Acids-----------Alkalis-and-Bases-CM.pptx

All about acids and bases

1. Types of Acids, Alkalis and Bases Define the following words: concentration, dilute, acid and alkali.

2. Acids, Alkalis and Bases An acid is a substance that has a pH lower than 7 . For example, hydrochloric acid, sulphuric acid, citric acid, ethanoic acid (vinegar). A base is a substance that neutralises an acid – those that dissolve in water are called alkalis. An alkali is a soluble substance with a pH higher than 7 . Acid + Base -> Salt + H 2 O Acid + Metal -> Salt + H 2 Acid + Carbonate -> Salt + CO 2 + H 2 O 

3. Calculating Concentration

4. Dilute and concentrated acids Acids contain a lot of hydrogen ions. If an acid has a higher concentration of hydrogen ions, its pH is lower. If the concentration of hydrogen ions in an acid increases by a factor of 10, the pH decreases by 1 (and vice versa).

5. Dilute and concentrated acids - questions How many times more concentrated is: an acid with a pH of 3 compared to an acid with a pH of 4? An acid with a pH of0 compared to an acid with a pH of 4?

6. Dilute and concentrated acids - answers 10x more concentrated 10 x 10 x 10 x 10 = 10 000x more concentrated

7. Strong and weak acids The pH of an acid also depends on the type of acid. The molecules in strong acids dissociate (break up) completely into ions when they dissolve in water, producing high concentrations of hydrogen ions. Strong acids have a low pH.

8. Neutralisation Do now: What does the word ‘neutral’ mean? How do antacids (such as Gaviscon) help to stop heartburn?

9. A base is any substance that reacts with an acid to form a salt and water only (neutralising that acid). All acids form hydrogen ions in solution, all alkalis for hydroxide ions in solution. All alkalis are bases, but not all bases are alkalis. A soluble base is known as an alkali.

10. Example question: Explain why all alkalis are bases, but not all bases are alkalis. Example answer: Alkalis are soluble bases, but only some bases are soluble in water, so these are not alkalis. e.g. copper oxide is a base, but is insoluble in water so is not an alkali. Sodium hydroxide is a base, which is also soluble in water – making it an alkali. Can the substance neutralise an acid (forming a salt and water)? Yes Is the substance soluble in water? No Yes Base Alkali

11. Your task: Zinc oxide reacts with hydrochloric acid to form zinc chloride and water. Zinc oxide is insoluble in water. State and explain whether zinc oxide is a base, an alkali, or both. Can the substance neutralise an acid (forming a salt and water)? Yes Is the substance soluble in water? No Yes Base Alkali

12. Answer: Zinc oxide is a base. It neutralises an acid, producing a salt and water, but is insoluble (doesn’t dissolve) in water.

13. A  neutralisation  reaction is a reaction between an acid and a base. Remember: acids in solution are sources of hydrogen  ions , H + alkalis in solution are sources of hydroxide ions, OH - In acid-alkali neutralisation reactions, hydrogen ions from the acid react with hydroxide ions from the alkali: H + ( aq ) + OH - ( aq ) → H 2 O(l) Pure water is  neutral  (its  pH  is 7). A neutral solution can be produced if the correct amounts of acid and alkali react together. The change in pH during a neutralisation reaction can be measured using a pH probe and meter, or estimated using  universal indicator  solution and a pH colour chart. An acid-alkali neutralisation is the reaction between hydrogen ions and hydroxide ions, forming water.

14. To name the salt formed, look closely at which acid is used. Hydrochloric acid forms chloride. Sulfuric acid forms sulfate . Phosphoric acid forms phosphate. Nitric acid forms nitrate. Name the salts formed by the following reactions: Acid Alkali Salt Hydrochloric Sodium hydroxide Sodium chloride Nitric Sodium hydroxide Sulfuric Lithium hydroxide Phosphoric Potassium hydroxide

15. Do now: Bee stings have a pH of around 3, wasp stings a pH of around 9. State and explain which sting could be soothed with vinegar, and why it would not work on the other sting.

16. Using Indicators We’re going to be having a look at the effect different indicators have on household chemicals. We’re then going to work out what colour changes we should expect for strong and weak acids, neutral solutions and strong and weak alkalis using each indicator. We’ll be using universal indicator, phenolphthalein, methyl orange and litmus paper (blue and red)

17. Conclusion Categorise each product under the following headings: Strong acid Weak acid Neutral Weak alkali Strong alkali State the colour changes for each category and indicator (e.g. universal indicator turns from green to red in a strong acid). Compare your results and expected colour changes to published data available online. Why do some of your results not fit the published data?

