Acids, pH and equilibrium

Question 1

What is the equilibrium law?

Answer 1

For the equilibrium:aA + bB⇔ cC + dD,Kc(Equilibrium constant) = ([C]cx [D]d)/([A]ax [B]b).

Question 2

What is a homogeneous equilibrium?

Answer 2

An equilibrium in which all the species making uop the reactants and products are in the same phase.

Question 3

What is a heterogeneous equilibrium?

Answer 3

An equilibrium in which species making up the reactants and products are in different phases.

Question 4

What is the significance of the Kcvalue?

Answer 4

1. A Kcvalue of one exactly indicates the position of equilibrium being in the middle.2. A Kcvalue of< 1 indicates the position of equilibrium is in favour of the reactants.3. A Kcvalue of> 1 indicates the position of equilibrium is in favour of the products.

Question 5

What effect on the Kcvalue does changing the temperature have?

Answer 5

• Increasing the temperature of an exothermic shifts the position of equilibrium to the left, in favour of the endothermic reverse reaction, decreasing value of Kc.- Increasing the temperature of an endothermic reaction shifts position of equilibrium to the right, in favour of the endothermic forward reaction, increasing value of Kc.

Question 6

What effect on the Kcvalue does changing the pressure/concentration have?

Answer 6

Changing the pressure/concentration doesn’t affect the value of Kc. However, it does change the concentration of reactants and products, which would change the Kcvalue, thus position of equilibrium shifts to maintain a constant Kcvalue.

Question 7

What effect on the Kcvalue does adding a catalyst have?

Answer 7

Value of Kcis unaffected by the addition of a catalyst.

Question 8

What is k (rate constant)?

Answer 8

Constant determining the rate of a reaction under specific conditions.

Question 9

What is the relationship between k and Kcduring a compromise?

Answer 9

During a compromise, a big value of Kcresults in a small value of k, and vice versa. This means that a balance needs to be struck so that a big enough k value is obtained so that the reaction isn’t too slowwithout compromising decreasing the value of Kctoo much so that a low yield is obtained.

Question 10

What is a Brønsted-Lowryacid?

Answer 10

A proton, H+,donor.

Question 11

What is a Brønsted-Lowry base?

Answer 11

A proton, H+, acceptor.

Question 12

What is an alkali?

Answer 12

A water soluble base that releases hydroxide (OH-) ions when in solution.

Question 13

What is a neutralisation reaction?

Answer 13

A chemical reaction in which an acid and a base react together to produce a salt and water.

Question 14

What is a monobasic acid?

Answer 14

An acid whereby each molecule is able to release up to one proton each. E.g. HCL.

Question 15

What is a dibasic acid?

Answer 15

An acid whereby each molecule is able to release up to two protons each. E.g. H2SO4.

Question 16

What is a tribasic acid?

Answer 16

An acid whereby each molecule is able to release up to three protons each. E.g. H3PO4.

Question 17

Why is H2SO4acidic whendissolved in H2O but not when liquid?

Answer 17

For a compound like H2SO4to be acidic, it needs to releaseprotons (dissociate). However, it would only release a proton if there is another compound present to accept the proton (a base). In water, water molecules act as bases and accept protons from acid:H2SO4+ H2O→ HSO4-+ H3O+As liquid, no molecules are able to act as bases, so H2SO4is not acidic.

Question 18

What is a conjugate acid?

Answer 18

A species formed when a proton is added to a base.

Question 19

What is a conjugate base?

Answer 19

A species formed when a proton is removed from an acid.

Question 20

What is an acid-base pair?

Answer 20

Apair of two species that transform into each other by the gain or loss of a proton.

Question 21

What is an example ofacid-base pairs?

Answer 21

HCl + H2O→ H3O++ Cl-In this equation:HCl is the acid, Cl-is its conjugate base (since Cl-becomes HCl by gaining proton).H2O is the base, H3O+is the acid (since H3O+becomes H2O by losing proton).

Question 22

What is the formula of pH?

Answer 22

pH = -log10[H+(aq)]

Question 23

What is a strong acid?

Answer 23

An acid that completely dissociates in solution.

Question 24

What is a weak acid?

Answer 24

An acid that partially dissociates in solution.

Question 25

What is the ionic product of water (Kw)?

Answer 25

Kw = [H+(aq)][OH-(aq)]at 298K, Kw = 1 x 10-14 mol2dm-6.

Question 26

What is significant about Kw?

Answer 26

At a given temperature, Kw is constant no matter the pH of the solution in question.

Question 27

What is the only factor that changes Kw?

Answer 27

Temperature.

Question 28

What is the acid dissociation constant (Ka)?

Answer 28

The actual extent to which an acid dissociates in solution. The Ka of an acid HA is defined as:Ka = [H+(aq)][A-(aq)]/[HA(aq)]

Question 29

What is the significance of the Ka value?

Answer 29

A large Ka value means that the acid dissociates readily and thus is a stronger acid.A small Ka value means that the acid dissociates less and thus is a weaker acid.

Question 30

What is a buffer solution?

Answer 30

A mixture that minimises pH change on addition of small amounts of acid or base.

Question 31

What does a buffer solution consist of?

Answer 31

A weak acid and an excess of its salt (conjugate base).E.g. CH3COOH (lots) ⇔ H+(few) + CH3COO-(lots)The presence of a high amont of salt shifts position of equilibrium to the left, resulting in a very small amount of H+ ions.

Question 32

How does a buffer solution work?

Answer 32

Consider the buffer solution:CH3COOH ⇔ H+ + CH3COO-Suppose we add some acid (H+ ions), position of equilibrium would shift to the left to decrease  [H+] as H+ reacts with conjugate base, minimising pH change.Suppose we add some alkali (OH- ions); the reaction H+ + OH- → H2O would occur, decreasing [H+], position of equilibrium shifts to the right to increase [H+] as acid dissociates, minimising pH change.

Question 33

How does the carbonic acid - hydrogencarbonate buffer system work?

Answer 33

The buffer system has the equation: H2CO3 ⇔ H+ + HCO3-Suppose an acid was added (H+ ions). The excess H+ ions react with the conjugate base (HCO3-), shifting the position of equilibrium to the left, decreasing [H+] and minimising pH change.Suppose an alkali was added (OH- ions). The reaction H+ + OH- → H2O occurs, decreasing [H+]. The acid (H2CO3) dissociates, shifting the position of equilibrium to the right, increasing [H+] and minimising pH change.

Question 34

What is the equivalence point of a titration?

Answer 34

The point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution.

Question 35

What is the end point of a titration?

Answer 35

The point in a titration at which there are equal concentrations of the weak acid and conjugate base form of the indicator.

Question 36

How is a suitable indicator chosen for a titration?

Answer 36

The indicator is suitable if the range over which its colour changes overlaps with the sudden jump in pH which encompasses the equivalence point of the titration but also indicates that an excess of acid suddenly became an excess in alkali.