redox reactions and galvanic/fuel cells - U3 AOS 1 Flashcards
redox reactions
- reactions that involve the transfer of electrons from one species to another
oxidation
- the loss of electrons
- electrons will be on the products side of equation
reduction
- the gain of electrons
- electrons will be on the reactants side of the equation
oxidising agent
- the species that is causing another species to be oxidised
- is itself reduced
reducing agent
- the species that is causing another species to be reduced
- is itself oxidised
oxidation state
- an increase in oxidation state means the substance has been oxidised
- a decrease in oxidation state means the substance has been reduced
oxidation state of a free element
0
oxidation state of a simple ion
equal to the charge of the ion
oxidation state of hydrogen
+1 unless it is a metal hydride where it is -1
oxidation state of oxygen
-2 unless it is a peroxide where it is -1
acidic conditions
- solutions with greater concentration o H= ions compared to OH- ions\
- hydrogen atoms need to be balanced by adding H+ ions
basic conditions
- 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
steps to balance half equations
- 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
overall redox reaction
- ensure electrons of each half equation are balanced
- add the two equations together and cancel out
galvanic cell
- an electrochemical cell in which energy from spontaneous redox reactions in converted into electrical energy
galvanic cell set up
- 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
anode
- the negative electrode where oxidation occurs
cathode
- the positive electrode where reduction occurs
salt bridge
- 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
internal circuit and external circuit
- the flow of ions in salt bridge (internal)
- the flow of electrons through wires (external)
top left of electrochemical series
- strongest oxidising agent
- the substance being reduced
- will always occur at the cathode
bottom right of electrochemical series
- strongest reducing agent
- the substance undergoing oxidation
- will always occur at the anode
limitations of the electrochemical series
- does not provide information about the rate of reaction
- electrochemical series can only be used under SLC
galvanic cells can be constructed so that a reaction takes place between
- a solid metal and aqueous ion (metal serves as electrode)
- an aqueous ion and an aqueous ion
- a gas and an aqueous ion
inert electrodes
- 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
determining cell voltage
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
fuel cell
- 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
evaluation of fuel cells
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
