redox reactions and galvanic/fuel cells - U3 AOS 1

Question 1

redox reactions

Answer 1

• reactions that involve the transfer of electrons from one species to another

Question 2

oxidation

Answer 2

• the loss of electrons
• electrons will be on the products side of equation

Question 3

reduction

Answer 3

• the gain of electrons
• electrons will be on the reactants side of the equation

Question 4

oxidising agent

Answer 4

• the species that is causing another species to be oxidised
• is itself reduced

Question 5

reducing agent

Answer 5

• the species that is causing another species to be reduced
• is itself oxidised

Question 6

oxidation state

Answer 6

• an increase in oxidation state means the substance has been oxidised
• a decrease in oxidation state means the substance has been reduced

Question 7

oxidation state of a free element

Answer 7

0

Question 8

oxidation state of a simple ion

Answer 8

equal to the charge of the ion

Question 9

oxidation state of hydrogen

Answer 9

+1 unless it is a metal hydride where it is -1

Question 10

oxidation state of oxygen

Answer 10

-2 unless it is a peroxide where it is -1

Question 11

acidic conditions

Answer 11

• solutions with greater concentration o H= ions compared to OH- ions\
• hydrogen atoms need to be balanced by adding H+ ions

Question 12

basic conditions

Answer 12

• solutions with a greater concentration of OH- ions compared to H+ ions
• OH- ions need to added to either side of the equation to neutralise the H+ ions. The H+ ions and OH- ions will then undergo a neutralisation reaction to form water

Question 13

steps to balance half equations

Answer 13

• balance key elements
• balance oxygen atoms with water
• balance hydrogen atoms with H+ ions
• balance H+ ions with OH- ions (basic conditions)
• add electrons
• add states

Question 14

overall redox reaction

Answer 14

• ensure electrons of each half equation are balanced
• add the two equations together and cancel out

Question 15

galvanic cell

Answer 15

• an electrochemical cell in which energy from spontaneous redox reactions in converted into electrical energy

Question 16

galvanic cell set up

Answer 16

• two containers
• one anode and one cathode
• reduction occurs at the cathode (top left)
• oxidation occurs at the anode (bottom right)
• wires with voltage and electrons flow
• salt bridge connecting solution
• add states

Question 17

anode

Answer 17

• the negative electrode where oxidation occurs

Question 18

cathode

Answer 18

• the positive electrode where reduction occurs

Question 19

salt bridge

Answer 19

• a connection that allows for the flow of ions
• cations will flow into the electrode where electrons are gained
• anions will flow into the electrode where electrons are lost

Question 20

internal circuit and external circuit

Answer 20

• the flow of ions in salt bridge (internal)
• the flow of electrons through wires (external)

Question 21

top left of electrochemical series

Answer 21

• strongest oxidising agent
• the substance being reduced
• will always occur at the cathode

Question 22

bottom right of electrochemical series

Answer 22

• strongest reducing agent
• the substance undergoing oxidation
• will always occur at the anode

Question 23

limitations of the electrochemical series

Answer 23

• does not provide information about the rate of reaction
• electrochemical series can only be used under SLC

Question 24

galvanic cells can be constructed so that a reaction takes place between

Answer 24

• a solid metal and aqueous ion (metal serves as electrode)
• an aqueous ion and an aqueous ion
• a gas and an aqueous ion

Question 25

inert electrodes

Answer 25

• used when reactions occur between ions in a solution or in a gas
• they must be solid, not react with any other species and be electrically conductive

Question 26

determining cell voltage

Answer 26

Ecell = E 0 of reduction reaction − E 0 of oxidation reaction
- rearrange equation to find the E0 of an electrode when given the cell voltage

Question 27

fuel cell

Answer 27

• electrochemical cell that continuously converts chemical energy into electrical
  energy via a redox reaction
• reactants are continuously supplied
• open systems and usually gaseous reactants
• porous electrodes - material with many holes (pores) used in a fuel cell to maximise the
  ability of gaseous reactants to come into contact with the electrolyte

Question 28

evaluation of fuel cells

Answer 28

advantages
- high efficiency: pours electrodes and electrodes are coated in catalysts
- energy transfer: they convert chemical energy directly into electrical energy which minimises energy loss as heat. combustion engines release a signifying amount of energy as heat.
- more environmentally friendly, if they do not release carbon dioxide, especially when using renewable feedstocks
- they generally realease just waster and not greenhouse gases or toxic waste
- quiet operation
- heat produced by the fuel cell can be used to heat the system

disadvantages
- hydrogen storage and availability
- production of hydrogen generally comes from fossil fuels
- sometimes high heat is requied to operate the fuel cell
- production of renewable feel stocks can often result in the use of fossil fuels