Acids and bases

Question 1

What is the Bronsted-Lowry definition of an acid?

Answer 1

a proton donor

Question 2

What is the Bronsted-Lowry definition of an base?

Answer 2

a proton acceptor

Question 3

Why are H3O+ (oxonium/hydroxonium) ions present in the solution instead of H+ only?

Answer 3

Because the H+ ion is attracted to the lone pair of electrons on a water molecule so a co-ordinate bond is formed.

Question 4

What are the assumptions made for K- acid disassociation constant?

Answer 4

• only a small amount of acid disassociates so HA at equilibrium is the same as the start
• The dissociation of acid is greater than the dissociation of water present in the solution so we assume all the H+ ions come from the acid

Question 5

What does it mean when acid is mono/diprotic?

Answer 5

They only donate one/two moles of protons per mole of acid

Question 6

What is the definition of pH?

Answer 6

the concentration of H+ ions in a solution -log10

Question 7

Define the term Kw constant and its value?

Answer 7

1 x 10-14

Kw = [H+][OH–]

Question 8

What is the point of pka? What happens what the value of pKa becomes lower?

Answer 8

its used to determine whether something is a weak or strong acid

the lower the value of pka, the larger the value of ka, the greater the ionisation of acid in the water so the stronger the acid

Question 9

What is a buffer solution?

Answer 9

pH changes by a small amount when acid/ alkali is added

Question 10

Why don’t strong acids have buffer solutions?

Answer 10

because they fully ionise

Question 11

What is a buffer? What is an acidic buffer and basic buffer?

Answer 11

A chemical that resists (not stop) the change in pH when small amounts of acid or base are added

Acidic = keep solution below ph7 and made from a weak acid and its salt

Basic - keep solution above ph&, made from weak base and its salt

Question 12

What is the difference in the position of equilibrium for weak acids vs salt of weak acid

Answer 12

Weak acids dissociate weakly = left

salt dissociate strongly = right

Question 13

What happens when acids (H+) and bases (OH-) are added to acidic buffers?

Answer 13

acid = the salt decreases and the acid increases (because H+ reacts with the salt to produce more acid)

Base = salt increases and acid decreases (cause OH reacts with H+ ions (for example CH3 COOH → CH3COO- + H+) so the equilibrium shifts to the right to oppose the decrease in H+, aka CH3COOH dissociate even more

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Question 14

What happens when acids (H+) and bases (OH-) are added to basic buffers?

Answer 14

Example = NH3 + H2O→ NH4 +OH-

Acid = reaction becomes more acidic and some of the H+ ions react with OH - to make H2o, causing equilibrium position to move to the right to oppose the change to make more OH- ions

Base = reaction more basic. some OH- react with NH4+ ions to from NH3 and H2o , so equilibrium with shift to the left to oppose the change

Question 15

Why does dilution have little to no effect on the pH value of a buffer solution

Answer 15

because the ration of the concentrations of the acid and its salt remain the same

Question 16

When a small quantity of hydrochloric acid is added to a buffer, the pH remains ALMOST constant. Why?

Answer 16

adding HCL, the H+ will react with the salt of the weak acid. So the concentration of salt will decrease and concentration of acid will increase slightly

Question 17

State and then explain what would happen to the pH of water, if the temperature were increased.

Answer 17

pH would decreae

increase in temp = equilibrium shifts to the right = endothermic reaction = causes an increase in H+ concentration

Question 18

Explain why chloroethanoic acid is a stronger acid than ethanoic acid.[2 marks]

Answer 18

M1 (Electronegative) chlorine withdraws electrons

M2 Stabilises/reduces the charge on COO

weakens O-H bond

makes O-H more polar

Question 19

Suggest a value for the pH of a 1.26 M solution of sodium chloride and explain your answer.

Answer 19

7 because concentration of H+ = concentration of OH-

Question 20

Explain why the expression for Kw does not include the concentration of water

Answer 20

• the concentration of H2O is very high and only dissociates very slightly.
• The equilibrium lies far to the left
• H2o is effectively constant

Question 21

Explain why the value of Kw increases as the temperature increases

Answer 21

Bond breaking is endothermic, so the equilibrium moves to the right-hand side to oppose increase in temperature

(h2o → H+ + OH-)

Question 22

State why calibrating pH meter just before it is used improves the accuracy of the pH measurement

Answer 22

Over time / after storage meter does not give accurate readings

Question 23

Describe how you would obtain the pH curve for titration? (5 marks)

Answer 23

• Measure pH (of the acid)
• Add alkali in known small portions (Allow 1 - 2cm3)
• Stir mixture
• Measure pH (after each addition)
• Repeat until alkali in excess (Allow 27 - 50cm3)
• Add in smaller increments near endpoint (Allow 0.1 - 0.5cm3)