R3.2 - electron transfer reactions Flashcards
What is oxidation?
- Gain of oxygen
- Loss of hydrogen
- Loss of electrons
- Increase in oxidation state
What is reduction?
- Gain of hydrogen
- Loss of oxygen
- Gain of electrons
- Decrease in oxidation state
What are limitations of the oxidation states model?
It can be hard to assign integer oxidation states to compounds with more than 1 atom of an element such as C3H8.
What is the oxidation state of carbon in C3H8?
-2.67
What is an oxidizing agent?
The reactant that accepts electrons and undergoes reduction.
What are some examples of oxidizing agents?
- O2
- O3
- MnO4-
- Cr2O7 2-
- OH-
- H2O2
- HNO3
- H+
What is a reducing agent?
The reactant that donates electrons and undergoes oxidation.
What are some examples of reducing agents?
- H2
- C
- CO
- SO2
- Reactive metals
What can act as both reducing and oxidizing agents?
H2O and H2O2
What is the role of half-equations?
Shows the electrons transfer process in redox reactions.
What are the steps involved in writing a redox reaction?
1) Write the unbalanced equation and assign oxidation states to each element.
2) Write the half equations.
3) Balance O by adding H2O to the opposite side of the equation.
4) Balance H by adding H+ ions to the opposite side of the equation.
5) Balance the number of electrons and check that the charge number is balanced.
6) Add the 2 half equations together and cancel out if anything is the same.
What is a redox titration used for?
Used to determine the concentrations of reactants and products in redox reactions.
What is the equation for the iodine-thiosulfate titration?
2I- (aq) + oxidising agent (clear) –> I2 (s) + other products (blue)
2S2O3 2- (aq) + I2 (aq) –> 2I- (aq) + S4O6 2- (aq)
What are the half-equations for the iodine-thiosulfate titration?
Oxidation: 2S2O3 2- –> S4O6 20 + 2e-
Reduction: I2 + 2e- –> 2I-
How does the iodine-thiosulfate titration work?
- Iodine ions are oxidised to iodide and titrated against sodium thiosulfate, which reduces iodide back to iodine, using starch as an indicator.
- The starch forms a deep blue colour with iodine molecules in the complex.
- As the I2 is reduced to I-, the blue colour disappears.
- Starch is added at intervals during the titration.
What is the equation for the titration with iron and manganese?
5Fe 2+ (aq) + MnO4- (aq) + 8H+ (aq) –> 5Fe 3+ (aq) + Mn 2+ (aq) + 4H2O (l)
What are the half-equations for the titration with iron and manganese?
Oxidation: Fe 3+ –> Fe 2+ + e-
Reduction: MnO4= + 8H+ + 5e- –> Mn2+ + 4H2O
What happens during titration of iron and manganese?
KMnO4 is used in acidic conditions to oxidise Fe2+ to Fe3+ and reduce MnO4- to Mn2+.
- Colour changes from deep purple to colourless.
- Reaction mixture acts as its own indicator.
What is the order of the reactivity series?
K, Na, Ca, Mg, Al, C, Zn, Fe, Sn, Pb, Cu, Ag, Au
Which non-metals are stronger oxidising agents?
More reactive non-metals
Which metals are stronger reducing agents?
More reactive metals
What happens when an acid reacts with a metal?
Produces salt and hydrogen
Why is H important in the reactivity series?
Anything less reactive than H will not react with acids.
What are electrochemical cells?
Umbrella term for voltaic and electrolytic cells.
What are voltaic cells?
Cells that generate electricity from chemical reactions.
What occurs at the anode and cathode of electrochemical cells?
Anode - oxidation
Cathode - reduction
What are electrolyctic cells?
Cells that use electricity to drive chemical reactions.
How is the type of reaction different for voltaic and electrolyctic cells?
Voltaic - spontaneous
Electrolytic - non spontaneous
Where does oxidation occur for voltaic and electrolyctic cells?
Anode
Where does reduction occur for voltaic and electrolyctic cells?
Cathode
What is the polarity of the anode and cathode at the voltaic cell?
Anode = -
Cathode = +
What is the polarity of the anode and cathode at the electrolyctic cell?
Anode = +
Cathode = -
What is a primary voltaic cell?
Electrochemical cell that converts energy from spontaneous redox reactions to electrical energy,.
E.g) Lithium batteries in remotes
- It is single use as chemicals are not renewed during this process.
What happens when 2 half-cells are connected by an external wire and a salt bridge?
Electrons will have a tendency to flow spontaneously through the external circuit from the more negative zinc half cell to the less negative copper half cell.
- This can be quantified by measuring potential difference between 2 electrodes with a voltmeter.
