galvanic/fuel cells AOS1

Question 1

are cells and batteries expensive?

Answer 1

they’re more expensive than energy from other sources like fossil fuels but this cost also allows for greater convenience

Question 2

what is a galvanic cell?

Answer 2

a galvanic cell (voltaic cell) is a type of electrochemical cell in which chemical energy is converted into electrical energy

Question 3

what type of reaction occurs in a galvanic cell?

Answer 3

spontaneous and exothermic reactions

Question 4

what’s an electrochemical cell?

Answer 4

a device in which chemical energy is converted into electrical energy

Question 5

what’s a battery?

Answer 5

strictly applies to a combination of cells which is done by connecting several cells in a series to obtain a higher potential difference or voltage

Question 6

what’s an external circuit?

Answer 6

the electric current produced in a cell that flows through the wire and light globe, in a Daniell cell the glob converts the electrical energy of the current into light + heat

Question 7

what does a flow of current mean?

Answer 7

it means that the chemical reaction (redox) is taking place

Question 8

what can occur (physically) because of this reaction in a galvanic cell?

Answer 8

in the oxidation half-equation, electrons are lost which prompts corrosion whilst in the reduction half-equation more deposits are added onto it as electrons are gained (sometimes colour loss occurs)

Question 9

what’s a galvanometer?

Answer 9

an instrument used for detecting electric current (also indicates direction)

Question 10

what’s a salt bridge?

Answer 10

a salt bridge is used to ensure that current flows between two halves of the cell and is part of the internal circuit of a galvanic cell so it allows a cell to reduce electricity by allowing the movement of ions between the 2 half-cells, preserves electrical connection

Question 11

what’s a salt bridge made of?

Answer 11

often made from filter paper soaked in a relatively unreactive electrolyte like potassium nitrate (KNO3)

Question 12

what’s an electrolyte?

Answer 12

a chemical substance that conducts electric current

Question 13

what’s an oxidising agent?

Answer 13

a chemical that causes another substance to undergo oxidation but it itself is reduced

Question 14

what’s a reducing agent?

Answer 14

a chemical that causes another substance to be reduced but it undergoes oxidation

Question 15

how to know if a galvanic cell is spontaneous?

Answer 15

the redox reaction can be described as a spontaneous reaction if it doesn’t need to be driven by an external source of energy (like the reaction occurring naturally)

Question 16

what happens if reactants are in direct contact with one another?

Answer 16

their chemical energy is transformed directly to thermal energy as opposed to galvanic cells where the half-reactions occur in seperate containers + electrons are transferred by the external circuit so that chemical energy is transformed into electrical energy

Question 17

how does a galvanic cell work?

Answer 17

half-reactions occur in 2 seperate compartments so since the reductants + oxidants don’t come into direct contact with each other, electrons can only be transferred though an external circuit connecting negative + positive electrodes, this flow of electrons then creates an electric current

Question 18

what does a half-cell include?

Answer 18

each half-cell contains an electrode in contact with a solution. the species (chemicals) present in each half-cell form a conjugate redox pair

Question 19

what are the conditions of electrodes?

Answer 19

if 1 member of the conjugate pair in a half-cell is a metal then it’s usually used as the electrode but some redo pairs don’t involve solid metals so if no metal is present an inert (unreactive) elected like platinum/graphite is used but platinum electrodes are very expensive

Question 20

do half-cells contain other species not involved in the reaction?

Answer 20

yes they do, these include spectator ions and the solvent, if a element is in aqueous form then water is often also present

Question 21

what happens if 1 of the conjugate pairs in a half-cell is a gas?

Answer 21

then a special ‘gas electrode is used, like a glass of the gas name to prevent it from being lost to the environment

Question 22

what’s a cathode?

Answer 22

electrode where reduction occurs, electrons are gained, is the positive terminal in a galvanic cell and is where cations in the salt bridge go

Question 23

what’s an anode?

Answer 23

electrode where oxidation occurs, electrons are lost/ released, negative terminal in a galvanic cell and is where anions in the salt bridge go

Question 24

what happens in a salt bridge?

Answer 24

it contains ions that move freely between the half-cells so that they can balance changers formed in the 2 compartments

Question 25

what would happen in a galvanic cell if there wasn't a salt bridge?

