2019 A Level H2 Chemistry Paper 1 Question 9 - Deduce Compound from QA Notes and Periodicity
Here's 2019 A Level H2 Chemistry Paper 1 Question 9.
Unknown compound X is split into 2 different substances solution Y and gas Z.
Y and Z are further reacted and the observations are given in the question.
We need to identify the element in Y and the element in Z that will allow us to determine the 2 elements in X.
By considering the options, we know that there is a metal element and non-metal element in X.
The metal element is either Mg or Al, while the non-metal element is either Si or P.
1. Identify element in solution Y
When NaOH(aq) is added to solution Y, a white precipitate insoluble in excess NaOH(aq) is formed.
This test should remind us of identifying metal cations in QA experiments.
Hence we can refer to the QA Notes in the Data Booklet to identify the metal.
In this case Mg2+ is the metal cation that will form white ppt with NaOH(aq), insoluble in excess.
Therefore the metal element in Y and X will be magnesium, Mg.
2. Identify element in gas Z
The product that we need to focus on is the unknown oxide which is a solid and insoluble in dilute acid or alkali.
Since the metal element has already been identified, the element in gas Z must be a non-metal.
Hence we will need to consider the 2 possible oxides, SiO2 and P4O10, and their reactions with dilute acid and alkali.
Both SiO2 and P4O10 are acidic oxides hence will not react with dilute acids.
They should both react with alkali, but since SiO2 is a giant molecule with strong and extensive covalent bonds that are hard to break, SiO2 only reacts with strong alkali instead of dilute alkali.
P4O10 will be able to react with dilute alkali via neutralisation.
Hence the non-metal element in Z and X will be silicon, Si.
Finally we can compare the options to determine the answer to be option D
Topic: Periodicity, Inorganic Chemistry, A Level Chemistry, Singapore
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