What must be complete in order for a reaction to occur?
The circuit must be complete in order for a reaction to occur.
What are the half-equations for the reaction zinc and copper?
Cu2+ (aq) + 2e- <–> Cu (s)
Zn (s) <–> Zn2+ (aq) + 2e-
What occurs at the anode and cathode for a reaction involving zinc and copper half cells?
Anode - oxidation of zinc occurs.
Cathode - reduction of copper occurs.
What is the relationship between reactivity and position of equilibrium?
The more reactive the metal, the more the equilibrium favours the formation of ions, and the greater the negative charge on the electrode as there is a greater build-up of electrons.
What connections do the half-cells need in a voltaic cell?
- An external electronic circuit, connected to the metal electrode in each half-cell.
- A voltmeter to record the voltage produced.
- A salt bridge to complete the circuit.
What is the direction of flow for electrons?
From the anode (-) to the cathode (+), through the wire.
What is a salt bridge?
A glass tube or a strip of absorptive paper that contains an aqueous solution of ions.
What does the salt bridge do?
Movement of aqueous ions in the salt bridge neutralizes any build up of charge and maintains the potential difference.
What happens to anions in the salt bridge?
Anions move in the salt bridge from cathode to anode, opposite to the direction of flow of electrons in the circuit.
What happens to cations in the salt bridge?
Cations move in the opposite direction of the anions in the salt bridge, from anode to cathode.
Why is aqueous NaNO3 or KNO3 chosen as solutions?
These do not interfere with the reactions at the electrodes.
What are the features of a cell diagram?
The cell diagram shows the arrangements of electrodes in a voltaic cell and show the direction of the 2 half-equations.
What are the polarities of the anode and cathode in a voltaic cell?
Anode = -
Cathode = +
What happens if there is no salt bridge?
The circuit is incomplete and no voltage is generated.
What are the 5 rules for drawing a cell diagram?
1) Vertical lines represent a state boundary.
2) Double vertical lines represent the salt bridge.
3) Aqueous solutions are placed next to the salt bridge.
4) Anode is on the left and cathode is on the right.
5) Electrons flow from the anode to cathode, left to right.
What do the negative ions in the salt bridge do?
Negative ions neutralize the metal ions at the anode to force equilibrium to the right, so that more electrons are made to generate more electricity and keep the reaction going.
What do the positive ions in the salt bridge do?
The positive ions neutralize the negative ions in the salt solution to free the metal ion so that it can bind to electrons.
How is voltage determined in a voltaic cell?
Difference in the reducing strength of 2 metals.
Larger difference in electrode potentials = greater voltage produced.
How can a primary voltaic cell lose it’s battery?
A primary voltaic cell can lose its battery if the electrodes are used up, so no longer submerged in the solution, or the solution is used up, so the electrodes cannot reach the solution.
What is a secondary cell?
A cell that can be reversed or recharged when an external current is applied.
When can the system be reversed?
If one of the products is insoluble as the product cannot disperse away from the electrode.
Why are lithium-ion batteries useful?
- Low density
- High reactivity
- Stores lots of electrical energy per unit of mass.
What electrolyte is used in a lithium-ion battery?
Non-aqueous polymer-based electrolyte.
What problems are there with the lithium-ion battery?
Due to its high reactivity, it forms lithium oxide when in contact with oxygen, preventing it from coming into contact with the electrolyte.
How can the problems in a lithium-ion battery be resolved?
- Lithium anode is mixed with graphite.
- Lithium cathode is placed in a lattice with a metal oxide (MnO2).
What are the half-equations for discharging in a lithium-ion battery?
Anode (-) = oxidation = Li –> Li+ + e-
Cathode (+) = reduction = Li + MnO2 + e- –> LiMnO2
What are the half-equations for recharging in a lithium-ion battery?
Anode (-) = reduction = Li + e- –> Li
Cathode (+) = oxidation = LiMnO2 –> MnO2 + Li+ + e-
What happens at the anode and cathode in discharging?
Anode (-) = oxidation
Cathode (+) = reduction
What happens at the anode and cathode in recharging?
Anode (-) = reduction
Cathode (+) = oxidation
Why are hydrogen fuel cells useful?
- Reduces dependence on fossil fuels
- Produces a lot of heat energy when combined with oxygen.
What are the equations for a hydrogen fuel cell in an acidified environment?
Overall: H2 + 0.5O2 –> H2O
Anode (-) = oxidation = 2H2 –> 4H+ + 4e-
Cathode (+) = reduction = 4H+ + 4e- + O2 –> 2H2O
What are the equations for a hydrogen fuel cell in an alkaline environment?