18. Summary Universal indicator changes colour across a range of pHs , it’s why it’s so useful. Red and blue litmus only show alkali (blue), acid (red) and neutral (green) – they do not show strength. Methyl orange is red in the presence of strong acids (up to pH = 3), orange around pH = 4 and yellow in all other conditions. Phenolphthalein is colourless until pH = 9, where it turns purple (remaining so throughout other alkaline colours). Many of our household liquids were dissolved in water so that they could be easily tested, this means they will/may have been closer to neutral than they would if tested without water. This is because the concentration of H+ and OH- ions were varied.

19. Defining Acids and Bases We’ve talked about how acids and bases can be used to neutralise one another, and also some generic reactions they will undergo with other groups. We can also think of acids as proton donors, and bases as proton acceptors.

20. More on Neutralisation Soil pH is often analysed and monitored. Why? Most plants favour a pH between 5 and 8. What might decrease the pH of soil? Crushed limestone (calcium carbonate), lime (calcium oxide) or slaked lime (calcium hydroxide) is often added to acidic soil. Explain, using a generic word equation, why for each lime is added.

21. More on Neutralisation What might decrease the pH of soil? The breakdown of biological material produces acidic compounds, overuse of fertilisers (containing acidic ammonium salts) and acid rain all often cause acidic soil. Crushed limestone (calcium carbonate), lime (calcium oxide) or slaked lime (calcium hydroxide) is often added to acidic soil. Explain, using a generic word equation, why limestone is added. Calcium carbonate, calcium oxide and calcium hydroxide are all bases. When reacted with acidic soil, they will create neutral products. For limestone (calcium carbonate): Metal carbonate + acid -> salt + water + carbon dioxide. For lime (calcium oxide): Metal oxide + acid -> salt + water. For slaked lime (calcium hydroxide): metal hydroxide + acid -> salt + water.

22. Oxides Metal + Oxygen -> metal oxide We saw that these metal oxides would tarnish and often decrease the structural integrity of the metal. How can metals be protected from the formation of metal oxides? Sacrificial protection (e.g. galvanising) - electroplating Painting or other substance coated

23. Acidic Oxides Acidic oxides are formed when a  nonmetal  element combines with oxygen. They react with bases to form a salt and water (neutral products). When dissolved in water they produce an  acidic  solution with a  low   pH. Common examples include SO 2  and SiO 2.

24. Basic Oxides Basic oxides are formed when a  metal  element combines with oxygen. They react with acids to form a salt and water. When dissolved in water they produce a  basic  solution with a  high   pH. Hydroxides are usually alkaline oxides. Common examples include NaOH, KOH and Ca(OH) 2.

25. Acidic or Basic Oxide? Generally, as we move from left to right across the periodic table we move from strongly basic oxides to strongly acidic oxides. Predict whether the following are acidic or basic oxides: CaO SO₂ K₂O Na₂O CO₂ MgO

26. Acidic or Basic Oxide? Generally, as we move from left to right across the periodic table we move from strongly basic oxides to strongly acidic oxides. Predict whether the following are acidic or basic oxides: CaO SO₂ K₂O Na₂O CO₂ MgO

27. Neutral Oxides Some oxides do not react with either acids or bases and thus are said to be  neutral . Neutral oxides tend to have low solubility in water. Neutral oxides will not form salts when reacted with acids or bases. Examples include N 2 O, NO and CO.

28. Amphoteric Oxides Amphoteric oxides are a group of oxides that can behave as  both   acidic  and  basic , depending on whether the other reactant is an acid or a base. In both cases a salt and water is formed. The two most common amphoteric oxides are  zinc   oxide  and  aluminum   oxide. The  hydroxides  of both of these elements also behave amphoterically

29. Titrations Titrations are accurate ways of reacting solutions together. In neutralisation reactions, we can accurately note the endpoint of the reaction using indicators . Using the volumes from a titration, we can then mix our solutions in the right proportions to produce soluble salts . Define the terms underlined. Once a soluble salt is created in solution, crystals can be created using crystallisation. Explain how and why crystallisation is used.

30. Method Use the pipette to add a measured volume of sodium hydroxide solution to a clean conical flask. Add a few drops of universal indicator and put the conical flask on a white tile. Fill the burette with hydrochloric acid and note the starting volume. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix. Stop adding the acid when the end-point is reached (when the indicator first permanently changes colour). Note the final volume reading. Repeat steps 1 to 5 until concordant titres are obtained. More accurate results are obtained if acid is added drop by drop near to the end-point.

