Anode: 2H2 (g) + 4OH– (aq) –> 4H2O (l)+ 4e–
Cathode: O2(g) + 2H2O (l) + 4e– –> 4OH– (aq)
Overall: 2H2(g) + O2(g) –> 2H2O (l)
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List 3 examples of exothermic reactions, and 3 examples of endothermic reactions.
Examples of exothermic reactions are:
– combustion of fuels
– rusting of iron
– neutralisation reaction
Examples of endothermic reactions are:
– photosynthesis
– thermal decomposition
– dissolving of some ionic compounds in water (eg NH4Cl)
This reaction, CH4 + 2O2 –> CO2 + 2H2O, is exothermic. Describe this exothermic reaction in terms of energy involved in bond breaking and bond making.
The heat energy absorbed to break the bonds in CH4 and 2O2 is less than the heat energy released to form the bonds in CO2 and 2H2O, so overall enthalpy change is negative.
Write down the 5 reactions in the blast furnace for the extraction of iron from haematite.
C (s) + O2 (g) –> CO2 (g)
CaCO3 (s) –> CaO (s) + CO2 (g)
C (s) + CO2 (g) –> 2CO (g)
Fe2O3 (s) + 3CO (g) –> 2Fe (l) + 3CO2 (g)
CaO (s) + SiO2 (s) –> CaSiO3 (l)
Give 3 advantages of recycling metals.
It helps to reduce pollution. Extraction of metals from their ores produces much more air, land and water pollution compared to recycling of metals.
It conserves energy resources. Extraction of metals from their ores uses up much more energy compared to recycling of metals.
It conserves limited land resources. Metal mining and smelting processes use up large area of land. Recycling metals reduces the need for these metal mines, and free up the land for alternative uses.
Explain why, going from left to right in the same Period, the atomic radii of the element decrease.
Elements in the same Period have the same number of electronic shells, but the proton number of the elements increase from left to right.
As number of protons increase, the attractive force of the positive protons on the negative electrons also increases, pulling the electronic shells closer together.
Hence atomic radius decreases from left to right in the same Period.
Explain why, going down Group VII, the melting point and boiling point of the element increases.
Group VII elements exist as diatomic molecules with simple molecular structure.
Going down Group VII, the molecular mass of the element increases.
An increasing molecular mass results in increasing intermolecular force of attraction.
The higher the intermolecular force of attraction, the higher the energy needed to break these bonds, hence higher melting point and boiling point.
Explain why, going down Group I, the melting point and boiling point of the element decreases.
Going down Group I, the number of electronic shells increases, and atomic radius increases. As the valence electrons get further from the nucleus, the attractive force between the positive protons in the nucleus and the negative valence electrons decreases.
Hence, going down Group I, the metallic bonding formed between the positive protons in the nucleus and the delocalized valence electrons becomes weaker.
The weaker the metallic bond, the lesser the energy needed to break these bonds, and the lower the melting point and boiling point of these elements.
Define relative molecular mass.
Relative molecular mass is the average mass of one molecule of a substance compared to the mass of 1/12 of a carbon-12 atom.
Define relative atomic mass.
Relative atomic mass is the average mass of one atom of an element compared to the mass of 1/12 of a carbon-12 atom.