Unit 1.7 - Simple Equilibria and Acid-Base Reactions

Question 1

1) What is an acid?

2) What is a base?

Answer 1

1) H+

2) OH-

Question 2

State how the following can be reversed:

1) Heating blue hydrated copper sulphate releases water and forms with anhydrous copper sulphate.
2) Adding alkali to orange potassium dichromate solution forms yellow potassium chromate.

Answer 2

1) CuSO4 . 6 H2O ⇌ CuSO4 + 6 H2O

2) K2Cr2O2 + OH- ⇌ 2 KCrO4 + H+

Question 3

1) Define dynamic equilibrium.
2) What must happen for this to occur?
3) State an example of dynamic equilibrium.

Answer 3

1) A system is in the chemical equilibrium when the rate of forward reaction is equal to the rate of the back reaction.
2) Closed system - no net gain or loss of matter, processes continue to occur at molecular level, overall properties are constant.
3) Haber process - N2 + 3 H2 ⇌ 2 NH3

Question 4

What happens to the reaction in a closed system?

Answer 4

Reaction won’t go to completion because as soon as products begin to form the reverse reaction will take place.

Question 5

1) What does the rate of the forward reaction depend on?

2) What does the rate of the back reaction depend on?

Answer 5

1) Concentration of reactants.

2) Concentration of products.

Question 6

1) Define homogenous equilibrium.

2) State an example.

Answer 6

1) All the reactants and products are in the same physical state.
2) Nitrogen, hydrogen, ammonia all gas:
N2 (g) + 3 H (g) ⇌ 2 NH3 (g)

Question 7

1) Define heterogeneous equilibrium.

2) State an example.

Answer 7

1) There are two different physical states present in the system.
2) Decomposition:
CaCO3 (s) ⇌ CaO (s) + CO2 (g)

Question 8

What is Le Chatelier’s principle?

Answer 8

If a system in equilibrium is subjected to any change the position of equilibrium will shift in a direction that opposes/counteracts the change.

Question 9

1) What is meant by equilibrium position?

2) What happens to the equilibrium when the yield of forward reaction is higher than the yield of the back reaction?

Answer 9

1) Balance between yield of forward and reverse reaction.

2) Position lies to the right.

Question 10

1) What happens to the equilibrium when the yield of forward reaction is less than the yield of the back reaction?
2) What type of reaction is this?

Answer 10

1) Position lies to the left.

2) Exothermic.

Question 11

State 4 factors which affect the rate of a reaction.

Answer 11

Temperature, concentration, catalysts and pressure.

Question 12

1) What happens when you increase the concentration of a reactant?
2) What happens when you decrease the concentration of a reactant?

Answer 12

1) Equilibrium shifts to right, remove added reactants, form more products and restore the equilibrium.
2) Equilibrium shifts to the left to make more.

Question 13

1) What happens when ethanol or ethnic acid is added to a system?
2) What happens when the equilibrium is restored?

Answer 13

1) Increases rate of forward reaction, more ester is formed.

2) Shifts to the right, increasing the yield of ester.

Question 14

1) What happens if you increase temperature?

2) What happens if you decrease temperature?

Answer 14

1) Reaction shifts in endothermic direction (+ve), opposes change and brings temperature back down.
2) Reaction shifts in exothermic direction (-ve), opposes change and brings temperature back up.

Question 15

What does pressure change affect?

Answer 15

Only gaseous equilibrium systems.

Question 16

1) What happens if you increase pressure?

2) What happens if you decrease pressure?

Answer 16

1) Shifts to side with least number of molecules to decrease it, reduces volume, bringing molecules closer together and increasing their concentration.
2) Shifts to side with most number of molecules to increase it.

Question 17

1) What happens to the pressure and position of equilibrium if there is no mole change?
2) State an example.

Answer 17

1) External pressure changes will have no effect upon the position of equilibrium.
2) H2 (g) + I2 (g) ⇌ 2 HI (g)

Question 18

1) What does a catalyst allow?

2) How will this effect the position of an equilibrium?

Answer 18

1) Lowers activation energy by providing an alternate pathway, so increases rate of forward and back reactions.
2) No effect upon position of equilibrium.

Question 19

aA + bB ⇌ cC + dD

What is the general equation for equilibrium constant (Kc)?

Answer 19

[C]c[D]d [Products]
————– = ————— [ ] = concentration
[A]a[B]b [Reactants

Question 20

1) When is Kc constant?
2) What is it unaffected by?
3) What does the magnitude imply?

Answer 20

1) Temperature is constant.
2) Unaffected by changes in concentration of either reactants or products.
3) Extent of reaction - low value indicates high proportion of reactants to products.