Overall: H2 + 0.5O2 –> H2O
Anode (-) = oxidation = H2 + 2OH- –> 2H2O + 2e-
Cathode (+) = reduction = 2H2O + O2 + 4e- –> 4OH-
What are the pros and cons of general primary cells?
Pros:
- Not expensive
- Lightweight
- Convenient
- Good shelf life
Cons:
- Can only be used once
- Large amounts of waste as battery needs to be recycled
- Batteries put into landfill sites have sever environmental impacts
- Only delivers small currents
What are the pros and cons of primary fuel cells?
Pros:
- More efficient than combustion as more chemical energy is converted to useful electrical energy.
- No pollution if hydrogen is the fuel
- Low density of hydrogen
Cons:
- Hydrogen is an explosive gas
- High transportation and storage costs
- Expensive
- Can only deliver small currents
- Low reaction rates and low power output
- Technical problems due to catalytic failures, leaks and corrosion
What are the pros of general secondary cells?
Pros:
- Materials can be regenerated
- Can deliver a high current
What are the pros and cons of a lead-acid secondary cell?
Pros:
- Large amounts of energy in a short time
Cons:
- Heavy mass
- Pollution
What are the pros and cons of a cadmium-nickel secondary cell?
Pros:
- Longer life than lead-acid batteries
Cons:
- Cadmium is toxic
- Produces a low voltage
- Very expensive
What are the pros and cons of a lithium ion secondary cell?
Pros:
- Low density of lithium
- High voltage
- No toxic heavy metal
Cons:
- Expensive
- Limited life span
What are electrolytic cells?
An electrochemical cell that converts electrical energy to chemical energy by bringing about a non-spontaneous reaction, using an external power source.
- It is opposite to a voltaic cell.
What are the components of an electrolytic cell?
- Power source
- Electric wires to connect electrodes to the power supply.
- Electrodes, which are placed in the electrolyte and are connected to the power supply.
What are the properties of electrodes?
- Made out of conducting materials, like metals or graphite.
- They are inert and do not take part in the redox reaction.
- They are electrically conducting.
What is an electrolyte?
A substance which does not conduct electricity in the solid state, but does conduct electricity when molten as the ions are free to move.
- They are ionically conducting.
What is the polarity of the anode and cathode of an electrolytic cell?
Anode = +
Cathode = -
What occurs at the anode and cathode of an electrolytic cell?
Anode = oxidation - negative non-metals are attracted here.
Cathode = reduction - positive metals are attracted here.
What are the half equations for the anode and cathode of an electrolytic cell?
Anode = A- –> A + e-
Cathode = M+ + e- –> M
How is electrolysis carried out in an industrial scale?
The salt that will be electrolysed is dissolved with another molten salt (cryolite) to lower the melting point of the mixture.
What is an example of a common oxidising agent?
Acidified potassium dichromate (K2Cr2O7)
What colour is K2Cr2O7?
Bright orange, which turns dark green if reduced to Cr3+ when alcohol is oxidised.
What is a reflux reaction?
A reaction with constant cycling of boiling, condensing and evaporating.
What happens during the oxidation of primary alcohols?
Primary alcohol –> Aldehyde –> Carboxylic acid
- Aldehyde = loses H
- Carboxylic acid = gains O
What happens when ethanol undergoes oxidation?
- Ethanol loses H to make ethanal.
- Ethanal gains O to make ethanoic acid.
How can ethanal be obtained?
Distillation
What happens during the oxidation of a secondary alcohol?
Secondary alcohol –> Ketone
What happens during the oxidation of a tertiary alcohol?
Tertiary alcohol = no reaction
What are the colour changes for primary, secondary and tertiary alcohols?
Primary = orange –> green
Secondary = orange –> green
Tertiary = orange –> orange
What is the reagent and conditions for reduction reactions?
- LiAlH4 (contains H- ions)
- Heated in dry ether
What happens in the reduction of a ketone and carboxylic acid?
Ketone –> Secondary alcohol
Carboxylic acid –> Aldehyde –> Primary alcohol
What does reduction of unsaturated compounds do?
Lowers the degree of unsaturation
What are examples of unsaturated hydrocarbons?
- Alkenes
- Alkynes
What does saturated mean?
An atom where the molecules are linked by single bonds.
What does unsaturated mean?
An atom where the molecules are linked by double or triple bonds.
What reactions can unsaturated compounds undergo?
- Hydrogenation
- Reduction
- Addition
What happens if propene is reduced?
Propene –> Propane
What is hydrogenation?
Adding of H
What is an example of hydrogenation?
Liquid oil –> Solid fat
Oil –> Margerine
What is the relationship between saturation and melting point?
More saturation = straighter chains = higher melting points = solids
