Wjec Chemistry Ch5-Topic 15 Redox And Standard Electrode Potential

Front 1

What does the acronym OILRIG stand for?

Back 1

Oxidation Is Loss Reduction Is Gain. This means that something that has been oxidized has lost electrons and something that has been reduced has gained electrons.

Front 2

Go practice writing and balancing 2 half equations. Don't skip!!! Practice!!!

Back 2

Make sure the number of electrons on both sides are equal and therefore do you have to multiply one half equation by anything to ensure that it is balanced.

Front 3

Define the term Standard Electrode Potential.

Back 3

This is the electrode potential of a half cell under standard conditions measured relative to the hydrogen half cell under standard conditions which is set at 0.

Front 4

What is a half cell?

Back 4

A half cell is generally a solid rod of an element immersed in a solution of it's own ions with the rod connected to an electrical wire.

Hint 4

A half cell on it's own does nothing but when you set it up in opposition with another half cell an electrical current is produced. Many cells in a series makes a battery.

Front 5

What is the colour change for Cu2+ -> Cu(s).

Back 5

Cu2+ is blue
Cu(s) is salmon pink

Front 6

What is the colour change for Zn2+ -> Zn(s).

Back 6

Zn2+ is colourless
Zn(s) is white

Front 7

Is there a colour change for H+ -> H2(g)

Back 7

No, they are both colourless.

Front 8

What is the colour change for Fe3+ -> Fe2+?

Back 8

Fe3+ is orange
Fe2+ is pale green

Front 9

What is the colour change for MnO4- -> Mn2+?

Back 9

MnO4- is purple
Mn2+ is an extremely pale pink

Front 10

What is the colour change for Cl2 to Cl-?

Back 10

Cl2 is green
Cl- is colorless

Front 11

What is the colour change when Br2 -> Br-?

Back 11

Br2 is red/brown
Br- is colourless

Front 12

What is the colour change form I2 -> I-?

Back 12

I2 is purple
I- is colourless

Front 13

Write the cell statement for the Daniell Cell of Zn2+/Zn(s) -0.76V and Cu2+/Cu(s) +0.34V.

Back 13

Zn(s)/Zn2+//Cu2+/Cu(s)

Hint 13

The most negative Eθ value goes on the left. Sometimes the // in the middle is drawn as two dotted lines.

Front 14

When two half cells are set up in opposition as well as the electrical wire a salt bridge joins both flasks of ions. What does it contain and what is it's purpose?

Back 14

The salt bridge contains a saturated solution of electrolite and this contains ions that move to allow the flow of electrons therefore completing the circuit.

Front 15

What are the standard conditions that all the half cells should be measured under?

Back 15

298K, 100kPa/1atm and a solution of 1 moldm-3

Front 16

Draw a standard hydrogen electrode.

Back 16

Best picture is half way down this page http://www.chemguide.co.uk/physical/redoxeqia/introduction.html

Front 17

Describe the half cell that you would draw for Fe3+(aq)/Fe2+(aq).

Back 17

This would have a Platinum electrode in a solution of 1moldm-3 Fe3+ and 1 moldm-3 Fe2+

Front 18

Calculate the cell potential for Zn2+/Zn -0.76V and Cu2+/Cu +0.34 V.

Back 18

We start by writing the cell statement, The most negative electrode potential goes on the left and is therefore reversed i.e in this case Zn2+/Zn becomes Zn/Zn2+ which also means we must change the sign of the electrode potential so it is now +0.76. So Zn/Zn2+//Cu2+/Cu. We then put the numbers in and add them together 0.76 + 0.34 = +1.10V.

Front 19

What does e.m.f or Eθ stand for?

Back 19

Electromotive force.

Front 20

How would you predict the feasibility of a cell reaction occurring?

Back 20

By calculating the cell potential, if the resulting value is +ve the reaction will occur but if the value is -ve the reaction won't occur.

Hint 20

Remember that the reaction may not occur this way round but if you reversed the reaction it may occur. Also check the concentrations and conditions because if it is not under standard conditions it may give an unexpected result.

Front 21

Write the half equation showing Dichromate (VI) reducing to Chromium (III).

Back 21

(Cr2O7)2- + 6e + 14H+ -> 2Cr3+ + 7H2O

Hint 21

6 electrons are needed to reduce two Cr6+ to two Cr3+ and 14H+ because there are 7O so we need 14H+ to use up all the O.

Front 22

Write the equation that shows (Cr2O7)2- oxidizing Fe2+.

Back 22

Start with the two half equations
Fe2+ -> Fe3+ + e
(Cr2O7)2- + 6e + 14H+ -> 2Cr3+ + 7H2O.
The electrons need to be balanced so because there are 6e in the Cr half equation the Fe half equation has to be x6 to balance it. Then add the two together and cancel out anything that is the same on both sides.

Hint 22

(Cr2O7)2- + 14H+ + 6Fe2+ +6e-> 2Cr3+ + 6Fe3+ + 6e
Cancel the same.
(CrO7)2- + 14H+ + 6Fe2+ -> 2Cr3+ + 6Fe3+

Front 23

Give the equation and colour change for when an acidic solution is added to chromate(VI)

Back 23

2CrO42- + 2H+ <--> Cr2O72- + H2O and the colour change here is yellow to orange, (note: the reaction is an equilibrium)

Front 24

Describe the reaction between acidified MnO4- and Fe2+.

Back 24

1) write the two half equations and combine to produce:
MnO4- + 8H+ 5Fe2+ -> Mn2+ 4H2O + 5Fe3+

Front 25

Describe the redox reaction between Cu2+ and I-

Back 25

2Cu2+ + 4I- ->2CuI + I2

Front 26

What reactions occur in a hydrogen fuel cell?

Back 26

At the anode the H2(g) is turned into H+ and electrons
H2(g) <--> 2H+ + 2e
At the Cathode hydrogen ions react with the oxygen to form water molecules.
O2 + H+ <--> H2O
Put these two reactions together:
2H2(g) + 2O2(g) <--> 2H2O

Front 27

What are the advantages of fuel cells?

Back 27

1) They are much cleaner, producing only water as a waste gas.
2) The engines would be very quiet.

Front 28

What are the disadvantages of fuel cells?

Back 28

1) It would be necessary to store H2(g) in the car, which is a huge explosion and fire hazard.
2)Electricity is required in the first place to make the hydrogen gas, electricity that may come from burning fossil fuels so that would negate the effect of reduced CO2 emmisions from the car.
3)Petrol stations would also have to be adapted in order to provide H2.