module 5 (chapter 23) - electrode potentials and redox Flashcards
1
Q
electrochemical cells
A
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
2
Q
half cells
A
- 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
3
Q
metal-metal ion half cells.
A
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
4
Q
ion-ion half cells.
A
- 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
5
Q
electrode potentials
A
- 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.
6
Q
standard electrode potentials
A
- 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
7
Q
standard conditions
A
- Concentration of exactly 1moldm-3
- 298K
- 100Kpa
8
Q
general rules for measuring a standard electrode potential
A
- 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.
9
Q
standard cell potential equation
A
E(positive electrode) – E(negative electrode)
10
Q
limitations of electrode potentials
A
- 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
11
Q
salt bridge
A
- allows ions to flow and contains a solution that doesn’t react with the half cell solutions
- e.g. filter paper socked in KNO3
12
Q
by convention. how do we write half cell equations?
A
-reduction as the forwards reaction
13
Q
over time why does the measured cell potential change?
A
- the concentration of the electrolyte chances
- this means that the conditions are standard
14
Q
how do primary cells work?
A
- 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
15
Q
what are primary cells used for?
A
- 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
16
Q
advantages of primary cells
A
- They have high specific energy
- Light weight so can be carried around on aircrafts
- Cheap to make and buy
- Long storage times
17
Q
disadvantages of primary cells
A
- non-renewable and therefore can only be used once
- Made from toxic materials (heavy metals)
- Difficult to dispose of and often sent to landfill.
18
Q
how do secondary cells work?
A
- 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.
19
Q
what are secondary cells used for?
A
- 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.
20
Q
advantages of secondary cells
A
- 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
21
Q
disadvantages of secondary cells.
A
- more expensive
- Don’t have a huge lifespan
- Extremely sensitive to high temperatures and therefore are prone to exploding and burning
22
Q
how do hydrogen fuel cells work?
A
- 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
23
Q
fuel cells
A
- 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.
24
Q
advantages of fuel cells
A
- 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.
25
Q
disadvantages of fuel cells
A
- 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.
26
Q
how do methanol fuel cells work?
A
-Water and methanol react as the cathode whilst oxygen and H+ ions react at the anode to produce water
27
Q
positives and negatives of methanol fuel cells
A
- used in consumer goods
- Limited power so they can’t power vehicles
- Methanol is very toxic and highly flammable
28
Q
how do you do redox equations?
A
- work out what has been oxidised and what has been reduced
- balance the electrons
- balance any remains atoms and predict further species
29
Q
what can manganate redox titrations be used for?
A
- the analysis of different reducing agents
- irons ions Fe2+
- ethanedioic acid (COOH)2
30
Q
how do you analyse the purity of an iron compound?
A
- 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
31
Q
how to analyse your results (iron and manganate)?
A
- 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
32
Q
what happens in iodine/thiosulphate titrations?
A
- thiosulfate ions are oxidised
- iodine is reduced
33
Q
what can iodine/thiosulphate titrations be used for?
A
- the ClO- content in household bleach
- the Cu2+ content in copper compounds
- the Cu content in copper alloys
34
Q
how is the iodine/thiosulphate titration done?
A
- 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.
35
Q
iodine/thiosulphate and copper
A
- 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
