GCE O Level Tuition

Everything you want to know about acids

1. What is an acid?

A: An acid is a substance that ionizes in water to produce H+ as the only positive ion.

2. What is an alkali?

A: An alkali is a substance that ionizes in water to produce OH- as the only negative ion.

3. What causes a solution to be acidic?

A: Hydrogen ions, H+.

4. What causes a solution to be alkaline?

A: Hydroxide ions, OH-.

5. What is a strong acid?

A: Strong acids are acids that undergoes complete ionization of hydrogen ions in water.

6. What is a weak acid?

A: Weak acids are acids that undergoes partial ionization of hydroxide ions in water.

7. What are indicators?

A: An indicator is a chemical substance added to determine the pH of a substance.

8. What are the common indicators?

A: Universal indicator - Red at pH 1 for strong acids, Orange and yellow for weak acids, Green for neutral solution, blue for weak alkali and violet for strong alkali.

9. What exactly is neutralization?

A: Process where acids and bases react with form salt and water only.

10. What is a base?

A: A substance that reacts with acids to form salt and water. When it is soluble, it forms an alkali.

11. What are the products when acids react with metals?

A: Salt and hydrogen gas.

12. What are the products when acids react with metal carbonate?

A: Salt, water and carbon dioxide.

13. What are the products when acids react with metal oxide, also known as a base?

A: Salt and water.

14. What are the products when acids react with metal hydroxide, also known as a base?

A: Salt and water.

15. What is produced when bases dissolved in water?

A: Alkalis

16. Write the ionization equations of all common acids, including state symbols.

A: HCl --> H+ + Cl-, CH3COOH --> H+ + CH3COO-, HNO3 --> H+ + NO3-, H2SO4 --> 2H+ + SO42-. State symbols all aqueous

17. Write the ionization equations of common alkalis, including state symbols.

A: NaOH --> Na+ + OH- or Ca(OH)2 --> Ca2+ + 2OH- or NH4OH --> NH4+ + OH- and again all state symbols are aq.

18. Describe a simple test to prove that water is needed for the ionization of acids.

A: Dissolve nitric acid into water and ethanol separately. Connect a light bulb and a battery to the solution. The acid that dissolved into water will enable the light bulb to light up and the acid that dissolves into ethanol will not light up. It is because in the presence of water, acid dissociates to produce hydrogen ions. These ions are free to move to conduct electricity therefore the bulb that is connected to acid dissolved in water lights up. Ethanol is an organic solvent and therefore it will not allow acid to ionize.

19. Which oxides are non-metal oxides?

A: Acidic oxides and neutral oxide.

20. Which oxides are metal oxides?

A: Basic oxides and amphoteric oxide.

21. What is an acidic oxide?

A: Acidic oxides are non metal oxides when dissolved into water forms acids.

22. What is a neutral oxide?

A: Neutral oxides are non metal oxides that have no effect on the pH of a solution. All monoxides like carbon monoxides, nitrogen monoxide and water.

23. What is a basic oxide?

A: Basic oxide are metal oxides that, if soluble forms alkali. Bases also react with acids to form salt and water.

24. What is an amphoteric oxide?

A: Amphoteric oxides are metal oxides that react with both acids and alkalis to form salt and water. Common amphoteric oxides ZnO, Al2O3 and PbO. Collectively, they form the acronym ZAP.

25. When acid is spilled on the ground, suggest how they can be easily neutralized.

A: Calcium carbonate is used to neutralize spilled acid. As calcium carbonate is insoluble and neutral, when added in excess, will not cause the resulting mixture to be alkaline. Since it is insoluble, the excess calcium carbonate can be easily swept up.

26. Suggest a good substance to neutralized acidic soil.

A: Calcium oxide or lime can be used to neutralize acidic soil as it is a basic oxide. In some instance, calcium hydroxide is also used to neutralize the soil.

27. A farmer adds an alkali to neutralize acidic soil and ammonium sulfate to fertilize the soil. Explain why the farmer is wasting his time.

