module 5 (chapter 23) - electrode potentials and redox

Question 1

electrochemical cells

Answer 1

transfer chemical energy into electrical energy
-electrical energy is produced by the movement of electrons and so redox reactions are needed as these transfer electrons

Question 2

half cells

Answer 2

• contains a chemical species present in a redox half-equation
• the chemicals of the two cells are kept apart as if allowed to mix electrons flow uncontrollably releasing heat energy not electrical

Question 3

metal-metal ion half cells.

Answer 3

metal rod dipped into a solution of its aqueous metal ion

• the phase boundary is where the metal is in contact with its ions and this sets up an equilibrium
• phase boundary is shown by a vertical line in the equation
• in an isolate half cell, there is no net transfer of electrons either into or out of the cell.
• when there are two, the direction of flow depends upon the relative tendency of each electrode to release electrons

Question 4

ion-ion half cells.

Answer 4

• Contains ions of the same element in different oxidation states.
• An inert electrode made out of platinum is used. These help to transport electrons either into or out of the half-cell

Question 5

electrode potentials

Answer 5

• The electrode with more reactive metal loses electrons and is oxidised. This is the negative electrode
• The electrode with the less reactive metal gains electrons and is reduced. This is the positive electrode.

Question 6

standard electrode potentials

Answer 6

• The e.m.f of a half-cell connected to a standard hydrogen half-cell under standard conditions.
• A standard Half-cell contains hydrogen gas, a solution containing H+ ions and an Inert platinum electrode

Question 7

standard conditions

Answer 7

• Concentration of exactly 1moldm-3
• 298K
• 100Kpa

Question 8

general rules for measuring a standard electrode potential

Answer 8

• The more negative the electrode potential value, The greater the tendency to lose electrons and undergo oxidation.
• The more positive the electrode potential value, The greater the tendency to gain electrons and undergo reduction.
• Metals tend to have more negative electrode potential values than non-metals.

Question 9

standard cell potential equation

Answer 9

E(positive electrode) – E(negative electrode)

Question 10

limitations of electrode potentials

Answer 10

• They do not give an indication of rate of reaction of activation energy
• If the concentration is different to 1moldm-3, it won’t work.
• The actual conditions may be different to standard.
• standard electrode potentials apply to aqueous equilibria, many reactions take place that are not aqueous
• f the potential difference is less than 0.4V, the reaction is unlikely to happen

Question 11

salt bridge

Answer 11

• allows ions to flow and contains a solution that doesn’t react with the half cell solutions
• e.g. filter paper socked in KNO3

Question 12

by convention. how do we write half cell equations?

Answer 12

-reduction as the forwards reaction

Question 13

over time why does the measured cell potential change?

Answer 13

• the concentration of the electrolyte chances

- this means that the conditions are standard

Question 14

how do primary cells work?

Answer 14

• electrical energy is produced by redox reactions at the electrodes.
• however, the reactions cannot be reversed. this means they are non-renewable and therefore when chemicals are used up they will go flat

Question 15

what are primary cells used for?

Answer 15

• Low current, long-storage devices such as wall clocks and smoke detectors
• Most modern primary cells are alkaline based on Zinc and Magnesium dioxide, and a potassium hydroxide electrolyte solution

Question 16

advantages of primary cells

Answer 16

• They have high specific energy
• Light weight so can be carried around on aircrafts
• Cheap to make and buy
• Long storage times

Question 17

disadvantages of primary cells

Answer 17

• non-renewable and therefore can only be used once
• Made from toxic materials (heavy metals)
• Difficult to dispose of and often sent to landfill.

Question 18

how do secondary cells work?

Answer 18

• electrical energy produced by redox (simultaneous oxidation and reduction)
• They can be recharged as the cell reaction producing the electrical energy can be reversed
• The chemicals in the cell are regenerated and can be used again
• The recharging process is just the opposite of the discharging reaction so is essentially the redox reaction in reverse for recharging.

Question 19

what are secondary cells used for?

Answer 19

• Lead-acid batteries used in cars
• Nickel-cadmium cells form the batteries used in radios and torches
• Lithium ion cells are used in laptops, camera and mobile phones. They are also being developed for use in cars.