31. Soluble Salts What is the name of the salts formed in the following reactions ? Sodium hydroxide reacting with hydrochloric acid Zinc oxide reacting with sulphuric acid Calcium carbonate reacting with nitric acid Remember! The first part comes from the  metal ,  metal  oxide or  metal  carbonate used in the reaction. The second part comes from the  acid .

32. Preparing Salts Some salts are found naturally, and can be mined – others need to be created in the laboratory. Two questions must be answered first: Is the salt being formed soluble or insoluble? Is water of crystallisation present in the salt crystals?

33. Soluble or Insoluble? Insoluble salts are formed using precipitation reactions, soluble salts are often formed from neutralisation reactions (acid and alkali or acid and base/metal/carbonate). You must know the solubility of common salts off by heart (core and extended). Are the following soluble or insoluble? Silver chloride Sodium chloride Potassium nitrate Calcium hydroxide Copper (II) sulphate Magnesium carbonate

34. Soluble or Insoluble? Insoluble salts are formed using precipitation reactions, soluble salts are often formed from neutralisation reactions (acid and alkali or acid and base/metal/carbonate). You must know the solubility of common salts off by heart (core and extended). Are the following soluble or insoluble? Silver chloride Sodium chloride Potassium nitrate Calcium hydroxide Copper (II) sulphate Magnesium carbonate

35. Soluble Salts – adding acid to a solid metal, base or carbonate Heat acid until warm, then add metal/base/carbonate – stirring constantly. Stop adding metal/base/carbonate when it no longer disappears (added in excess). Filter mixture to remove excess base. Transfer solution to evaporating basin. Evaporate water from solution until crystals appear. Remove heat and allow filtrate to dry and crystallise. For each above, draw a labelled diagram showing how each step should be undertaken with the correct lab equipment . At extended/supplement level you would be expected to suggest appropriate methodology for producing salts. At core, you must know the method and be able to discuss and label appropriate equipment.

36. Soluble Salts – reacting an acid and alkali Add alkali and an appropriate indicator to a conical flask, using a pipette. Add acid to the burette, noting the starting volume (usually 50 cm³). Slowly, add acid to alkali until indicator shows a neutral solution. Calculate the volume of acid added. Repeat steps 1-4 without indicator. Transfer the solution from the conical flask to an evaporating basin and heat to partially evaporate the water. Remove the evaporating basin from the heat and allow the filtrate to dry and crystallise. Why are steps 1-4 first undertaken with indicator, and then again without?

37. Preparation of pure, hydrated CuSO ₄ (copper (II) sulphate). Add dilute sulfuric acid into a beaker and heat gently using a bunsen burner flame. Add copper (II) oxide (insoluble base), a little at a time to the warm dilute sulfuric acid and stir until the copper (II) oxide is in excess (stops disappearing). Filter the mixture into an evaporating basin to remove the excess copper (II) oxide. Leave the filtrate in a warm place to dry and crystallise. Decant excess solution. Blot crystals dry.

38. Homework Complete the Acids, Bases and Salts homework. Core students should not complete Q4. b).

39. Water of Crystallisation Water of crystallisation is water that is chemically bonded into a crystal structure. Here is the formula for hydrated copper (II) sulphate. Notice that water of crystallisation is separated from the main formula by a dot . It must be removed by heating the sample. CuSO₄.5H₂O ← water of crystallisation Hydrated means that the solid crystals contain water of crystallisation. Dehydration is the removal of water of crystallisation. An anhydrous substance contains no water of crystallisation. The degree of hydration is the number of moles of water of crystallisation chemically bonded in 1 mole of the compound. The degree of hydration in the example above is 5.

40. Core Students Complete Acids, Bases and Soluble Salts worksheet on the OneDrive (8.1). Then, self-assess your work based off the answer sheet.

41. Insoluble Salts – Supplement Only The insoluble salt formed will be the precipitate in a reaction. Add soluble salts to water and mix (the soluble salts must contain the elements you wish to create the insoluble salt with, e.g. lead (II) nitrate and potassium sulphate to form lead (II) sulphate). Filter to remove the precipitate from the mixture. Wash the precipitate (in the filter) with distilled water. Dry the precipitate in an oven. What equipment should be used to filter the precipitate? Why must the precipitate be washed? Why is distilled water used to wash the precipitate?

42. All Revise the topics of Electrolysis and Acids, Bases and Salts. Use Seneca Learning or past paper questions and answers (available on Save My Exams or Physics and Maths Tutor).