pH Curves and Indicators

Question 1

What happens when you add a strong base to strong acid?

Answer 1

The pH starts around 1, as there’s an excess of a strong acid. it finishes up around pH 13, when you have an excess of strong base

Question 2

What happens when you add a weak base to a strong acid?

Answer 2

The pH starts around 1, as there’s an excess of a strong acid. It finishes up around pH 9, when you have an excess of a weak base

Question 3

What happens when you add a strong base to a weak acid?

Answer 3

The pH starts around 5, as there’s an excess of weak acid. It finishes up around pH 13, when you have an excess of strong base

Question 4

What happens when you add a weak base to a weak acid?

Answer 4

The pH starts around 5, as there’s an excess of weak acid. It finishes up around pH 9, when you have an excess of weak base

Question 5

Why do pH curves have a vertical bit?

Answer 5

This is the equivalence point (or end point). At this point, a tiny amount of base/acid can cause a sudden, big change in pH - it’s here that the reaction is neutralized

Question 6

How would you see the end point in a weak acid/weak base reaction?

Answer 6

Because you don’t get such a sudden change in a weak acid/weak base titration, its colour change would be very gradual, and would be very hard to see the exact end point. You’re better of using a pH meter for this type of titration

Question 7

What happens to the pH curves if you titrate a base with an acid?

Answer 7

The shapes of the curves stay the same but they flip over

Question 8

How can you use pH curves to decide what indicator to use?

Answer 8

When using an indicator, you need it to change colour exactly at the end point of your titration. So you need to pick one that changes colour over a narrow pH range that lies entirely on the vertical part of the pH curve

Question 9

What indicator should you use for strong acids/strong base titrations

Answer 9

You can use either methyl orange or phenophthalein because there’s a rapid pH change over the range for both indicators

Question 10

What indicator should you use for strong acid/weak base titrations?

Answer 10

For strong acid/weak base only methyl orange will do - the pH changes rapidly across the range for methyl orange

Question 11

What indicator should you use for weak acid/strong base titrations?

Answer 11

For weak acid/strong base you use phenophthalein - the pH changes rapidly across the range for phenophthalein