Redox 2

Question 1

Steps to set up an electrochemical cell

Answer 1

1. Obtain metals under investigation and clean them with emery paper to remove surface impurities
2. wash surface with propanone, wear gloves
3. place each metal into the solution of its ion
4. make salt bridge from filter paper soaked in KNO₃ or KCl, link beakers together submerged in the solutions
5. connect the electrodes with wires, crocodile clips, and a voltmeter

Question 2

What is a half cell?

Answer 2

• a half cell is one side of an electrochemical cell

They can be made from:
* a metal dipped in its ions
* a platinum electrode with 2 aqueous ions

Question 3

why is platinum used in half cells with 2 aqueous ions?

Answer 3

it is intert but electrically conductive

Question 4

what is the flow of electrons in terms of reactivity of metals

Answer 4

electrons always flow from a more reactive metal to a less reactive one

Question 5

Describe the redox reaction happening in an electrochemical cell

Answer 5

For example, Zn and Cu

The zinc half cell loses electrons as its more reactive than copper
* OXIDATION: Zn(s) –> Zn²⁺(aq) + 2e⁻
* observation: zinc electrode becomes thinner as more Zn²⁺ is produced to make electrons

The copper half cell accepts the electrons made from zinc
* REDUCTION: Cu²⁺(aq) + 2e⁻ –> Cu(s)
* observation: copper electrode gets thicker as Cu²⁺ ions recieve electrons and turn into copper

Question 6

What is a salt bridge and why is it needed?

Answer 6

A salt bridge is filter paper with saturated KNO₃ solution.

Ions flow through which balances the charges

Question 7

What is an electrode potential?

Answer 7

Each half cell has an electrode potential (Eθ) value measured in volts

It tells you how easily the half cell gives up electrons (oxidised)

Question 8

How do we know which reaction will undego oxidation/reduction?

Answer 8

NO PRoblem

The most Negative half cell will undergo Oxidation
The most Positive half cell will undergo Reduction

whichever half cell is undergoing oxidation, we flip the equation (because e⁻ are being lost)

Question 9

what is the Standard Hydrogen Electrode? (SHE)

Answer 9

The SHE is used as a reference to measure standard electrode potential (Eθ)

The SHE’s Eθ = 0.00V

Question 10

What is the set up for measuring electrode potentials of half cells against a Standard hydrogen electrode?

Answer 10

In the SHE half cell:
* a platinum electrode with a glass bulb surrounding it
* hydrogen gas goes in at the side (at standard conditions)
* 1 mol/dm³ of H+ ions in solution, 1 mol/dm³ of the other half cell ions in solution
* the other general electrochemical cell components (e.g. salt bridge, voltmeter, etc)

Question 11

What are the standard conditions for Standard Electrode Potential?

Answer 11

1. temperature of 298K
2. pressure at 100kPa
3. concentrations of ions at 1 mol/dm³

Question 12

The concentration of acid required depending on the acid used, to produce standard condition of 1mol/dm³ of H⁺ ions

Answer 12

to get 1mol/dm⁺ of H⁺ ions, different acids require different concentrations

ex. HCl would need conc of 1mol/dm³ because its monoprotic
H₂SO₄ would need conc of 0.5mol/dm³ because its diprotic

Question 13

Interpreting the electrochemical series and how to use it

Answer 13

The half cell reactions at the top are the most likely to be reduced

N-negative O-oxidised
P-positive R-reduced OBLEM

Question 14

Calculating standard cell potential equation

Answer 14

Eθcell = Eθred - Eθoxid

Question 15

Method for calculating standard cell potential

Answer 15

• Out of the two half cells, identify which is the most positive (most likely to be reduced)
• This half cells standard electrode potential (Eθ) - the oxidised half cells standard electrode potential (Eθ) = overall Eθ cell

Question 16

What affects standard cell potential?

Answer 16

• if conditions change from standard conditions - equilibrium will change
• temperature, concentration and pressure

Question 17

Cell notation

Answer 17

red l ox ll ox l red

the most NEGATIVE half cell potential goes to the LEFT of the double line

• WHEN we have 2 aqueous ions:
  1. instead of a single line, a comma is used (because there isn’t a change in state)
  2. add a platinum electrode on the end ( l Pt(s) )

Question 18

What do the single and double bar lines mean in cell notation?

Answer 18

single lines - show a physical state change
double lines - represents the salt bridge

Question 19

Predicting reaction feasiblity (under standard conditions)

Answer 19

standard electrode potentials (Eθ) can be used to predict if the reaction is feasible under standard conditions

• combine the half equations (using the flipped version of the oxidised one) to get the feasible reaction
• compare it to the question if they match it is feasible
• confirm by calculating Eθ cell (all feasible reactions have a positive value)

Question 20

What are the links between cell potential, entropy and the equilibrium constant?

Answer 20

(cell potential) Eθ ∝ ∆s total

Eθ ∝ ln(k)

Question 21

How do Energy storage cells (batteries) and rechargable batteries link to overall cell equations?

Answer 21

example of an overall discharge equation:
Li + CoO₂ –> Li⁺[CoO₂]⁻

Rechargable batteries work by:
* plug them in to supply a current
* the current forces the electrons to flow in the opposite way
* we reverse the overall discharge equation to show battery recharging

Question 22

What are fuel cells?

Answer 22

Fuel cells generate electricty by a continuous external supply of chemicals rather than a ‘ready store’ like in batteries

Question 23

How do fuel cells work? (steps)

Answer 23

In fuel cells, the energy is released as electrical energy (voltage)

1. Hydrogen is fed into one side. It reacts with OH⁻ ions in solution. 2H₂(g) + 4OH⁻(aq) –> 4H₂O(l) + 4e⁻
2. Oxygen is fed into the other side. It reacts with water and the 4 electrons made in step 1 to make OH- ions.
   O₂(g) + 2H₂O(l) + 4e- –> 4OH-(aq)
3. combining the half equations:
   2H₂(g) + O₂(g) –> 2H₂O(g)

Question 24

Why are methanol and other hydrogen-rich fuels used in fuel cells?

Answer 24

Hydrogen reacts cleanly with oxygen to produce water.

Methanol contains lots of hydrogen atoms that can be used in the reaction.

They are efficient and produce fewer harmful emissions compared to fossil fuels.

Question 25

What is the purpose of **redox titrations?**

Answer 25

redox titrations are used to find out the concentration of a reducing or oxidising agent

Question 26

What is the general method of a **redox titration?**

Answer 26

1. a pipette is used to measure a known volume of solution being analysed into a conical flask 2. add dilute sulfuric acid to acidify solution and indicator (if needed) 3. fill burette with titrant and carry out tirations to find the volume required to each the end point 4. we know the volume of both solutions and the concentration of the titrant, so unknown concentration can be found

Question 27

Why is sulfuric acid used rather than hydrochloric acid when acidifying a solution being titrated?

Answer 27

Cl- can be oxidised