Answer 25

the solution in 1 compartment would accumulate negative charge whilst the solution in the other would accumulate positive charge as the reaction proceeded, this accumulation of charge would stop the reaction very quickly and would prevent further reaction

Question 26

how to write a balanced half and overall equation?

Answer 26

make the numbers of atoms of each element equal on both sides and the total charge on each side as well. the no. of electrons lost in the oxidation reaction must equal the number of electrons gained in the reduction reaction

Question 27

what is the flow of electrons in a galvanic cell?

Answer 27

electrons flow through the external circuit from the anode (negative) to the cathode (pos) and anions flow in the internal circuit to the anode and cation flow towards the cathode

Question 28

what's a primary cell?

Answer 28

galvanic cells that can't be recharged, designed to be disposable hence they 'go flat' when the cell reaction reaches equilibrium which is a point when there's no tendency for the quantities of reactants and products to change, the products also slowly migrate away from the electrodes/ are consumed by side reactions occurring in the cell, preventing the cells from being recharged

Question 29

example of a primary cell?

Answer 29

common commercial alkaline cells like the ones used for a torch, it is similar to a galvanic cell but the 2 half-reactions occur in seperate places in 1 container, it is cost-efficient for things that need high currents irregularly

Question 30

what's a secondary cell?

Answer 30

galvanic cells that are rechargeable that are designed to be re-used

Question 31

how to determine what species goes to what terminal?

Answer 31

in a galvanic cell, the stronger reducing agent is in the half-cell with the anode whilst the stronger oxidising agent is in the half-cell with the cathode

Question 32

why does current flow in a galvanic cell?

Answer 32

it flows because 1 half-cell has a greater tendency to push electrons into the external circuit than the other half-cell so a potential difference exists between the 2 half-cells

Question 33

what's a potential difference?

Answer 33

also called the electromotive force (emf) but is mainly the VOLTAGE which uses volts for units and is measured by a voltmeter + usually measured under SLC

Question 34

where does the standard electrode potential (E) come from?

Answer 34

comes from connecting a cell to the standard hydrogen half-cell (SHE)

Question 35

what's the standard reduction potential?

Answer 35

a given numerical measure of the tendency of a half-cell reaction to occur as a reduction reaction

Question 36

where are the strongest oxidants and reductants located in the electrochemical series?

Answer 36

strongest oxidants= top left strongest reductants=bottom right

Question 37

how to calculate the volage of a cell?

Answer 37

the E of a higher half-cell minus the E of a lower half-cell, different values for the cell voltage are obtained under non-SLC

Question 38

what occurs as a galvanic cell discharges?

Answer 38

the cell voltage eventually drops to 0 zinc the cell is referred to as 'flat' so equilibrium has been reached

Question 39

what doesn't the electrochemical series do?

Answer 39

give an indication of the rate of reactions

Question 40

what are direct redox reactions?

Answer 40

if the contents of the half-cells of a galvanic cells ere mixed, the reactants would react directly so energy would be released as thermal energy instead of electrical but are still spontaneous

Question 41

what's the standard electrode potential?

Answer 41

the voltage measured when a half-cell, at SLC is connected to a hydrogen half-cell

Question 42

what are fuel cells?

Answer 42

a type of galvanic cell that converts the chemical energy of reactants directly into electrical energy (so it's relatively efficient) generates electricity from redox reactions, unlike galvanic cells reactants are supplied continuously allowing for constant product of energy

Question 43

what's a hydrogen economy?

Answer 43

fuel cells play a large role in this, and it's where hydrogen could be a major source of energy which could replace fossil fuels

Question 44

what's a limitation of galvanic cells?

Answer 44

is that they contain relatively small amounts of reactants and once they have been consumed the cell has to be discarded/ recharged

Question 45

what's a key difference between a fuel and primary cell?

Answer 45

that the reactants aren't stored in the fuel cell so they must be continuously supplied from an external source

Question 46

what's the design of fuel cells?

Answer 46

a fuel cell has 2 compartments, gas compartments are separated from each other by porous electrons and an electrolyte solution, the electrolyte in a fuel cell carries ions from 1 electrode to the other

Question 47

how are fuel cells classified?

