Redox Equilibria

Question 1

What is a salt bridge?

Answer 1

this is a piece of filter paper soaked in potassium nitrate solution.
Its purpose is to complete the circuit and to replace cations and anions that leave the solution

Question 2

what is an electromotive force (e.m.f) value?

Answer 2

this is the difference between electrode potential between the two metals (electrodes)

Question 3

what is the standard electrode potential (SHE) of a standard half cell?

Answer 3

this is the voltage of that half cell relative to a standard hydrogen half-cell under standard conditions

Question 4

what is the standard cell potential?

Answer 4

this is the voltage developed under standard conditions when two standard half cells are joined

Question 5

what does the standard hydrogen electrode consist of?

Answer 5

this consists of

• H2 gas at 1 atm and 25°C bubbling on a platinised platinum electrode
• the electrode is immersed in 1moldm-3 solution of H+ ions

Question 6

draw the standard hydrogen electrode.

Answer 6

no, you actually have to do this. It’s a question that comes. It’s really easy to learn. (see page 184)

Question 7

what are the functions of the platinum electrode in the standard hydrogen electrode?

Answer 7

• it acts as an inert connection to the H2/H+ system. This means that it doesnt try to form ions with itself because it is unreactive
• It allows H2 gas to be aDsorbed (not aBsorbed) onto its surface .
• It is platinised which increases surface area which results in increased reaction rate

Question 8

what is the difference between the terms ‘absorbed’ and ‘adsorbed’?

Answer 8

aBsorbed- occurs when a liquid actually soaks into a medium

aDsorbed- occurs when a liquid just accumulates on a surface without soaking into it.

Question 9

what is the standard electrode potential of the platinum half cell?

Answer 9

0

Question 10

how can the standard electrode potential of a species be obtained?

what criteria must be met for this to occcur?

Answer 10

this species can be connected to the standard hydrogen half cell

• all solutions must have a concentration of 1moldm-3
• temperature must be 25°C
• any gases involved must have a pressure of 1atm
• platinum must be used as an electrode when the half cell system does not include a metal

Question 11

what are fuel cells?
where are they found?
what are they made of?

Answer 11

• fuel cells are electrochemical cells which convert chemical energy of a fuel to electrical energy
• fuel cells can be found in vehicles. they produce electricity which is used to power the motor of a car
• fuel cells are made up of a cathode, an anode and an electrolyte. the electrolyte consists of warm KOH solution which contain K+ and OH- ions (from KOH) and H+ and OH_ ions (from H2O)

Question 12

what happens at the anode of a fuel cell?

Answer 12

H2 gas enters at the anode and splits into H+ and electrons.

The electrons flow through an external circuit which generates a current

• The H+ stays in the KOH solution and combines with the OH- forming water

Overall equation is H2 + 2OH- = 2H2O +2e

Question 13

what happens at the cathode of a fuel cell?

Answer 13

O2 gas enters at the cathode.

It combines with water and electrons to form OH-

overall equation is )2(g) + 2H2O(aq) = 2H2O + 2e

Question 14

state some advantages of fuel cells

Answer 14

• pollution free
• they convert 70% of chemical energy into electrical energy. (they are efficient)
  (BONUS) they provide water, heat and electricity to astronauts

Question 15

state one disadvantage of fuel cells

Answer 15

• catalysts in fuel cells are rapidly poisoned

Question 16

state two uses of standard electrode potential

Answer 16

• predict the feasibility of a reaction

- calculate the standard cell potential