Topic 12Acid- base equilibria

Week 5 - 4 May 2020, to complete by Friday 8 May.

This week we are continuing Topic 12 – Acid Base Equilibria.

Please work your way through the powerpoint and make notes on each slide – youwill need your scientific calculator – if you need any help with the particularcalculator function buttons please get in touch.

This work will account for the 2 hours you would normally spend on my lessons.

By Friday – please do the attached question sheet to return to me:

Either take a photo of your work and email it to me, or send your answers viaa word document, again via email.

Good Luck!

Dr Osbornes.osborne@bishopchalloner.bham.sch.uk

Specification points for this section…

pH of strong acids

• Objectives:

• To know the difference between a strong and weak acid.

• To know the definition of pH.

• To understand what is meant by a monoprotic and a diprotic acid.

• To be able to calculate pH of a strong acid from hydrogen ion concentration and vice versa.

• To understand what the ionic product of water is.• To be able to calculate pH for a strong base using Kw.

Starter

• What is the difference between a strong and a weak acid?

• Write out an equation to illustrate a weak and a strong acid dissociating.

(Use HCl and CH3COOH as examples).

• Write the balanced chemical equation for the reaction between potassium carbonate and sulphuric acid. State symbols not needed.

• Ionic equation?

What can you remember from Year 11?

• What is the difference between a strong and a weak acid?

• A strong acid fully dissociates in water whereas a weak acid only dissociates to a slight extent.

• Write out an equation to illustrate a weak and a strong acid.

• Write the balanced chemical equation for the reaction between potassium carbonate and sulphuric acid. State symbols not needed.

• K2CO3 + H2SO4 K2SO4 + H2O + CO2

• Ionic equation?

• CO32- + 2H+

H2O + CO2

Notice the different symbols

needed for the arrows

Try these exam questions….

Try these exam questions….

C – can you explain

why?

D

pH of strong acids

• Objectives:

• To know the difference between a strong and weak acid.

• To know the definition of pH.

• To understand what is meant by a monoprotic and a diprotic acid.

• To be able to calculate pH of a strong acid from hydrogen ion concentration and vice versa.

Hydrochloric acid and nitric acid are strong acids and ionise fully.

HCl and HNO3 are monoprotic acids because they can donate one proton per

molecule. (mono = one)

Sulphuric acid is also a strong acid and ionises fully. It is a diprotic acid because it

can donate two protons per molecule.

Extra background information….

pH of strong acids

• Objectives:

• To know the difference between a strong and weak acid.

• To know the definition of pH.

• To understand what is meant by a monoprotic and a diprotic acid.

• To be able to calculate pH of a strong acid from hydrogen ion concentration and vice versa.

We can calculate the pH of a solution if its hydrogen ion concentration is known.

This is how we work out pH using the hydrogen ion concentration.

The [] show it’s a concentration.

You need to use the ‘log’ button on your calculator.

(not the Ln button as that’s a different type of log!!)

We can also rearrange the

equation to find H+

concentration if pH is known….s

(on your calculator you would

use ‘shift and log’ to do this – to

get the ‘10x’ function.

For a strong monoprotic acid like hydrochloric acid, the hydrogen ion concentration

is equal to the concentration of the acid itself – because every acid molecule is

thought to dissociate to a H+.

Q What is the pH of 0.10 mol dm–3 hydrochloric acid?

The hydrogen ion concentration will also be 0.10 mol dm–3.

This is because every HCl will produce 1 H+

So, now on your calculator, do:

‘-’ ‘log’ ‘0.1’ ‘=‘

(it’s the negative sign ‘-’ button, not minus!)

A: The pH will be 1.0 for 0.10 mol dm–3 hydrochloric acid.

Hopefully you get the answer ‘1’!!

(no units needed for pH)

For a strong diprotic acid like sulfuric acid, the hydrogen ion concentration is

equal to twice the concentration of the acid itself.

What is the pH of 0.10 mol dm–3 sulphuric acid? The hydrogen ion concentration

will be 0.20 mol dm–3.

The pH will be 0.69 for 0.10 mol dm–3 sulphuric acid.

What is the hydrogen ion concentration in nitric acid with a pH of 1.7?

Substitute the pH into the equation. Remember to enter ‘minus pH’ when you use a

calculator.

The hydrogen ion concentration for nitric acid with a pH of 1.7 will be 0.02 mol dm–3.

As it is a strong monoprotic acid, this will also be the concentration of the nitric acid

itself.

(on your calculator – ‘shift’ ‘log’ (to get 10x) then ‘-’ ‘1.7’)

Worked examples to review:

• Now have a go at these questions:

• Answers

16a)pH = 1.00 b) pH = 2.00 c) pH = 3.00

17 pH = 1.09

18a)5.01 10−4 mol dm−3

b) 3.98 10−6 mol dm−3

c) 1.99 10−7 mol dm−3

d) 3.16 10−11 mol dm−3

Now have a go at the following exercises to practise:

3

2

8

0.3

0.6

0.2

0.2

0.08

0.16

0.35

3.00 -0.17 3.60 11.35

0.0018 5.01x10-41.20x10-14 0.014 5.01

You may need to work through how pH is affected by dilution:

For Q3, concentration is moles

vol in dm3

pH of strong acids

• Objectives:

• To know the difference between a strong and weak acid.

• To know the definition of pH.

• To understand what is meant by a monoprotic and a diprotic acid.

• To be able to calculate pH of a strong acid from hydrogen ion concentration and vice versa.

• To understand what the ionic product of water is.• To be able to calculate pH for a strong base using Kw.

Units of Kw?

mol2 dm-6

19 a) Ionisation is endothermic.b) Extent of ionisation increases as the temperature rises and the

hydrogen ion concentration rises as the temperature rises. The number value of the pH falls.

c) No, because [H+(aq)] always equals [OH−(aq)].20 a) pH = 14.00

b) pH = 12.30 c) [OH−] in 0.0010 mol dm−3 Ba(OH2) = 2 × 0.0010 mol dm–3

So [H+(aq)] = = 5.0 × 10−12 mol dm–3

pH = 11.30

12 Acid–base equilibria

14 2 6

3

1.0 10 mol dm

2 0.0010 mol dm

© Andrew Hunt, Graham Curtis 2015

Plenary

© Andrew Hunt, Graham Curtis 2015

© Andrew Hunt, Graham Curtis 2015

© Andrew Hunt, Graham Curtis 2015

Well done!

Now complete the homework task and remember to send a copy of your answers back to me by Friday 8 May.