A: The alkali and ammonium sulfate reacts to produce ammonia gas and salt. The alkali thus fail to neutralize the acidic soil effectively. Ammonium salts are added to increase the nitrogen content of the soil. But the reaction produces ammonia gas, which causes the nitrogen to escape to the environment as a gas and thus unable to be absorbed by the plant.

28. Solution A has a pH of 1, B has a pH of 3, C has a pH of 7, D has a pH of 10 and E has a pH of 14. You are given potassium chloride, sodium hydroxide, nitric acid, ethanoic acid and aqueous ammonia. Assign the alphabets to the substances according to their pH.

A: pH 1- nitric acid. pH 3 - ethanoic acid. pH 7 - potassium chloride. pH 10 - aqueous ammonia, also known as ammonium hydroxide. pH 14 - sodium hydroxide.

29. Solution A has a pH of 1, B has a pH of 3, C has a pH of 14, D has a pH of 10. Which 2 solution when mixed together MUST produce an acidic solution.

A: Solution A and B since they are both acidic, the resulting mixture must be acidic.

30. Many plants do not grow well in acidic soil, calcium oxide is often used to solve this problem. What is the name of the chemical process?

A: Neutralization.

31. What is the difference between concentration of acids and the strength of acids?

A: Concentration measures the amount acid dissolved in the solvent but strength of acids measures the degree of ionization of the acid. So it is possible to have a high concentration acid and yet it is weak.

32. What are the products form when copper reacts with nitric acid?

A: Nothing. There is no reaction as copper is a non reactive metal. Together with silver and gold, they do not have reactions with acid. However, their carbonates, oxides or hydroxides still react with the acids.

33. Describe a reaction in which sulfuric acid is not acting as an acid.

A: In a precipitation reaction, sulfuric acid can react with barium nitrate to form barium sulfate precipitate. In order to at as an acid, sulfuric acid needs to react with metal, base or metal carbonate. In a precipitation reaction, it is merely acting as a soluble sulfate which caneasily be replaced by sodium sulfate.

34. Zinc oxide is added nitric acid and aqueous sodium hydroxide. State which reaction produce a salt. Explain why.

A: Both. Zinc oxide is amphoteric therefore is able to react with both acids and alkalis to form salt and water. The other possible oxides are aluminium oxides and lead (II) oxide.

35. The pH of sulfuric acid is 1. State and explain what happen to the pH of the acid when 20g of sodium chloride solution is added to it.

A: The solution remains at pH 1. Sodium chloride is a neutral salt and thus does not affect the pH of the solution.

36. There is an acid spill in the laboratory, state a reagent that can be added in excess without causing the resultant mixture to be alkaline.

A: Calcium carbonate.

37. State all the reactants that can be added to nitric acid to form silver nitrate.

A: Silver oxide, silver carbonate and silver hydroxide. Silver does not work as it is an unreactive metal and thus unable to react with acid.

38. State 2 ways to produce ammonia gas.

A: Heating a concentrate solution of ammonia. Warm an alkali with an ammonium salt.

39. Describe a simple to test distinguish between potassium carbonate and ammonium carbonate.

A: Since both salts contain carbonate, testing for carbonate will not be useful. Add sodium hydroxide to both solution, warm. The solution that produces a gas that turns moist litmus blue contains ammonium salt.

40. A substance known as potassium hydrogen phosphate is known as a salt and an acid. Explain why it is both an acid and salt.

A: It is a salt as it is made by the reaction between an acid, phosphoric acid and an alkali, sodium hydroxide. It is an acid as it is able to ionize to produce hydrogen ions in water.

41. When potassium hydroxide reacts with phosphoric, there are 3 possible salts formed. State the name and formula of the 3 salts.

A: Potassium phosphate, Na3PO4, potassium dihydrogen phosphate NaH2PO4, potassium hydrogen phosphate Na2HPO4.

42. Describe a simple experiment to test the strength of nitric acid and ethanoic acid. Explain why they display that way.

A: React both acids with magnesium metal, the reaction between nitric acid will be faster indicating that it is a stronger acid. As a strong acid, it undergoes complete ionization of hydrogen ions and thus with a higher concentration of hydrogen ions causes the reaction to be faster.

