Titration-curves.pptx

PH curves

1. Starter Answer these questions, you can use p.149 to prompt your memory. A solution of NaOH has a concentration of 0.15 mol dm -3 . What is the pH? A solution of Ca(OH) 2 has a concentration of 2 mol dm -3 . What is the pH? A solution of KOH has a concentration of 0.5 mol dm -3 . What is the pH?

2. Titrations What happens during a titration? Titrations measure the volume of one solution that reacts exactly with a known volume of the other solutions.

3. Key Terms When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralize" each other. When the indicator changes color, this is often described as the  end point  of the titration.

4. Key Terms In an ideal world, the color change would happen when you mix the two solutions together in exactly equation proportions. That particular mixture is known as the  equivalence point . This does not always happen at the end point .

5. Key Terms The neutral point of a reaction, is when the pH is 7. However, not all salts have a pH of 7 so this expression should be avoided.

6. Titration Curves The pH is measured successively after the addition of small volumes from the burette, using a pH meter. Alternatively the pH can be data logged continuously and a titration curve plotted by a computer.

7. Titration Curves Describe the shape of the curve:

8. Titration Curves Slight rise in pH : When the base is first added, the pH increases very slightly as the acid is in great excess. Sharp rise in pH : Within 1-2cm 3 of the equivalence point the pH starts to increase more quickly. Only small excess of acid. Eventually there is a very sharp increase in pH brought about by a very small addition of base (1 drop). This shows as a vertical section. The equivalence point is at the center of the vertical section. Slight rise in pH : As further base is added, there is little additional change in pH as the base is in great excess.

9. Titration Curves Sketch the titration curve for an acid being added to a base.

10. Titration Curves Decide whether the acid and alkali used in each titration was strong or weak. Weak acid – Strong base

11. Titration Curves Decide whether the acid and alkali used in each titration was strong or weak. Weak acid – Weak base

12. Titration Curves Decide whether the acid and alkali used in each titration was strong or weak. Strong acid – Strong base

13. Titration Curves Decide whether the acid and alkali used in each titration was strong or weak. Strong acid – Weak base

14. Indicators An acid-base indicator is a weak acid, often represented as HIn. An indicator has one color in its acid form (HIn) and a different color in its conjugate base form (In - )

15. Indicators For methyl orange, HIn is red and In - is yellow: HIn H + + In - When there are equal amounts of the weak acid and the conjugate base present: [HIn] = [In - ] The indicator is at its end point .

16. Indicators Most indicators have a change color over about two pH units. The end point is usually in the middle of this range.

17. Choosing an Indicator Indicators are chosen so that the pH value of the end point is as close as possible to the pH value of the titration’s equivalence point. A suitable pH indicator changes color within the pH range of the vertical section of the titration curve, often with just a single drop of the base.

18. Weak acid – Strong base For each titration curve state which indicator would be suitable (phenolphthalein or methyl orange) Phenolphthalein suitable Methyl orange unsuitable

19. Weak acid – Weak base For each titration curve state which indicator would be suitable (phenolphthalein or methyl orange) Neither indicator is suitable.

20. Strong acid – Strong base For each titration curve state which indicator would be suitable (phenolphthalein or methyl orange) Phenolphthalein suitable Methyl orange suitable

21. Strong acid – Weak base For each titration curve state which indicator would be suitable (phenolphthalein or methyl orange) Phenolphthalein unsuitable Methyl orange suitable