Chapter 6 - Chemical Equations : Test for Anions

Question 1

Reactants

Answer 1

Starting substances of a reaction

Question 2

Products

Answer 2

Substances formed in a reaction

Question 3

Anions

Answer 3

Negative ions

Question 4

Law of Conservation of Mass

Answer 4

States that the total mass of the products of a chemical reaction is the same as the total mass of the reactants

Question 5

Law of Conservation of Matter

Answer 5

Sates that in any chemical reaction, matter is neither created or destroyed, but merely changes from one form into another

Question 6

how do you balance a chemical equation?

Answer 6

1. Count the number of atoms of each element on the LHS and on the RHS
2. Edjust the numbers in front of each formula to make the LHS equal the RHS
3. Recheck again to ensure the number of atoms of each element on the LHS equals the number of atoms of that element on the RHS

Question 7

How do you test for the presence of chloride (Cl^-) ions in a aqueous solution?

Answer 7

1. Add ¹/₄ test tube of deionised water
2. Add a few crystals of chloride salt e.g. NaCl
3. Shake to dissolve
4. With a clean dropper, add a small amount of silver nitrate (AgNO₃) → a white cloudy precipitate appears
5. Add a ¹/₄ test tube of ammonia solution →precipitate should dissapear

Question 8

What is the chemical equation for the reaction of silver nitrate and a chloride salt?

Answer 8

Ag⁺ + Cl^- → AgCl↓

Question 9

How do you test for the presence of sulfate ions (SO₄^2-) and sulfite ions (SO₃^2-) in an aqueous solution

Answer 9

1. Add ¹/₄ test tube of deionised water to two test tubes
2. Add afew crystals of sulfate salt (e.g. Na₂SO₄) to the water in one of the tubes. Do the same with the sulfite salt (e.g. Na₂SO₃)
3. Shake to disslove
4. Add a few drops of barium chloride (BaCl₂) solution → both the sulfate and sulfite should produce a white precipitate
   
   1. Add ¹/₄ test tube dilute HCl → ​the sulfite precipitate dissolves, the sulfate doesn’t react

Question 10

What is the chemical equation for the reaction of barium chloride and a) a sulfate salt and b) a sulfite salt?

Answer 10

Ba²⁺ + SO₄^2- → BaSO₄↓

Ba²⁺ + SO₃^2- → BaSO₃↓

Question 11

What is the chemical equation for the reaction between barium sulfite and HCl?

Answer 11

BaSO₃ + HCl → BaCl₂ + SO₂ + H₂O

Question 12

What is the chemical equation for the reaction between barium sulfate and HCl?

Answer 12

BaSO₄ + HCl → no reaction

Question 13

How do you test for the presence of carbonate ions (CO₃^2-) and hydrogencarbonate ions (HCO₃^-​) in an aqueous solution?

Answer 13

1. Place 1cm of carbonate salt, e.g. NaCO_3, in a test tube
2. Set up the apparatus so that the test tube has a stopper with a tube leading out of it into a container of limewater
3. Remove the stopper and add a ¹/₄ test tube of HCl (and quickly replace the stopper)
4. Repeat with the hydrogencarbonate salt, e.g. NaHCO₃ → fizzing turns the limewater milky
5. Add carbonate salt to ¹/₄ test tube of deionised water
6. Add ¹/₄ test tube of magnesium sulfate (MgSO₄) solution → white precipitate is formed
7. Repeat witht the hydrogencarbonate salt → ​no change, but boiling gives a precipitate

Question 14

What is the chemical equation for the reaction of carbonate/hydrogencarbonate salt with HCl?

Answer 14

CO₃^2- + 2H⁺ → CO₂ +H₂O

HCO₃^- + H⁺ → CO₂ +H₂O

Question 15

What is the chemical equation for the reaction of carbon dioxide with limewater?

Answer 15

Ca(OH)_{2 +} CO₂ → CaCO₃↓ + H₂O

Question 16

What is the chemical equation for the reaction of calcium carbonate and carbondioxide and water? (i.e. if you continue to pass carbondioxide through limewater)

Answer 16

CaCO₃ + CO₂ + H₂O → Ca(HCO₃) … clear again

Question 17

How do you test for the presence of nitrate ions (NO₃^-) in an aqueous solution

Answer 17

1. Add nitrate salt, e.g. NaNO₃ to ¹/₄ test tube of deionised water
2. Shake to dissolve
3. Add ¹/₄ test tube of iron (II) sulfate solution (FeSO₄)
4. In a fume hood, add 3ml concentrated sulfuric acid (H₂SO₄) down the side of the slanted test tube → a brown ring appears at the boundary of the two liquids
5. Wash down the sink with lots of water

Question 18

How do you test for the presence of phosphate ions in an aqueous solution?

Answer 18

1. Add phosphate salt, e.g. Na₂HPO₄, to ¹/₁₀ test tube of deionised water
2. Shake to dissolve
3. Fill up to half way mark with ammonium molybdate solution (NH₄)₂MoO₄
4. In the fume hood, add about 5 drops of concetrated nitric acid → a yellow precipitate froms (if not ward to 60^oC in a water bath)