U3AOS1: options for energy production - electrochemistry Flashcards
define oxidation
the loss of electrons
where does oxidation occur
anode
how can you identify oxidation?
-loss of electrons
-increase in oxid number
-gain oxygen
-loss of hydrogen
define reduction
the gain of electrons
where does reduction occur
cathode
how can you identify reduction?
-gain of electrons
-decrease in oxid number
-lose oxygen
-gain of hydrogen
what happens to the reductant?
-lose electrons bc undergoes oxidation
-causes another substance to be reduced (the oxidant)
what happens to the oxidant?
-gain electrons bc undergoes reduction
-causes another substance to be oxidised (the reductant)
KOHES in basic conditions
do normal KOHES, then add OH- ions to each side to balance out H+ ions and form water molecules
what is a direct redox reaction?
when reactants are directly in contact with each other, so a spontaneous reaction has occurred without providing an external energy source - generally exothermic, release heat energy
what is an indirect redox reaction?
when the reactants are not in contact and are in separate containers, connected by a wire which facilitates the reaction - releases electrical energy and a little bit of heat energy
does a direct or indirect reaction produce a voltage?
indirect
potential impacts and observations of redox reaction
-reaction may not be observed bc too slow
-another product may form at an electrode
-different voltage produced
what are redox reaction predictions impacted by?
-changes in temperature
-higher/lower concentrations than 1.0M
-conditions and experimental design
define galvanic cell
an electrochemical cell where chemical energy is converted into electrical energy via sponatneous reactions
function of salt bridge
-allows the flow of ions between two half cells
-maintains electrical neutrality/even distribution of charge
-completes the circuit
function of electrolyte
the source of ions, can allow for flow of electrical charge within the cell
how to calculate electric potential
E(cathode) - E(anode)
define fuels cells
a type of galvanic cells that converts chemical energy from a fuel to electrical energy
what happens to the fuel and oxygen in fuel cell?
fuel is oxidised at anode
oxygen is reduced at cathode
electrolyte in fuel cell
-can be solid or liquid
-allows the movement of charged ions, and reactant species must have access to electrolyte
electrodes in fuel cell
porous electrodes so:
-greater surface area for reactions to occur
-may contain a catalyst
-allow reacting gases/ions to access electrolyte
difference between fuel cells and other galvanic cells
constant supply of reactants vs finite quantity of fuel
porous electrodes vs non-porous electrodes
advantages of fuel cells
-higher energy conversion efficiency bc less intermediate, energy conversions
-low chemical pollution
-quiet operation
-low running costs
-unlimited electrical energy produced w constant fuel supply
disadvantages of fuel cells
-manufacturing processes + materials are expensive
-need reliable and continual supply of fuel
-transportation and storage of fuel = difficult
-need to maintain at high temp
-expensive electrodes bc catalysts = exp
energy efficiency in fuel cells vs combustion
-coal fired power stations have many intermediate energy conversions so 30%
-fuel cells directly convert chemical -> electrical so 60% efficiency
why are fuel cells expensive in some aspects?
-porous electrodes and catalysts = exp
-fuel cell has to be run at high temps
-pure hydrogen is difficult to obtain
how is hydrogen used in a fuel cell renewable and carbon neutral?
-can be produced from electrolysis of water
-electricity used for this can be renewable eg solar/hydro powered
-no CO2 produced
how is hydrogen fuel cell non-renewable?
-hydrogen gas can be sourced from fossil fuels, releasing CO2 or CO in reactions = non renewable resources
-CO2 released from these reactions, and in transport or storage of fuel
safety and storage of hydrogen fuel
-highly flammable and ignited easily
-flame = almost invisible, colourless and odourless gas
-lightness t/f would dissipate into air
-high energy content so release high energy
-can be stored in metal hydrides, as a liquid but at very low temps, and in high pressure tanks
define primary cells
-a type of galvanic cell, non-rechargeable electrochemical cell where chemical reaction generates electrical energy
-amount of energy supplied is limited to amount of reactants available
why are primary cells non-rechargeable?
products do not remain in contact w electrodes
as a result, reaction can reach equilibrium and all reacts may be consumed
features of a dry cell:
-elongated anode to ^SA:V ratio
-electrolytic paste = balance charge, source of ions for reactants
-porous separator: prevent a spontaneous reaction from occurring b/w unwanted products, allows certain ions to pass through and prevent accumulation of charge (act as a salt bridge)
why is a paste better than an aqueous solution in a primary, dry cell?
-more concentrated so ^ ROR
-more practical
-aq solutions can leak
-water can react w products
features of an alkaline button cell:
-electrolyte can be liquid as water is produced
-greater life span than dry cells
-anode does not corrode easily bc alkaline = better as acids corrode metals easily
factors impacting primary cell selection
-initial and operating costs
-size and shape
-mass
-memory effect
-voltage provided eg. aq sols can do no more 2V
-discharge curve
-shelf life
-current-ease of disposal and env factors
define secondary cell
type of galvanic cell that is rechargeable and converts chemical energy to electrical energy
what is needed to recharge a secondary cell?
-need a higher voltage to recharge than is produced in discharge
-the pos terminal of power supply needs to be connected to pos terminal of cell and vice versa
-products in contact w electrodes
why are secondary cells rechargeable?
-in reversible form
-products of discharge reaction (SPECIFIC) stay in contact w electrodes
discharge of secondary cells
-acts as a galvanic cell
-produces electrical energy from chemical energy
-anode is negative, oxidation occurs
-cathode is positive, reduction occurs
-electrons flow from anode -> cathode
recharge of secondary cells
-acts as a electrolytic cell
-produces chemical energy from electrical energy
-anode is positive, OIL
-cathode is negative, RIG
-electrons still flow from anode to cathode -> must be pushed so e- can be consumed at cathode
define battery life
number of discharge/recharge cycles before a battery becomes unusable and essentially dies
how does temp affect battery life?
HIGH TEMP - ROR of side reactions also increases, compromising battery’s functionality. while ROR of discharge also increases, overall battery life decreases
LOW TEMP - cell reaction rate also decreases of discharge, less electricity generated. expands battery life but slow rate
factors affecting battery life
-temp
-reactants and products = detached from electrodes
-impurities in cell react w products and reactants
-build of products = polarisation, accum of solids at electrodes so less effectively recharged
-corrosion of internal components
-leakage of electrolyte sol. decrease contact b/w reactants
what is self discharge
-loss of the electrical capacity of a battery due to the deterioration of battery components
-also caused by side reactions even when batter X used
-to decrease self discharge, should be stored at low temp