Acids and bases

1. Acids and bases

2. What are acids, alkalis and bases? ACIDS taste sour can be corrosive have a pH lower than 7 turn blue Litmus paper red turn UI yellow – orange – red BASES taste bitter, feel soapy can be corrosive have a pH higher than 7 turn red Litmus paper blue turn UI blue – purple ALKALIS are soluble bases

3. More on acids • All acids form H+ ions when dissolved in water • It is the H+ ions that make a solution acidic. • pH stands for ‘power of hydrogen’ • What pH do they have? • common acids are: • hydrochloric acid HCl • sulphuric acid H2SO4 • nitric acid HNO3

4. More on bases • Bases are usually: • metal hydroxides contain OH • metal oxides contain O • metal carbonates contain CO3 • All alkalis form hydroxide ions (OH-) when in solution. • It is the OH- ions that make a solution alkaline. • What pH do they have?

5. Acid reactions • PRACTICAL • -> Look at the reactions of two different acids, compare them and discuss the products formed • Make sure you follow the instructions and record your results. • What did you find out?

6. Acid reactions • acid and base/alkali acid + base/alkali  salt + water • acid and metal acid + metal  salt + hydrogen • acid and metal carbonate acid + metal carbonate  salt + water + CO2

7. Which salt? • In all three types of reactions, a salt is formed. • What type of salt depends on the acid: • Hydrochloric acid makes chlorides. • Sulphuric acid makes sulphates. • Nitric acid makes nitrates. • What type of salt would phosphoric acid make?  phosphates

8. Acid reactions • PRACTICAL • -> Make a soluble salt (copper sulphate) using an acid and a base • Make sure you follow the instructions and work safely.

9. So far so good… (hopefully) Rule for crystallisation: The longer it takes, the larger the crystals. This is because the particles have more time to align themselves and bond. What size crystals would you get if you heated the solution over a Bunsen burner and evaporated the water?

10. Making Copper Sulphate-questions • Why was it important to warm the acid? • Why is it important to have some unreacted copper sulfate left at the end of the reaction? • Write a word equation for your reaction. • You have made copper(II) sulfate. Write a symbol equation for your reaction. • Do you think it would be possible to make copper sulfate using a MASH reaction? Explain your answer. Forgotten the formula for sulphuric acid? Try: Johnny liked his drink, But, lo! He’ll drink no more, For what he thought was H2O Was H2SO4!

11. Making salts from metals or bases The generic equation M.A.S.H. metal + acid salt +hydrogen magnesium + hydrochloric acid magnesium chloride + hydrogen solution Mg (s) + HCl (aq) MgCl2 (aq) + H2 (g) Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g) acid+insoluble base salt +water hydrochloric acid + iron(iii) oxide iron(iii) chloride + water solid solution HCl (aq) + Fe2O3 (s) FeCl3 (aq) + H2O (l) 6HCl (aq) + Fe2O3 (s) 2FeCl3 (aq) + 3H2O (l)

12. Making Insoluble Salts If the salt we are trying to make does not dissolve in water, this makes our experiment quicker. We can use a precipitation reaction. A precipitate is a solid which is formed when two solutions are mixed. The insoluble salt ‘appears’ in the previously clear solution and ‘falls’ to the bottom of the container (precipitates)

13. Ammonium Salts as Fertilizers Ammonia dissolves in water to produce an alkaline solution. It is used to produce ammonium salts. Ammonium salts are important as fertilisers.

14. Removing unwanted ions from solution by precipitation Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed. Precipitation can be used to remove unwanted ions from solutions, for example in treating water for drinking or in treating effluent.

15. Making an insoluble salt An insoluble salt can be made by combining two solutions. This reaction is called a precipitation reaction. The insoluble salt ‘appears’ in the previously clear solution and ‘falls’ to the bottom of the container (precipitates) Carry out the practical by following the instructions on the sheet. Wear goggles.

16. What is electrolysis? Electrolysis means ‘splitting up using electricity’. An electrical current is used to decompose (break down) a substance made of ions into simpler substances. The substance that is broken down is called the electrolyte.

17. Ionic compounds - recap • Ionic bonding: • between metals and non-metals • metal atom donates electrons • non-metal atom accepts electrons • they become positively and negatively charged (ions) • strong electrostatic forces between the ions of opposite charge  bonds • huge lattice is formed

19. Ionic compounds - recap • Properties of ionic compounds: • Solids at room temperature • Very high melting and boiling points • Conduct electricity when molten or dissolved in water • WHY?

20. Electrolysis – the set up Electrodes are conducting rods that are dipped into the electrolyte. They are usually made of an unreactive (inert) substance like graphite or platinum. The positive electrode is called the anode. The negative electrode is called the cathode.

21. Electrolysis – the process • Electrolysis of copper bromide: • needs to be dissolved so ions are free to move • copper ions move to the cathode • bromide ions move to the anode • CuBr2(aq) Cu (s) + Br2(g)

22. Homework • Some useful preparation for your ISA!!! • Plan an experiment to measure how quickly electrolysis occurs in a copper sulphate solution. • Include: • What you will measure (change in mass in cathode might be good), what you will control, a brief method, equipment, a risk assessment.