Chem5 - Redox & Electrochemical Cells

Question 0

What makes the best oxidising agent? (1)

Answer 0

Something that itself is reduced and therefore has the highest E°

Question 1

How to calculate Emf/voltage

Answer 1

E°= E°RHS - E°LHS

Question 2

What makes the best reducing agent? (1)

Answer 2

Something that itself is oxidised and therefore has the lowest E°

Question 3

Give the components of the standard reference electrode. State the condition under which the stanard reference electrode is 0.00V (6)

Answer 3

Components
~Platinum electrode
~Hydrogen gas
~H+ (aq) or HCl or H2SO4

Conditions
~298K or 25°C
~1atm or 100 kPa
~1.00 mol dm-3 of [H+]

Question 4

How to make full redox equations

Answer 4

~multiply so electrons on both sides are the same ¥ cancel

~merge half equations and remove anything that cancels

Question 5

How to balance ionic equations

Answer 5

~make sure correct number of elements both sides
~then balance Oxygen with H2O
~balance Hydrogen with H+
~then balance to the side with the lowest charge with e-

Question 6

Fe2+ –> Fe3+ + e-

This is the reaction which occurs at the negative electrode. Deduce what change in concentration of Fe3+ would cause an increase in the Emf of the cell. Explain your answer. (3)

Answer 6

Change in conc: decrease
Explanation: equilibrium shifts to the right to oppose the change (and form more Fe3+ so its the same concentration as before) hence causing more electrons donated from the Fe2+ so electrode potential for Fe2+/Fe3+ electrode is more negative

Question 7

Rule for feasible reactions with HCl (2)

Answer 7

~in HCl its H+ so can only be reduced
~therefore if the other molecules Emf is a -ive emf the reaction with HCl would not be feasible (Emf=0.00-(+x) ) = -emf. ¥ not feasible

Question 8

Why does a fuel cells Emf stay constant? (1)

Answer 8

There is a constant supply of reagents

Question 9

What type of cell has a constant V/Emf? (1)

Answer 9

Fuel cell

Question 10

In a cell two reactions occur, molecule X loses electrons while Y gains electrons, which is the anode and which makes up the cathode? (1)

Answer 10

X is cathode (negative electrode) and Y is anode (positive electrode)

Question 11

Elements D and E react in a cell, D has an Emf/V of -0.46 and E has an Emf of +1.89, in the reaction which occurs which element is reduced and which is oxidised? (1)

Answer 11

E is reduced and D is oxidised

One with highest Emf is always reduced

Question 12

Element U has an Emf of +1.24 and V has an emf of +0.54, is this a feasible reaction?

U(s) + V3+ –> U3+ + V(s) (2)

Answer 12

~ because that would mean
Emf = +0.54 - (+1.24) = -0.70V
~Emf is negative ¥ reaction is not feasible

Question 13

How do you know if a reaction isn’t feasible? (1)

Answer 13

Has a negative Emf/V

Question 14

what is Emf? (1)

Answer 14

~the potential difference between two half cells