Question 20

advantages of secondary cells

Answer 20

• They use less toxic materials and therefore toxic waste.
• Sometimes generates hydrogen gas at the cathode when charging
• Produce less waste because they can be recharged
• Motion can be used to recharge batteries

Question 21

disadvantages of secondary cells.

Answer 21

• more expensive
• Don’t have a huge lifespan
• Extremely sensitive to high temperatures and therefore are prone to exploding and burning

Question 22

how do hydrogen fuel cells work?

Answer 22

• Anode is where oxidation takes place (electrically negative), cathode is where reduction takes place (electrically positive)
• Electrodes are porous and made of platinum materials reinforced carbon – acts as catalysts
• can have either an acid or alkali electrolyte

Question 23

fuel cells

Answer 23

• use the energy from a reaction of a fuel with oxygen to create voltage
• the fuel and oxygen flow into the fuel cell and the products flow out. the electrolyte remains in the cell
• they don’t need to be recharged and can operate continuously along as they are supplied.

Question 24

advantages of fuel cells

Answer 24

• Produced in an environmentally manner
• Water and heat are the only by-product
• More efficient than combustion engines
• Less noise
• Never run flat if you have a constant supply of fuel and therefore are more reliable. Secondary cells eventually become less efficient, primary cells are non-renewable as you have to replace the cell when it runs out.

Question 25

disadvantages of fuel cells

Answer 25

• expensive to manufacture
• There is a lack of infrastructure to support the distribution of hydrogen. This means it is not yet, widely available.
• It needs to be pressurised into a liquid as a gas is too dispersed. This is expensive and potentially explosive.

Question 26

how do methanol fuel cells work?

Answer 26

-Water and methanol react as the cathode whilst oxygen and H+ ions react at the anode to produce water

Question 27

positives and negatives of methanol fuel cells

Answer 27

• used in consumer goods
• Limited power so they can’t power vehicles
• Methanol is very toxic and highly flammable

Question 28

how do you do redox equations?

Answer 28

• work out what has been oxidised and what has been reduced
• balance the electrons
• balance any remains atoms and predict further species

Question 29

what can manganate redox titrations be used for?

Answer 29

• the analysis of different reducing agents
• irons ions Fe2+
• ethanedioic acid (COOH)2

Question 30

how do you analyse the purity of an iron compound?

Answer 30

• prepare a volume if impure hydrated iron sulphate in a volumetric flask
• measure a volume of this into a conical flask and add a smaller amount of 1moldm-3 sulphuric acid
• titrate this using a standard 0.02moldm-3 solution of potassium manganate
• analyse your results to determine percentage purity

Question 31

how to analyse your results (iron and manganate)?

Answer 31

• calculate the amount of managante that reacted.
• determine the amount of iron that reacted (equation and molar ratio)
• scale up to find mole of iron in standard solution
• find the mass of hydrated iron sulphate in the impure sample
• place this as a percentage of the impure sample

Question 32

what happens in iodine/thiosulphate titrations?

Answer 32

• thiosulfate ions are oxidised

- iodine is reduced

Question 33

what can iodine/thiosulphate titrations be used for?

Answer 33

• the ClO- content in household bleach
• the Cu2+ content in copper compounds
• the Cu content in copper alloys

Question 34

how is the iodine/thiosulphate titration done?

Answer 34

• prepare a solution of the oxidation agent and add to a conical flask
• add an excess of potassium iodide. the oxidising agent reacts with iodide ions to produce iodine, which turns the solution a yellow-brown colour
• titrate with thiosulphate
• the iodine is reduced to black ions and the yellow colour fades
• when the end point is being reached, add starch. this forms a deep blue colour to assist the identification of the end point
• once all of the iodine has reacted all of the blue/black disappears.

Question 35

iodine/thiosulphate and copper

Answer 35

• can be used to determine the copper content in copper salts or alloys
• for soluble copper salts dissolve in water, for insoluble react with acid.
• Cu2+ ions react with I- to form a solution of iodine and a white ppt of copper iodide
• the mixture appears brown
• the iodine is then titrated with a standard solution of sodium thiosulphate