43. A titration between aqueous barium hydroxide against sulfuric acid is performed. A graph of electrical conductivity against volume of sulfuric acid is plotted. The shape of the graph is a V - shaped graph, at the lowest point of the V - shape graph, it corresponds to no electrical conductivity. Suggest the implication of the volume of sulfuric acid at the lowest point of the V - shape graph. Explain, in terms of ions, why the electrical conductivity of the mixture decreases to zero as more sulfuric is added and subsequently increases again when more sulfuric acid is added.

A: Initially, when there only barium hydroxide alkali in the solution, the solution has free moving hydroxide and barium ions, hence it conducts electricity. When sulfuric acid is added, it forms the insoluble barium sulfate and water. The number of free moving ions decreases and the electrical conductivity decreases. When all the barium hydroxide has been neutralized by the sulfuric acid, the mixture contain only barium sulfate precipitate and water. There are no free moving ions and electrical conductivity goes to zero. The ions in insoluble barium sulfate are held by strong electrostatic force of attraction and therefore cannot conduct electricity. When excess sulfuric acid is added to the mixture, it introduces free moving hydrogen ions, this causes the electrical conductivity to increase. Hence the shape of graph is a v shaped graph. The significance of the lowest point of the v shape graph is the point where the volume of sulfuric acid needed to neutralize the barium hydroxide. This volume can be used for mole calculation.

44. State, as many as you can, an oxide that can react with acid and alkali.

A: Aluminium oxide, zinc oxide and lead (II) oxide.

45. State an oxide which contribute to acid rain.

A: Sulfur dioxide, nitrogen dioxide and carbon dioxide. This is because they are non metal oxides and can dissolve in rain clouds to form acid rain.

46. Metals are basic oxide and are likely to react with acid to form salt and water. State 3 metal basic oxide which DO NOT react well with sulfuric acid.

Lead (II) oxide, barium oxide and calcium oxide. This is because the 3 oxides form insoluble sulfates. These insoluble sulfates forms an insoluble layer around the oxides preventing reaction with acids. The reaction comes to a premature end. In fact this also prevents their carbonates, hydroxides or metals to react well with acid. They are insoluble salts so can work well when made with the precipitation method. Similarly, silver chloride and lead (II) chlorides are insoluble too. Their oxides, hydroxides, carbonates and metals will have problem reaction with hydrochloric acid too.

47. State an oxide that do not change the colour of universal indicator.

A: Carbon monoxide, nitrogen dioxide. They are neutral oxides and so they have no reaction with an acid. On a side note, water is also a neutral oxide.

48. A student came up with a report detailing the steps to make silver nitrate solution. The following is his report. Add silver carbonate to a fixed amount of nitric acid. If the silver carbonate reacted, add more silver carbonate until no more silver carbonate dissolves. The mixture is then filtered. The filtrate is then heated to approximately to one third of its original volume and the hot solution is allowed to cool. The crystals are then filtered and washed with small amount of cold distilled water and dried by pressing between filter paper. Do not evaporate the solution to dryness. Explain the terms that are underlined and their roles in the salt making process.

A: More silver carbonate till no more dissolves is to ensure all the nitric acid is used up. Filtering is to remove excess unreacted silver carbonate. Heating to one third of solution is to obtain a saturated solution. Cooling is to allow crystals to form. Small amount of cold distilled water is to prevent soluble from dissolving in the distilled water since they are soluble. Crystals have poor solubility at cold temperature. Evaporating the solution to dryness will heat away the water of crystallization preventing the formation of the crystals.

49. Suggest a good indicator for titration for mixture that changes colour in the pH zone of 3 to 5.

Methyl orange - When alkali added to acid (with methyl orange), it changes from red to yellow. When acid added to alkali (with methyl orange), it changes from yellow to red.

50. Suggest a good indicator for titration for mixture that changes colour in the pH range of 9 to 11.

A: Phenolthalein. When alkali added to acid (with phenolthalein), it changes from pink to colourless. When acid added to alkali (with phenoolthalein), it changes from colourless to pink.