Unit 10 Flashcards
Oxidizing agent
- substance that oxidises another atom or ion by causing it to lose electrons
- gets reduced – gains electrons
- the oxidation number of the oxidising agent decreases
Reducing agent
- substance that reduces another atom or ion by causing it to gain electrons
- gets oxidised – loses/donates electrons
- oxidation number of the reducing agent increases
Redox titrations
- an oxidising agent is titrated against a reducing agent. Electrons are transferred from one species to the other
- Indicators used to show the endpoint of the titration but most transition metal ions naturally change colour when changing the oxidation state. Common redox titrations:
- Manganate Titrations
- Iodine - Thiosulfate Titrations
Relative ease of oxidation and reduction
Metal more or less likely to be oxidised
- Down Group 1/2 metals the reaction becomes even more vigorous and violent, thus relative ease of oxidation increases
Reduction of halogens
- Halogens oxidise the metal by removing an electron from the metal
- Halogens become reduced as they gain an extra electron from the metal atom
- Oxidising power of the halogens decreases going down the group
Metal and Metal Ion reactions
- The more reactive metal acts as a reducing agent
- This allows metals to be ranked from most reactive strongest reducing agents to least reactive
Voltaic Cells
Spontaneously convert chemical energy into electrical energy.
- Anode (-) is where oxidation occurs
- Cathode (+) is where reduction occurs
- Electrons travel from anode to cathode
phase boundary
Represented by a solid vertical line, it is an interface between a solid and a solution
Salt bridge
concentrated solution of a strong electrolyte. The high concentration allows ions to diffuse out of it. The ions in a salt bridge must be inert
- Provides physical separation of reduction and oxidation processes
- Provides electrical continuity for anions and cations
- Reduces voltage generated when two different solutions come into contact with each other
Fuel Cells
Electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
- As B fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
Benefits of fuel cells
- Only product is water so environmental advantages over other types of cells
- bond energy is converted into electrical energy instead of heat and light as reaction takes place at room temperature without combustion
- no harmful oxides of nitrogen produced
- product can be used as drinking water
Disadvantages of fuel cells
- Hydrogen is a highly flammable gas and the production and storage of hydrogen carries safety hazards
- Thick walled cylinders and pipes are needed to store hydrogen which is expensive
- Relies on a non-renewable, finite resource
- Widespread use of hydrogen in fuel cells will be limited
Electrolytic Cells
Unspontaneously convert electrical energy into chemical energy.
Requirements
- electric current reverses the normal directions of chemical change and this is non-spontaneous
- Anode (+) is where oxidation occurs
- Cathode (-) is where reduction occurs
- Electrons travel from anode to cathode
Secondary Cells
Employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction
Lead Acid Batteries
Consist of six cells joined together in series
- use lead metal as the negative electrode and lead(IV) oxide as the positive electrode with electrolyte as sulfuric acid
- designed to produce a high current for a short period of time, hence their use in powering a starter motor in car engines
Electrolyte
liquid/solution which has ions to conduct electricity
Lead Acid Battery Half Equations
Pb (s) + SO42- (aq) → PbSO4 (s) + 2e-
PbO2 (s) + 4H+ (aq) + SO42- (aq) + 2e- → PbSO4 (s) + 2H2O (l)
Overall reaction:
PbO2 (s) + 4H+ (aq) + 2SO42- (aq) + Pb (s) → 2PbSO4 (s) + 2H2O (l)
Nickel Cadmium reactions
- negative electrode consists of cadmium and the positive electrode is made of a nickel(II) hydroxide-oxide system
Nickel Cadmium Half Equations
Cd (s) + 2OH- (aq) → Cd(OH)2 (s) + 2e-
NiO(OH) (s) + H2O (l) + e- → Ni(OH)2 (s) + OH-
Overall reaction:
2NiO(OH) (s) + 2H2O (l) + Cd (s) → 2Ni(OH)2 (s) + Cd(OH)2 (s)
Lithium Ion Cell
Very low density and relatively high electrode potential. Consists of
- A positive lithium cobalt oxide electrode
- A negative carbon electrode
- A porous polymer membrane electrolyte
The cell consists of different layers of lithium cobalt oxide and carbon
Lithium Ion cell Half equations
Li (s) → Li+ (s) + e–
Li+ (s) + CoO2 (s) + e– → Li+ (CoO2) – (s)
Overall Equation:
Li (s) + CoO2 (s) → Li+ (CoO2) – (s)
Adv / Dis of Lead Acid Batteries
Adv:
- Can deliver large amounts of energy over short periods
Dis:
- Heavy mass
- Lead and sulfuric acid could cause pollution
Cadmium Nickel Adv / Dis
Adv:
- Longer life than lead-acid batteries
Dis:
- Cadmium is very toxic
- Produces a low voltage
- Expensive
Lithium Ion Adv / Dis
Adv:
- Low density of lithium
- No toxic heavy metals
-High voltage
Dis:
- Limited life span
- Expensive
