of combustion of a liquid alkane is different from an obtained value

Answer 29

- alkane is not gaseous or - equation relates to gaseous alkanes or - that water/alkane/substances are gaseous in calculations using bond enthalpies

Question 30

Give the meaning of the term concordant tires

Answer 30

Titres that are within 0.1 cm3 of each other -> NEED the units

Question 31

Use your knowledge of carbocations to explain why this rearrangement takes place

Answer 31

Changes from secondary to tertiary carbocation to make it more stable