Physical: Electrode Potential and Electrochemical Cells

Question 1

What is the basis of an electrochemical cell

Answer 1

When a rod of metal is dipped/placed in a solution of its own metal ions an equilibrium is set up
There is a tendency for the metal to form +ve ions in the solution
Or gain electrons and form a metal atom

Question 2

What happens if the equilibrium lies to the left

Answer 2

Metal acquires -ve charge due to the build up of e- on the metal
The electrode has a -ve charge

Question 3

What happens if the equilibrium lies to the right

Answer 3

+ve charge builds up in the metal as the electrons have been used up to form the metal ions
Electrode has a +ve potential

Question 4

What determines wither a metal wants to become an atom or ion

Answer 4

Reactivity and position of equilibrium

Question 5

Do more reactive metals tend to form atoms or ions

Answer 5

Ions - eq shifts left as easier to lose an electron

Question 6

Do less reactive metals tend to form atoms or ions

Answer 6

Metals - eq shift right easier to gain

Question 7

What is a half-cell / electrode

Answer 7

Metal in a solution of its ions

Question 8

What are the 3 types of electrode

Answer 8

Metal - metal surrounded by solution of its ions
Gas- gas and solution of ions - inert metal is actual electrode to allow flow of electrons
Redox-two ions of same element with inert metal electrode

Question 9

Give an example of a half-cell

Answer 9

For example:
Zn (s) + CuSO4 (aq)→ Cu (s) + ZnSO4 (aq)

Instead of electrons being transferred directly from the zinc to the copper ions, a cell is built which separates the two redox processes
For example:
Zn (s) ⇌ Zn2+ (aq) + 2e–

If a rod of metal is dipped into a solution of its own ions, an equilibrium is set up
Each part of the cell is called a half cell

The Zn atoms on the rod can deposit two electrons on the rod and move into solution as Zn2+ ions:
Zn(s) ⇌ Zn2+(aq) + 2e–

This process would result in an accumulation of negative charge on the zinc rod
Alternatively, the Zn2+ ions in solution could accept two electrons from the rod and move onto the rod to become Zn atoms:
Zn2+(aq) + 2e– ⇌ Zn(s)

This process would result in an accumulation of positive charge on the zinc rod

Question 10

How do u measure the potential of a half cell

Answer 10

Measure the potential difference between the 2 half cells

Question 11

What factors does the p.d depend on

Answer 11

the nature of the ions in solution
the concentration of the ions in solution
the type of electrode used
the temperature

Question 12

What is the electrode potential

Answer 12

Show how easily a substance is reduced
Half equations must be wrote with the electrons on the left

The more positive (or less negative) an electrode potential, the more likely it is for that species to undergo reduction
The equilibrium position lies more to the right

The more negative (or less positive) the electrode potential, the less likely it is that reduction of that species will occur
The equilibrium position lies more to the left

Question 13

What is the standard hydrogen electrode

Answer 13

Potential that is used to measure against the potentials produced so that they can be measured
This is assigned to potential of O
And is known as the primary standard

The standard hydrogen electrode is a half-cell used as a reference electrode and consists of:
Hydrogen gas in equilibrium with H+ ions of concentration 1.00 mol dm-3 (at 100 kPa)
2H+ (aq) + 2e- ⇌ H2 (g)

An inert platinum electrode that is in contact with the hydrogen gas and H+ ions
When the standard hydrogen electrode is connected to another half-cell, the standard electrode potential of that half-cell can be read off a high resistance voltmeter

Question 14

What is an electrochemical cell

Answer 14

Two half cells
- two metals joined by a wire
-solutions joined with a salt bridge - ions flow through and go into each solution
Voltmeter used to collect he p.d

Question 15

What is the salt bridge

Answer 15

-either
-a piece of filter paper soaked in a solution if unreactive irons
-tube containing unreceptive ions in an Agar gel
-compound s such as KNO3 are used as ions re unreactive

Question 16

What are the standard conditions of an electrode potentials

Answer 16

Ion concentration of 1.00 mol dm-3
A temperature of 298 K
A pressure of 100 kPa
It is written as E dot
Needs to be standard conditions as a change in conditions could effect the equilibrium

Question 17

How is the EMF calculated

Answer 17

standard cell potential (Ecellꝋ) can be calculated by subtracting the less positive Eꝋ from the more positive Eꝋ value
The half-cell with the more positive Eꝋ value will be the positive pole
By convention this is shown on the right hand side in a conventional cell diagram, so is termed Erightꝋ
The half-cell with the less positive Eꝋ value will be the negative pole
By convention this is shown on the left hand side in a conventional cell diagram, so is termed Eleftꝋ
Ecellꝋ = Erightꝋ - Eleftꝋ

Since oxidation is always on the left and reduction on the right, you can also use this version
Ecellꝋ = Ereductionꝋ - Eoxidation

Question 18

Example of calculating the EMF/ standard potential of a cell

Calculate the standard cell potential for the electrochemical cell below and explain why the Cu2+ / Cu half-cell is the positive pole. The half-equations are as follows:

Cu2+(aq) + 2e- ⇌ Cu(s) Eꝋ = +0.34 V

Zn2+(aq) + 2e- ⇌ Zn(s) Eꝋ = −0.76 V

Answer 18

Step 1: Calculate the standard cell potential. The copper is more positive so must be the right hand side.

Ecellꝋ = Erightꝋ - Eleftꝋ

Ecellꝋ = (+0.34) - (-0.76)

= +1.10 V

The voltmeter will therefore give a value of +1.10 V

Step 2: Determine the positive and negative poles

The Cu2+ / Cu half-cell is the positive pole as its Eꝋ is more positive than the Eꝋ value of the Zn2+ / Zn half-cell

Question 19

What is the conventional notation of electrochemical cells

Answer 19

A solid vertical (or slanted) line shows a phase boundary, that is an interface between a solid and a solution
A double vertical line (sometimes shown as dashed vertical lines) represents a salt bridge

The substance with the highest oxidation state in each half cell is drawn next to the salt bridge
The cell potential difference is shown with the polarity of the right hand electrode

The cell convention for the zinc and copper cell would be
Zn (s)∣Zn2+ (aq) ∥Cu2+ (aq)∣Cu (s) E cell = +1.10 V

This tells us the copper half cell is more positive than the zinc half cell, so that electrons would flow from the zinc to the copper
The same cell can be written as:
Cu (s)∣Cu2+ (aq) ∥Zn2+ (aq)∣Zn (s) E cell = -1.10 V

The polarity of the right hand half cell is negative, so we can still tell that electrons flow from the zinc to the copper half cell

Question 20

What are secondary standards

Answer 20

The standard hydrogen electrode potential is difficult to use so other standards are used to calibrate against the SHE

Question 21

In electrochemical cells which one is the anode and which is the cathode

Answer 21

Anode - metal that is oxidised
Cathode - metal that is reduced

Question 22

In electrolytic cells which one is the anode and the cathode

Answer 22

Cathode = -ve
Anode = +ve