module 5 (chapter 23) - electrode potentials and redox Flashcards
electrochemical cells
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
half cells
- 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
metal-metal ion half cells.
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
ion-ion half cells.
- 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
electrode potentials
- 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.
standard electrode potentials
- 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
standard conditions
- Concentration of exactly 1moldm-3
- 298K
- 100Kpa
general rules for measuring a standard electrode potential
- 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.
standard cell potential equation
E(positive electrode) – E(negative electrode)
limitations of electrode potentials
- 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
salt bridge
- allows ions to flow and contains a solution that doesn’t react with the half cell solutions
- e.g. filter paper socked in KNO3
by convention. how do we write half cell equations?
-reduction as the forwards reaction
over time why does the measured cell potential change?
- the concentration of the electrolyte chances
- this means that the conditions are standard
how do primary cells work?
- 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
what are primary cells used for?
- 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
advantages of primary cells
- They have high specific energy
- Light weight so can be carried around on aircrafts
- Cheap to make and buy
- Long storage times
disadvantages of primary cells
- non-renewable and therefore can only be used once
- Made from toxic materials (heavy metals)
- Difficult to dispose of and often sent to landfill.
how do secondary cells work?
- 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.
what are secondary cells used for?
- 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.
advantages of secondary cells
- 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
disadvantages of secondary cells.
- more expensive
- Don’t have a huge lifespan
- Extremely sensitive to high temperatures and therefore are prone to exploding and burning
how do hydrogen fuel cells work?
- 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
fuel cells
- 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.
advantages of fuel cells
- 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.