1. Bronsted-Lowry acids and bases

2. The difference between dissociation and ionisation
Dissociation refers to a reaction where a molecule or substance breaks apart into smaller units.
The units are not necessarily ions, although this is often the case.
Ionization generally refers to a reaction which forms ions from an uncharged species.

3. Defining acids and bases
In chemistry, the Brønsted-Lowry theory is an acid-base theory, proposed independently by Johannes Nicolau Brønsted and Thomas Martin Lowry in 1923

4. Bronsted-Lowry definition of acid
A substance behaves as an acid when it:
donates a proton (H+) to a base.
Acids are proton donors.
When acids react with water, hydronium
(H3O+) ions are produced. H+ ions cannot exist by themselves
HCl + H2O  H3O+ + Cl-
acid base
H+ is attracted to the negative end of H2O to become H3O+

5. Bronsted-Lowry definition of a base
A substance behaves as a base when it:
accepts a proton from an acid (Bases are proton acceptors.
When bases react with water, hydroxide (OH−) ions are produced.

6. Acids and bases HCl is an acid because it donates H+
NH3 accepts H+ and therefore is the base
(NH4+ and Cl– then form an ionic compound)
Lewis acid: electron pair acceptor
Lewis base: electron pair donor H N Cl + -

7. Acid/base conjugate pairs
Conjugate means joined together
When an acid and a base react together a conjugate acid and base are formed
HNO3 + H2O  H3O+ + NO3-
acid base acid base 2
The conjugate pairs are (HNO3 /NO3-)
(H2O / H3O+)
They differ by a H+

8. Acid/base conjugate pairs
HCN(l) + H2O  CN–(aq) + H3O+(aq)
HCN is acid, H2O is base
HCN(l) + H2O  CN–(aq) + H3O+(aq)
H3O+ is acid, CN– is base
A conjugate acid-base pair are two substances that differ from each other by just one proton (H+)
HCN and CN– and H2O and H3O+ are conjugate acid-base pairs

9. Questions
Pg 245 Q 1,2,3

10. Hydrolysis
Hydrolysis is a chemical reaction during which an anion reacts with water to produce OH– or a cation reacts with water to produce H3O+
H2O + Cl-  OH- + HCl
H2O + H+  H3O+

11. Acid and base strength Acids and bases have different strengths
Some acids donate protons more readily than others
The strength of an acid is its ability to donate hydrogen ions to a base.
The strength of a base is its ability to accept hydrogen ions from an acid.
Strength is different from concentration (pg 248 Figure 14.9)

12. Strong acids and bases
A strong acid will completely ionise in solution (producing many ions)
A strong base will accept protons (H+) easily

13. Weak acids and bases
A weak acid does not ionise to any great extent and so contains a larger number of molecules compared with the number of ions produced in solution.
Completely ionised
Partially ionised

14. Reversible reaction
Reversible arrows in an equation show that the products on the right can react together and produce the left hand side.
A chemical equation without a double arrow isn't reversible and can only go in one direction.

15. Polyprotic acids
Polyprotic acids are acids capable of donating more than one proton (H+).
Monoprotic
HCl
2. Diprotic
H2SO4
3. Triprotic
H3PO4

16. Amphiprotic substances
Some substances act as acids and bases
They can donate or accept protons and are called amphi (meaning both) protic (hydrogen ions)
Example water, ammonia and amino acids

17. Write the formulae of the conjugate bases of the following acids:
H2SO H2S HS NH4+
Write the formulae of the conjugate acids of the following bases:
OH HCO H2O CN-

18. pH and pOH
pH stands for the potential of hydrogen or concentration of H+
pOH is a measure of the hydroxide ion concentration
The acidity of a solution is a measure of the concentration of H+
In a neutral solution there is the same concentration of H+ or H3O+ and OH-
Basic solutions have a lower concentration of H3O+ than OH-
Acidic solutions have a greater concentration of H3O+ than OH-

19. Ionic product
[H3O+ ] [H+ ] represents the concentration of H3O+ or H +
[OH-] represents the concentration of OH-
Experiments show that all aqueous solutions contain H + and OH- that the product of their molar concentrations is 10-14M2 at 25C
The ionic product is:
[H3O+ ]x[OH-] =
In pure water [H3O+ ]=[OH-]
10-7M = 10-7M

20. At 25C a solution is: Acidic if [H3O+ ] > 10-7M [OH-] < 10-7M
Neutral if [H3O+ ][OH-]
Basic if [H3O+ ] < 10-7M [OH-] > 10-7M

21. Calculating pH and pOH pH is calculated using the following formula:
pH = -log [H+]
pOH is calculated using the following formula:
pOH = -log [OH-]
pH + pOH = 14.0
[H+] = 10 -pH
[OH-] = 10 -pOH

22. pH calculations for weak acids
Can we use the pH calculation for weak acids? No
Why?
Because weak acids have not fully ionized so we do not know the H+ concentration
You have to wait till year 12 and you do the equilibrium constant

23. Calculating pH and pOH
Find the pH of a solution of sodium hydroxide that has a pOH of 2
pH = 14 – pOH
pH = = 12
Find the pOH of a solution of hydrochloric acid that has a pH of 3.4
pOH = 14 – pH
pOH = = 10.6

24. Questions
Find the pH of 25.0 mL of a M (mol/L) solution of HCl. What is the pOH?
Note that HCl is a strong monoprotic acid which means that... [HCl] = [H+]= M pH = -log [H+] pH = -log pH = 1.35
The pOH is given by pH + pOH = 14 We substitute in the pH of 1.35 and get: pOH = 14 So, pOH = 12.65

25. Questions a) Find the pOH of 0.000685 M solution of NaOH.
b) What is the pH of the solution?
a)Note that NaOH is strong and monobasic which means that...
[NaOH] = [OH-] = M hence, pOH = -log [OH-] pOH = -log pOH = b) pOH is The pH can be found by using pH + pOH = 14. Substituting in gives us pH = 14 So, pH =

26. Question What is the H+ concentration in the solution of pH 3.47
[H+] = 10-pH =
= 3.39x10-4 mol/L or M

27. Questions
What is the concentration OH- ions in a solution of pH of 10.47
Find concentration of H+ ions
[H+] = 10-pH =
= 2.51x10-11
Find OH- concentration
[H+ ] x [OH-] = 10-14
[OH-] = 10-14
2.51x10-11
= 3.98 x 10-4 mol/L or M

28. Find the pH of a 0.2 mol L-1 (0.2M) solution of H2SO4
Write the balanced equation for the dissociation of the acid
H2SO4  2H+(aq) + SO42-(aq)
Use the equation to find the [H+]: 0.2 mol L-1 H2SO4 produces 2 x 0.2 = 0.4 mol L-1
Calculate pH:
pH = -log[H+]
pH = -log [0.4] = 0.4

29. Pg 254 Q 9 a,b,c, 10 a,b 11