Understanding pH Scale and the Effects of Dilution on Acids and Alkalis

1. S2 Science pH Scale

2. Learning Intentions • We are learning why solutions have different pH’s. • We are learning about the definitions, differences and similarities between strong and weak acids. • We are learning how to show the effect of soluble oxides on the pH of water. • We are learning about the sources of natural and man-made non-metal oxides and their effects on the environment.

3. Success Criteria • I can explain the different pH’s of solutions in terms of concentration of hydrogen and hydroxide ions. • I can explain the effects of dilution on the pH of acids and alkali. • I can state that soluble non-metal oxides make acidic solutions. • I can state that soluble metal oxides make alkaline solutions. • I can explain the sources of both natural and man-made soluble non-metal oxides. • I can give examples of soluble oxides that contribute to acid rain. • I can describe the effect of soluble non-metal oxides on the environment.

4. The pH Scale • The pH scale is a numbered scale used to measure how acidic or alkaline something is. • The scale runs from about 1 to 14 • pH 7 is neutral • Below pH 7 is acidic • Above pH 7 is alkaline

5. The pH Scale pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 very slightly neutral slightly very acidic acidic alkaline alkaline

6. Burning Elements in Oxygen • The element was set alight and placed inside the glass jar to collect the fumes. • Water and pH indicator was added Glass jar Burning element

7. Burning Elements in Oxygen ElementColourpHacid/alkali Magnesium sulphur

8. Burning Elements in Oxygen ElementColourpHacid/alkali Magnesium blue >7 alkali Sulphur red <7 acid When an element burns in oxygen, the two elements combine to form an oxide. Sulphur dioxide is very soluble in water.

9. Burning Elements in Oxygen Conclusion • When metal oxides dissolve in water alkalis are formed • When non metal oxides dissolve in water acids are formed.

10. The Effect of Dilution on pH

11. The Effect of Dilution on pH

12. The Effect of Dilution on pH

13. The Effect of Dilution on pH • As an acidic solution is diluted the pH of the solution rises towards 7 • As an alkaline solution is diluted the pH of the solution falls towards 7

14. Water • Water is normally thought of as a covalent molecule but in reality water exists with a tiny trace of ionic water. H2O(l) H+(aq) + OH-(aq) ionic covalent

15. Water • Pure water is about 99.9999998% covalent. • The concentration of H+ ions equals the concentration of OH- ions

16. Water • The pH of a substance is related to the concentration of H+ ions present. • In pure water the concentration is 1 x 10-7 mol/l • This would give a pH of 7

17. Water • The pH of a substance is related to the concentration of H+ ions present. • In pure water the concentration is 1 x 10-7 mol/l • This would give a pH of 7

18. Acids and Alkalis • Acids have more H+ ions than OH- ions. • Their pH would be below 7

19. Acids and Alkalis • Acids have more H+ ions than OH- ions. • Their pH would be below 7 • Alkalis have more OH- ions than H+ ions. • Their pH would be above 7

20. Strong and Weak Acids • Hydrochloric acid can be made by dissolving hydrogen chloride gas in water. HCl(g) + water H+(aq) + Cl-(aq) • The conversion to ions is 100%. This makes HCl(aq) a strong acid

21. Strong and Weak Acids • Ethanoic acid can be made by dissolving hydrogen ethanoate in water. CH3COOH(l) + water CH3COOH(aq) CH3COO-(aq) + H+(aq) • Only 0.4% dissociate into ions. This makes ethanoic acid a weak acid.

22. Strong and Weak Acids • A strong acid is an acid that is completely dissociated into ions in dilute solution (eg hydrochloric, sulphuric and nitric acids) • A weak acid is an acid that is only partially dissociated into ions in dilute solution (eg ethanoic, citric and other carboxylic acids)

23. Comparing Strong and Weak acids

24. Comparing Strong and Weak acids

25. Comparing Strong and Weak acids • The weak acid has a higher pH and is less acidic. This is due to the lower concentration of H+(aq) ions. • The lower concentration of ions also explains the lower conductivity of the weak acid.

26. Comparing Strong and Weak acids • The rate of reaction is also determined by the concentration of H+(aq) ions. 2H+(aq) + Mg(s) H2(g) + Mg2+ (aq) • The greater the concentration of H+(aq) the faster the reaction

27. Strong and Weak Bases • A strong base is a base which is fully ionised to hydroxide ions in dilute solution (eg solutions of metal hydroxides such as potassium hydroxide) • A weak base is a base which is only partially ionised to hydroxide ions in dilute solution (eg ammonia solution)

28. Strong and Weak Bases • Sodium hydroxide is an example of a strong base Na+OH-(s) + water Na+(aq) + OH-(aq) • The resulting solution is fully ionic.

29. Strong and Weak Bases • Ammonia solution is a weak base NH3(g) + water NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) • 99.6% of the ammonia remains as molecules. The position of the equilibrium lies well to the left.

30. Comparing pH and Conductivity

31. Comparing pH and Conductivity

32. Comparing pH and Conductivity • The weak base has a lower pH because it has a lower concentration of OH-(aq) ions • The lower conductivity can also be explained by the lower concentration of ions. • To make it a fair test, the two solutions must be equimolar