Questions on Ionic Equations, Colours of Cations & Anions, Acid, Bases, Alkalis, Salts

Q1. Zinc Nitrate reacts with Aqueous Ammonia (NH4OH) to form a salt and a base.
       (i) Name the salt and base.
       (ii) Describe the observation for the reaction.
       (iii) Write a chemical equation for the reaction (include state symbols).
       (iv) Write an ionic equation for the reaction.

Ans: (i) The salt is ammonium nitrate (NH4NO3) and the base is zinc hydroxide ( Zn(OH)2 )
        (ii) A white precipitation is form. The white precipitate is soluble in excess to give a colourless solution.
        (iii)  Zn(NO3)2 (aq) + 2 NH4OH (aq) = Zn(OH)2 (S) + 2 NH4NO3 (aq)
           (iv) Zn²⁺ (aq) + 2 OH^- (aq) = Zn(OH)2 (S)

Q2. Ammonium Sulfate is heated with Sodium Hydroxide.
       (i) Write a chemical equation (include state symbols) for the reaction.
       (ii) Describe a test for the gas.

Ans: (i) (NH4)2SO4 (S) + 2 NaOH (aq) = Na2SO4 (S) + 2 NH3 (G) + 2 H2O (L)
        (ii) Hold a piece of dump red litmus paper over the mouth of the test tube. The ammonia gas will turn the damp red litmus paper blue.

Q3. An unknown green solution is heated with a piece of aluminum foil and sodium hydroxide solution.
       (i) The gas produced turns damp red litmus paper blue. Name the gas evolved.
       (ii) This is a confirmatory test for anion. Name this anion.
       (iii) Give a possible cation which gives the green solution.

Ans: (i) The gas evolved is ammonia (NH3).
        (ii) The anion is nitrate (NO3^-).
        (iii) The possible cation is iron(II) (Fe²⁺).

Q4. Sulfuric acid is titrated with potassium hydroxide in the preparation of potassium sulfate salt.
       (i) Explain why this method is recommended for the preparation for this salt.
       (ii) Write an ionic equation for the reaction.

Ans: (i) Because potassium sulfate is soluble salt, so it cannot be prepared by precipitation method. Titration method is for group (I) and ammonium salt only and potassium sulfate is also a group (I) salt, hence it can be prepared using titration method.
        (ii) H⁺ (aq) + OH^- (aq) = H2O (L)

Q5. Excess zinc carbonate is added to hydrochloric acid in the preparation of zinc chloride salt.
       (i) Write an ionic equation for the reaction.
       (ii) Why excess zinc carbonate is used?
       (iii) Briefly explains how the zinc chloride crystals can be obtained.

Ans: (i) ZnCO3 (S) + 2 H⁺ (aq) = Zn²⁺ (aq)  + CO2 (G) + H2O (L)
                (ii) Because the excess zinc carbonate is used to react with hydrochloric acid to prepare the zinc chloride salt as zinc chloride salt is soluble but not group(I) or ammonium salt. The zinc carbonate is used to excess tp make sure that is has completely reacted with hydrochloric acid to form calcium chloride.
        (iii) Firstly, add the excess zinc carbonate solid to a beaker of dilute hydrochloric acid until there is excess zinc carbonate left behind.
              Secondly, filter to remove the excess unreacted zinc carbonate solid.
              Thirdly, evaporate the zinc chloride solution to remove the water and make a saturated salt solution.
              Lastly, leave the hot saturated solution to cool down and it will form zinc chloride crystals.

comments on 3 answers

Hi Mr Tan, I have given some comments of other people's answer (in my blog?) as told. So people, please pardon me for any negative comments...Thx.

1) First of all, for Wan Ling (16)'s answer, I think that her answer was clear and accurate. She also includes some diagrams to add on to her explaination. This is what I think we should all learnt from. I guess one mistake that I find is that the relative mass of electrons should be 1/1840 (according to the textbook). Also, for the diagram for sulfur ion, the 2- should be outside the bracket. To conclude, her answer was nice and quite accurate with few mistakes.

2) Secondly, for Su (24)'s answer, she has very detailed and great answer. She also included some examples in her answer. I think we should learnt from it. For the first question, I was quite amezed at the length of the answer and the details. I have learnt from her answers. However, for the question 5, I think she can add on with the physical properties. Also, for the sulfur ion, there should be a 2- outside bracket. To conclude, I liked her answers as it is nice and interesting.

3) Lastly, for Aleen (27)'s answer, her answer are very details with pictures and easy to understand. She also answered the questions very well. Her drawings are also very clear and neat. But I think she write wrongly for the sodium ion, it should be Na not S and outside the brackets of sodium ion should be + not 1+. In conclusion, she had done a great work with detailed explaination and clear answers.

Atomic Structure Assignment

1. What does an atom looks like? What are the sub-atomic particles inside it.....? (Talk about electrons, neutrons, protons, electron shells, nucleus...)

Ans: Atom is the smallest foundation unit which builds up a matter. The structure of an atom is made up of neutrons, protons and electrons. It also has a nucleus in the center of the atom. The neutrons and protons are found inside the nucleus, packing closely together while electrons are found outside the nuclues, surrounding the neutrons and protons. The electrons outside the nucleus moves rapidly around the nucleus in the electron shells. In an atom, the number of protons is always equals to the number of electrons. Atoms are always neutral because each proton carries one positive charge, +1 while each electron carries one negative charge, -1 and neutron have no charges. Since every atom have the same number of protons and electrons, atoms will always be neutral. Each proton has a relative mass of 1, each neutron also has a relative mass of 1 while each electron has a relative mass of 1/1840.

2. Draw the atomic structure of a sodium atom and a sodium ion....explain why you draw it this way.

Ans:

Because sodium, Na is found in group I, it has a electronice configuration of (2.8.1), hence sodium atomic structure will have 2 electrons on the first shell, 8 electrons on the second shell and 1 electron on the valency shell. Since it is in group I, sodium ion  will have lose 1 electron in order to complete the valency shell and become stable.

3. Draw the atomic structure of a sulfur atom and a sulfide ion....explain why you draw it this way.

Ans:

Correction:

Because sulfur, S is found in group VI, it has a electronic configuration of (2.8.6), hence the sulfur atomic structure will have 2 elecctrons in the first shell, 8 electrons in the second shell and 6 electrons in the thrid shell. Since it is in group VI, the sulfur ion atomic structure will have to gain 2 electrons in order to complete the valence shell and become stable.

4.Chlorine-35 atom and Chlorine-37 atom are called isotopes...Use these two examples to explain what is 'isotopes'.

Ans: Isotopes are different atoms of the same element which have the same number of protons but different number of neutrons. Both Chlorine-37 and Chlorine-35 have the same number of protons, 17 Protons. However, Chlorine-37 have 20 neutrons (37-17=20), however, Chlorine-35 have 18 neutrons (35-17=18). Since they have same number of protons and different number of neutrons, Chlorine-35 and Chlorine-37 are isotopes.

5. Sodium is a metal and sulfur is a non-metal....why we classify them this way??

Ans: Metals can conduct electricity and loses electrons in order to complete the valence shell while non-metals cannot conduct electricity and gains electrons in order to complete the valence shell. Sodium, Na is a metal because it is a good conductor of electricity and it loses one electron to complete the valence shell to make it stable and to form positive ion, Cat ion. In contrast, Sulfur, S is a non-metal because it is an insulator of electricity and it gains two electrons to complete the valence shell to make it stable and to form negative ion.