Electrochemical cells

Question 1

Oxidation states

Answer 1

• in simple ions, oxidation state = charge
• in compounds of just one element = 0
• total of all states in complex compounds = overall charge of the charge

Question 2

Half equations

Answer 2

• need balanced charges on both sides, and one element must change oxidation state
• balance electrons by adding e, balance positive charge by adding H, balance oxygen’s by adding H2O

Question 3

Combining Half equations

Answer 3

• balance electrons on both sides by multiplying all coefficients so that electrons can be cancelled out on both sides

Question 4

Redox

Answer 4

• when one or more species change oxidation states
• disproportionation is when one species is both reduced and oxidised
• oxidising agents cause oxidation by being reduced
• reducing agents cause reduction by being oxidised`

Question 5

Electrode potentials

Answer 5

• caused by metals in a solution of metal ions
• ions at the anode are oxidised and form positive ions in solution, ions at the cathode are reduced and form solid metal
• if a + charge builds, metal has a + electrode potential, if a - charge build vice versa

Question 6

Building cells

Answer 6

• connecting 2 half cells (a metal in a solution of its ions) with a wire from the two metals and a salt bridge connecting the two solutions allows electrons to flow to the more positive half cell (drawn on the right hand side usually)
• negative cathode = reduction occurs
• positive anode = oxidation occurs

Question 7

Calculating E.M.F of cells

Answer 7

• Electrode potential of right hand cell - potential of left
• Standard Hydrogen Electrode has electrode potential of 0V under standard conditions
• if S.H.E is left cell, E.M.F of whole cell = electrode potential of right cell (can be used to find potential of other cells)

Question 8

Writing cells

Answer 8

• more negative cell on left e.g Zn I Zn2+ II (more oxidised states are next to double line - represents salt bridge)
• more positive cell on right, e.g II Cu2+ I Cu

Question 9

Standard conditions

Answer 9

• 298k
• 100 kPa
• 1.00 mol dm^-3

Question 10

unusual electrodes

Answer 10

• if element is a gas (e.g hydrogen), a solid electrode must be made using an inert metal (usually platinum), and the gas must be bubbled through the solution
• if element is actually two ions both in solution, inert solid metal electrode is still used and the solution just contains both species
• written as Pt I H, H+ II

Question 11

Electrochemical series

Answer 11

• writes all the electrode potentials of the reduction reactions taking place in every half-cell (written as reversible)
• more reactive metals lose electrons more easily, and have more negative electrode potentials as are more easily oxidised
• lower electrode potential = more easily oxidised = better reducing agent
• higher electrode potential = more easily reduced = better oxidising agent

Question 12

Writing chemical equations for cells

Answer 12

• the half equation with most positive electrode potential is written as a reduction reaction (as it is in the electrochemical series)
• the more negative half equation is oxidised, and as such is written (backwards) when the two are combined

Question 13

types of battery

Answer 13

• rechargeable = reversible cells as electrons can be forced back in
• in lithium cells, the positive electrode is Li+ + CO2 -> li+[CO2]-, the negative electrode is Li -> Li+ + e-
• non-rechargeable = non reversible, negative electrode lines the casing of the battery and gets thinner as it is oxidised so the reaction cannot be reversed

Question 14

Fuel cells

Answer 14

• chemicals are kept separate and fed in when electricity is required
• do not need to be electrically recharged, but they generate an electric current
• fuel cells are a more efficient source of energy than internal combustion engines
• they have fewer waste products so are better for the environment
• producing hydrogen for the fuel cells requires electricity, produced from fossil fuels. also hydrogen is dangerous and flammable. refueling stations are very rare as the infrastructure isn’t yet there to provide it on a large scale

Question 15

Hydrogen-Oxygen fuel cell

Answer 15

• hydrogen is fed to negative electrode, oxygen to positive
• electrodes are thin layers of platinum over porous ceramic
• anion exchange membrane allows OH- anions and water to pass through only
• electrolyte is KOH solution
• when hydrogen is fed to negative electrode it forms H2O with the OH-, and 2e- that flow to the positive electrode
• at the positive electrode Oxygen and water react with 4e- to make 4OH-
• cell has e.m.f of of +1.23V, waste product is only water