Electrochemistry Flashcards
Oxidation numbers
Total number of electrons that an atom gains or loses in order to form a chemical bond with another atom
What does it mean if a compound is not neutral
It is a polyatomic ion
Group 1 fixed oxidation number
+1
Group 2 fixed oxidation number
+2
Flourine fixed oxidation number
-1
Hydrogen with non-metal fixed oxidation state
+1 (hydrogen acts as the metal)
Hydrogen with metal or boron fixed oxidation state
-1 (hydrogen acts as the non metal)
Oxygen fixed oxidation state
Exceptions (3)
-2 except in H2O2 and Na2O2 where it is -1, or bonded with fluorine
Group 7 fixed oxidation state
-1 except Cl Br and I when bonded to O or F
What elements can have varying oxidation numbers (7)
Nitrogen
Phosphorus
Oxygen (H2O2, Na2O2)
Sulphur
Transition metals
Chlorine
Hydrogen
Applications of redox reactions/electrochemistry (3)
Batteries in computers
Electric cars
Remote controls
REDOX reactions
Reactions that involve the transfer of electrons
OILRIG
What happens when electrons are lost
The element/compound is oxidised and the oxidation number increases to become less negative
What happens when electrons are gained
The element/compound is reduced, the oxidation number decreases/ becomes more negative
Reducing agent
Substance that is oxidised
Oxidising agent
Substance that is reduced
Hydrogen-oxigen fuel cells
Hydrogen and oxygen react in a hydrogen-oxygen fuel cell to produce electricity, water and small amounts of heat. The electrochemical converts chemical energy to electricity which is useful in cars, spacecraft and is being developed for portable electronic devices
What is the result of hydrogen fuel cells called
Clean fuel
3 equations within a hydrogen oxygen fuel cell
Advantages of hydrogen fuel cells (5)
Renewable energy source
Plentiful cheap supply of reactants
High efficiency process (>60% compared to 33% for petrol)
Similar energy content
Produces zero carbon emmisions
Disadvantages of hydrogen fuel cells (5)
Lack of facilities and methods for widespread use
Compressed gases are harardous
Reactants are gases so must be compressed for storage
Hydrogen sourced from methane uses a non-renewable source
Hydrogen sourced from water is expensive to produce
Electrolysis
Decomposition of an ionic compound, when molten or in an aqueous solution by the passage of an electric current
Positive electrode
Anode
Negative electrode
Cathode
Electrolyte
Molten/aqueous substance that undergoes electrolysis
Describe the transfer of charge during electrolysis
There is movement of electrons in the external circuit from positive to negative, positively charged ions in the electrolyte move to the cathode (loses electrons) where they accept electrons. Negatively charged ions move to the anode (gains electrons) where they transfer electrons
Why metals are electroplated
To improve their appearance and resistance to corrosion
Electrode materials
Platinum or carbon/graphite because they are inert
Electrolysis of lead (II) bromide
Insert inert electrodes made from platinum or carbon/graphite
Molten PbBr2 electrolyte
Bromide ions attracted to anode (2Br-) - bromine gas forms (brown gas bubbles) 2Br- -> Br2 + 2e
Lead ions attracted to cathode (Pb2+) - grey metal accumulates on the surface (molten lead) -> Pb2+ + 2e -> Pb
What forms at the cathode
What forms at the anode
Metals + hydrogen
Non metals - hydrogen
Identify the products formed at the electrodes and describe the observations made during the electrolysis of concentrated aqueous sodium chloride
Electrolyte:NaCl (aq)
Na goes to and is deposited at the cathode
Cl goes to anode (green gas bubbles)
Identify the products formed at the electrodes and describe the observations made during the electrolysis of dilute sulfuric acid
Cathode : hydrogen (Bubbles)
Anode : OH (becomes water + oxygen) more dilute and bubbles