Answer 47

they are mainly classified by the kind of electrolyte (carries ions from one electrode to the other) they use, there are 2 key types: acidic and alkaline fuel cells

Question 48

what does a fuel cell using hydrogen produce?

Answer 48

electricity, water, heat and very few other emissions

Question 49

what are acidic fuel cells?

Answer 49

most common type is a phosphoric acid fuel cell, H+'s are removed from the overall eq like for alkaline fuel cells which remove the OH-'s from the overall equation

Question 50

what happens to a fuel in a fuel cell?

Answer 50

it's oxidised at the anode and oxygen is reduced at the cathode, different electrolyte can be used in cells

Question 51

what are the uses of fuel cells?

Answer 51

they're a useful source of continuous electricity and could be used to provide energy for vehicles, buildings and even cities, they don't involve as many energy transformations as with galvanic cells so it's more efficient as less energy is lost which also means a reduction in greenhouse gas emissions

Question 52

how efficient are fuel cells?

Answer 52

around 40-60% efficient, and some use they waste heat to produce steam which increases the efficiency to up to 85% which can be used to heat/operate a turbine

Question 53

what us the purpose of porous electrodes?

Answer 53

allows reactants t diffuse through them to react with ions in the electrolyte and they often contain catalysts to increase the rate of reaction, common electrolyte is KOH

Question 54

what is the use of fuel cells?

Answer 54

they have better fuel efficiency and lower emissions of greenhouse gases and other pollutants than traditional combusting fuels, some scientists believe that fuel cells will play a key role in the transition from a dependence on fossil fuels to a sustainable supply of energy but challenges around cost/performance remain as improvements are stil required to make hydrogen fuels truly competitive

Question 55

what are the advantages of a fuel cell?

Answer 55

hydrogen fuel cells produce water + heat as by-products when hydrogen is a fuel so no greenhouse gases are release, can use a variety of fuels, can generate electricity for as long as the fuel is supplied, electricity can be generated on-site and users aren't reliants on connection to an electricity grid

Question 56

what are the disadvantages of a fuel cell?

Answer 56

they require constant fuel supply, expensive -> electrodes/ catalysts/itself, in transport requires an extensive network of hydrogen filling stations, hydrogen is mainly sourced from fossil fuels, issues with storage/safety of hydrogen fuel and some types of cells use toxic electrolytes and their electrodes contain expensive/harmful material

Question 57

make up of a fuel cell?

Answer 57

the electrodes in fuel cells allow direct contact between the gases and the electrolyte, they have a high SA and are porous to ensure high cell efficiency, catalysts are also often part of an electrode to increase the rate of a fraction

Question 58

what are biofuels?

Answer 58

they're fuel from plants or organic waste that can be used as an alternative energy source to fossil fuels because it produces a lot less greenhouse gas emissions but this process of using it, would take a long time + the fuel produced is typically of low quality so it doesn't produce much energy when burned

Question 59

how is hydrogen produced for energy?

Answer 59

through a fuel cell which uses hydrogen, it can be described as a 'zero-emission' device because water is almost the only product apart from electricity and heat but it must be produced using renewable energy, currently 95% of hydrogen is produced from fossil fuels like natural gas through the process of steam reforming where steam reacts with the fossil fuel at high temp which produces hydrogen with a lower energy content than the original fuel as some of the original chemical energy of these exothermic reactions is last as waste heat during production + the process produces greenhouse gases

Question 60

how to make using hydrogen more sustainable?

Answer 60

capturing the greenhouse gases at its source and storing it which prevents its release into the atmosphere

Question 61

what are the 2 current practical methods of sustainable generating hydrogen?

Answer 61

using electrical energy to convert water to hydrogen and collecting biogas from landfill sites and converting the methane in the gas to hydrogen by steam reforming

Question 62

issues of hydrogen fuels?

Answer 62

very expensive, changes to production methods are needed, storage due to hydrogen becoming a gas at room temp and its needed in liquid form (materials-based storage is suggested) and hydrogen is extremely flammable and potentially explosive so some studies state that it's more dangerous than petrol

Question 63

use of fuel cells for sustainability?

Answer 63

they're being developed in response to society's energy needs so innovations in the designs of these cells can be viewed in terms of the principales of green chemistry (using renewable feedstocks + design for energy efficiency), example of one is microbial fuel cells