Electrochemical Cell

Question 1

Describe the set up of an electrochemical cell

Answer 1

• 2 diff. metal ions connected by wire (external circuit) - e^- flow from more reactive metal to less reactive
• Dipped in salt solution of own ions connected by salt bridge (filter paper dipped in KNO_{<strong>3</strong>} )
• Voltmeter show v btw 2 half cells - EMF
• Solutions of aqueous ions uses Pt electrode

Question 2

Explain the function of the salt bridge

Answer 2

• Allows ions to flow
• Maintain charge balance

Question 3

State a requirement of the soluble ionic compound used to make a salt bridge (1)

Answer 3

Must not react w/ electrolyte/ions in sol

Question 4

Why is platinum used?

Answer 4

Inert- won’t react w/ ions

Question 5

Which two reactions occur w/i an electrochemical cell?

Answer 5

• Oxidation
• Reduction

Question 6

Are reactions that occur at each electrodes irreversible or reversible?

Answer 6

Reversible

Question 7

How are electrochemical half-equations always written like?

Answer 7

Reduction reaction going in forward reaction

Question 8

What happens when 2 half-equations are joined?

Answer 8

• More -ve electrode potential (easily oxidised) is flipped
• More +ve electrode potential (reduced) stays the same

Question 9

What are electrode potentials measured against?

Answer 9

Standard hydrogen electrodes

Question 10

Define standard electrode potential of a half cell

Answer 10

Voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode

Question 11

What are the standard conditions?

Answer 11

• Conc: 1.00 mol dm^-3
• Temp: 298k (25^oC)
• Pressure: 100kPa

Question 12

Best oxidising agent

Answer 12

Most +ve (left w/ charge)

Question 13

Worst oxidising agent

Answer 13

Most -ve (left w/ charge)

Question 14

Best reducing agent

Answer 14

Most -ve (right)

Question 15

Worst reducing agent

Answer 15

Most +ve (right)

Question 16

EMF

Answer 16

EMF = R - L

Question 17

Outline how to draw electrochemical cells

Answer 17

R | O || O | R

• Double lines: salt bridge
• Lines: state change
• More -ve on left exp. w/ standard H
• O = ions

Question 18

Most -ve

Answer 18

• Anode
• Most reactive
• Oxidised
• Loses e^-

Question 19

Most +ve

Answer 19

• Cathode
• Least reactive
• Reduced
• Gains e^-

Question 20

Sulfate ions: +0.17v Bromide ions: +1.07v

Justify why sulfate ions should not be capable of oxidising bromide ions (1)

Answer 20

E(Br₂/Br^-) > E(SO₄^-2/SO₂)

Question 21

Outline the uses of lithium batteries

Answer 21

• Rechargeable
• Mobile phones, laptops + cars

Question 22

What are lithium batteries made up of?

Answer 22

• Lithium cobalt oxide (LiCoO₂) electrode + graphite electrode
• Electrolyte: lithium salt in organic solvent

Question 23

What are the half equations for a lithium battery?

Answer 23

• -ve electrode : Li → Li⁺ + e^-
• +ve electrode : Li⁺ + CoO₂ + e^- → Li⁺ [CoO₂]^-

Question 24

What happens when a battery is recharged?

Answer 24

Current force e^- to flow in opposite direction round the circuit + reverse the reactions

Question 25

Why can't non-rechargable batteries be recharged?

Answer 25

Reaction **can't** be **reversed**

Question 26

Give a use of an alkaline hydrogen-oxygen fuel cell

Answer 26

**Power electric vehicles**

Question 27

Outline the structure of an alkaline hydrogen-oxygen fuel cell

Answer 27

* **H₂** + **O₂ **fed to **2 seperate Pt containing** **electrodes** * Electrodes **seperated** by **anion-exchange membrane** that **allows anions** (OH^-) + **water** to **pass** through it **but not H** + **O gas** * **Electrolyte**: **aqueous alkaline** solution (KOH) * **Anions flow** towards **-ve electrode**

Question 28

In an alkaline hydrogen-oxygen fuel cell which electrode is hydrogen fed to?

Answer 28

**-ve electrode**

Question 29

In an alkaline hydrogen-oxygen fuel cell which electrode is oxygen fed to?

Answer 29

**+ve electrode**

Question 30

What is the half equation for the reaction that occurs at the positive electrode for a hydrogen-oxygen fuel cell that operates in alkaline conditions?

Answer 30

**O₂ + 2H₂O + 4e^- → 4OH^-**

Question 31

What is the half equation for the reaction that occurs at the negative electrode for a hydrogen-oxygen fuel cell that operates in alkaline conditions?

Answer 31

**2H2 + 4OH- → 4H2O + 4e-**

Question 32

What is the overall equation for reaction that occurs at a hydrogen-oxygen fuel cell that operates in alkaline conditions?

Answer 32

**2H₂ + O₂ → 2H₂O**

Question 33

What is the half equation for the reaction that occurs at the negative electrode for a hydrogen-oxygen fuel cell that operates in acidic conditions?

Answer 33

**H₂ → 2H⁺ + 2e^-**

Question 34

What is the half equation for the reaction that occurs at the positive electrode for a hydrogen-oxygen fuel cell that operates in acidic conditions?

Answer 34

**O₂ + 4H⁺ + 4e^- → 2H₂O**

Question 35

How is a current generated in a hydrogen-oxygen fuel cell?

Answer 35

* **Hydrogen produces e^-** * **Oxygen accepts e^-**

Question 36

What are the advantages of using fuel cells in cars than the internal combustion engine?

Answer 36

* **More efficien**t - **greater proportion** of **energy available** from hydrogen-oxygen reaction is **converted** into **kinetic energy** * **Waste** product is **H₂O** - **no toxic chem** * **Don't** need to be **recharged**

Question 37

What are the disadvantages of using a fuel cell?

Answer 37

* **Need energy** to supply hydrogen + oxygen (electorlysis - uses electricity) * **H** is **highly flammable/explosive**

Question 38

Explain why a fuel cell doesn't need to be recharged (1)

Answer 38

**Hydrogen/fuel** is **continuously supplied**