Question 21

State the effect on equilibrium position and constant for the following:

1) Increase temperature for forward exothermic direction.
2) Increase temperature for forward endothermic reaction.
3) Decrease temperature for forward exothermic direction.
4) Decrease temperature for forward endothermic direction.

Answer 21

1) Moves endothermic direction (back) reactants, Kc decreases.
2) Moves to right to take away heat in endothermic direction, Kc increases.
3) Moves in exothermic (forward) direction, Kc increases.
4) Moves to left in endothermic direction, Kc decreases.

Question 22

State the effect on equilibrium position and constant for the following:

1) Increase concentration of reactants.
2) Decrease concentration of reactants.

Answer 22

1) Moves to right to remove increased reactants, Kc unchanged.
2) Moves to left to replace reactants, Kc unchanged.

Question 23

Brønsted Lowry Theory - 1923

1) Define acid.
2) Define base.

Answer 23

1) Proton donor (H+)

2) Proton acceptor (H+)

Question 24

What happens during proton transfer?

Answer 24

Acid gives up a proton to a base which accepts it. In the process, the acid itself forms the conjugate (joins together) base of the acid and the base forms its conjugate acid.

Question 25

1) What is required for the Brønsted Lowry Theory?

2) State the general equation.

Answer 25

1) 2nd acid-base pair.
2) HA (aq) + H20 (l) ⇌ H3O+ (aq) + A- (aq)
HA (aq) ⇌ H+ (aq) + A- (aq)

Question 26

What is the name given to substances which can behave either as Brønsted-Lowry acids or bases?

Answer 26

Amphoteric

Question 27

1) State an example of a amphoteric substance.

2) State the equation of the ionisation of this substance.

Answer 27

1) Water can react with itself.
2) H2O (l) + H2O (l) ⇌ H3O+ (aq) + OH- (aq)
H2O (l) ⇌ H+ (aq) + OH- (aq)

Question 28

How can the position of the equilibrium indicate the strength of an acid?

Answer 28

Further lies to the right, more hydrogen ions are present and the stronger the acid is considered to be.

Question 29

1) What are strong acids?
2) State 3 examples.
3) State the general equation.

Answer 29

1) Acids which donate all their protons to water, completely ionised and equilibrium is to the right.
2) HCl, H2SO4 and HNO3.
3) HA (aq) > H+ (aq) + A- (aq)

Question 30

1) What are weak acids?
2) State an example.
3) State the general equation.

Answer 30

1) Donate some of their protons to water, only partly ionised and equilibrium is to the left.
2) Ethanoic acid - not fully dissociated.
3) HA (aq) ⇌ H+ (aq) + A- (aq)

Question 31

1) What are strong bases?

2) What are weak bases?

Answer 31

1) Sodium hydroxide or potassium hydroxide are completely ionised in water.
2) Ammonium hydroxide is only partly ionised in water.

Question 32

What does concentrated and dilute refer to?

Answer 32

Amount of substance is present in mol dm-3

Question 33

What is the relationship between a donating proton and concentration of hydrogen ions?

Answer 33

Greater ability of an acid to donate a proton, the greater the concentration of hydrogen ions.

Question 34

pH

Answer 34

pH= -log10 [H+ (aq)]

Question 35

How do you calculate hydrogen ion concentration?

Answer 35

[H+ (aq)] = 10 -pH

-pH - power of

Question 36

Standard solution

Answer 36

Solution of known concentration.

Question 37

Describe how to create a standard solution (5).

Answer 37

1) Weighed amount of solid is dissolved in 25cm3 of distilled water using a beaker and stirring rod.
2) Solution transferred to a volumetric flask using a funnel.
3) Any solution remaining in funnel is rinsed with water into volumetric flask.
4) Make up to the line on the bottom of the meniscus at eye level.
5) Stopper placed in flask and mix thoroughly/invert to insure solution is homogenous.

Question 38

State the 4 features of a primary standard.

Answer 38

High purity, stability (low reactivity), low hygroscopic (minimise weight changes due to humidity), high molar mass (minimise weighing errors).

Question 39

Describe how to carry out a titration.

Answer 39

1) Pipette 25cm3 of the alkali into a volumetric flask.
2) Fill the burette with the acid using a funnel.
3) Add a few drops of indicator to the solution in the flask.
4) Run the acid from the burette into the solution in the conical flask, swirling the flask.
5) Stop when the indicator just changes colour.

Question 40

Salt

Answer 40

Compound that forms when a metal ion replaces the hydrogen ion in an acid.