Chemistry wrong answers

Explain why the boiling point of hydrogen is lower than the boiling point of oxygen. 2

Hydrogen has only one electron

Weaker Van der waals forces

(c) Explain the difference in the reaction when sodium, rather than magnesium, is reacted with dilute hydrochloric acid. 2

Sodium is more reactive
Sodium only needs to lose one electron

(i) Give two physical properties of calcium chloride. 2

High melting point

Solube in water

(iii) Explain why the reaction of calcium with chlorine is a redox reaction.

You must refer to changes in oxidation number in your answer. 4

Calcium has been oxidised
Oxidation number of calcium has changed from 0 to +2
Chlorine has been reduced
Oxidation number of chlorine has changed from 0 to -1

Discuss the trends in electronegativity values shown in Figure 3.

Your answer should include why there are no values for the noble gases. 6

Groups

Number of electron shells increase down the group
Atomic radius increases down the group

Periods

Same number of shells
More protons in the nucleus

Noble gases

Do not form bonds

Noble gases have full outer shells

(ii) Explain why carbon monoxide can also be formed during combustion reactions of carbon with oxygen. 2

Incomplete combustion because insufficent oxygen

Carbon dioxide contains double bonds.

Carbon monoxide contains a triple bond.

(i) Explain the difference in strength between the carbon to oxygen bonds in the two molecules shown in Figure 1.

Carbon monoxide has a stronger bond because there are three pairs of electrons
Triple bonds are stronger than double bonds

(ii) Explain why carbon monoxide has permanent dipole-dipole forces of attraction but carbon dioxide does not. 4

Oxygen is more electronegative
Electrons pulled (more strength) towards oxygen (forming dipole)
There is an oxygen at each end of carbon dioxide
Dipoles cancel out in carbon dioxide

(iii) Describe the structure of solid scandium sulfate.

Giant ionic structure
Strong electrostatic attractions between oppositely charged ions

A lattice between ions in all directions

Discuss the difference in melting points of the elements shown in Figure 2.

In your answer, refer to the type of bonding and structure of the elements.

Metals

Li and Be are metals - have metallic bonding, positive ions in a sea of delocalised electrons

Li donates 1 electron - forms +1 ion, Be donates 2 - forms +2 ions

Higher charge ions & more electrons = stronger attraction to sea of electrons = stronger metallic bonding = Higher amount of energy to break metallic bond

Non metals

B, N and Ne are non mentals

B and N have covalent bonding - electrons are shared between atoms

Using the information in Table 1, explain why the C—— O bond is stronger than the C—H bond. 4

C=O has a double bond

C=O has two shared pairs of electrons

Stronger attraction between shared pairs of electrons and nuclei of atom

so therefore more energy to break the bond

(ii) Explain the trend in the boiling points of the halogens in Table 2. 3

Boiling points increase down the group

Larger atoms / more electrons

Stronger van der Waals forces

(ii) Explain why a molecule of hydrogen chloride is polar. 2

Large difference in electronegativity, electrons are more attracted to the Cl

so H is ∂+ and Cl ∂–

Iron can be extracted from iron oxide.

(b) Give the oxidation number of iron in Fe and in Fe2O3

Fe = 0

Fe2O3 = +3

lithium

• fizzing

• metal floats and moves around the surface

of the water

• universal indicator turns purple

sodium

• rapid fizzing

• metal floats and moves quickly around the surface of the water

• universal indicator turns purple

potassium

• very rapid fizzing

• lilac flame and popping sound

• metal floats and moves very quickly around the surface of the water

• universal indicator turns purple

Explain the similarities and differences between these three reactions.

Similarities

All turn universal indicator purple because they all produce a hydroxide

All float because the density is less than water

All fizz because they produce hydrogen gas

Differences

potassium moves most quickly because it is the most reactive

only potassium produces a flame as it is most reactive

the metals get more reactive as you move down the group

(ii) Describe, in terms of structure and bonding, why copper is ductile.

Giant lattice of atoms held together by delocalised electrons
Can move over one another
So the copper can be stretched

In industry, scrap iron can be used to extract copper from solutions of waste copper compounds. A reaction in which iron is used to extract copper is

Fe + CuSO4 → FeSO4 + Cu

(b) Explain what type of reaction is taking place. 2

Redox
Iron is oxidised and copper ions are reduced

ii) Describe the principles that have been applied when completing the electron-in-box model in Figure 1.

Aufbau principle

Electrons fill the orbitals of lowest energy first

Orbitals with the same energy must all be filled singly before electrons are paired

Each orbital can only hold 2 electrons

Spins (of paired electrons) are opposed

Aluminium, Al, can form aluminium ions, Al3+.

(a) Describe how aluminium atoms form aluminium ions. 2

Electrons are lost from outer shell
Three electrons

Table 1 shows three of the elements in period 3, the formulae of their oxides and the

melting point of the oxides.

element sodium silicon sulphur

formula of oxide Na2 O SiO2 SO2

melting point of oxide (°C) 1132 1710 –73

Discuss how the structure and bonding of the oxides shown in Table 1 change across

period 3.

You should use the melting point data to support your answer.

Na is a metal and O is a non-metal

Large difference in electronegativities between the Na and O

Electrostatic attraction between ions = large amounts of energy required to break bonds so has a high melting point

Si, S and O are non-metals
Small difference in electronegativities between the Si/S and O
Weak intermolecular forces in SO2 that require a small amount of energy to overcome, so has a low melting point.

What is a dative covalent bond

Shared pair of electrons

Electrons come from one atom

(c) Explain the trend in the size of the ionic radius from Li+ to B3+. 4

Greater electrostatic attraction between electrons and protons
Ionic radius decreases
Positive ions have the same number of electrons
Number of protons is increasing

Explain how the pattern of ionization energies shown in Figure 4 can be used to determine the group and period of the element. 6

14 points on group = 14 electrons = 14 protons = atomic number 14
1st section has 4 points, which is 4 electrons in outermost shell
So silicon is in group 4
2 jumps in graph so therefore 3 shells of electrons
So silicon is in period 3
Electron configuration 2.8.4

(a) (i) A physical property of group 1 metals is that they have a low melting point.

Give one other physical property of group 1 metals.

Shiny when cut

(ii) Give one use of sodium hydroxide.

oven cleaner

(ii) Give one use of sodium hydroxide.

oven cleaner

Explain how the electronegativity changes from gallium to bromine 4

Electronegativity increases from gallium to bromine
Greater number of protons in the nucleus
But the same amount of shielding
So more attraction for the bonding pair of electrons

An iron nail and a copper coin are placed in separate solutions of copper(II) sulfate.

The nail and the coin are removed from the copper(II) sulfate solutions after one hour.

Table 2 shows the recorded observations.

object observations

iron nail brown coating, solution loses blue colour

copper coin no coating, solution remains blue

Explain, in terms of oxidation and reduction, the observations shown in Table 2.

You may include equations to support your answer.

Iron nail
Displacement reaction
Iron is more reactive than copper
Iron loses electrons to form iron ions = therefore oxidised

Copper coin

Copper metal in the coin = same reactivity as the copper ions and so the copper is not displaced = no oxidation or reduced occurs
Copper sulfate solution is blue