Unit 3.5.3 - Redox Equilibria Flashcards
What are the standard conditions in an electrochemical cell?
298K, solutions of concentration 1.00moldm-3, and a pressure of 100kPa
In a lead acid cell what do both electrodes end up being covered in?
Lead (II) sulphate
Do oxidation states increases or decreases as electrons are lost?
Increase
What does a reducing agent do and what happens to itself?
It reduces something to be reduced and is oxidised itself
If the position of equilibirum moves to the right, is the forward or backward reaction faster? And do the amount of reactants or produts increase?
Forward - products
What do half equations show?
Reduction or oxidation
What are the other oxidation states of oxygen and when does it have them?
-1 when bonded to peroxides +1 or +2 when bonded to fluorides
What happens to the electrode potential of the half cell if the position of equilibrium is shifted to the left in a oxidation reaction?
More products are formed, therefore there will be less electrons and so the electrode potential increases
What happens to the position of equilibrium is the concentration of the reactants is increased?
The system seeks to oppose the increase in concentraion of the reactants by getting rid of the extra reactants, favouring the forwward reaction, shifting the position of equilibrium to the right and so producing more products
What is the half equations for the reaction at the anode of a hydrogen-oxygen fuel cell?
H2(g) → 2H⁺(aq) + 2e⁻
What happens to the electrode potential of the half cell if the position of equilibrium is shifted to the left in a reduction reaction?
More products are formed, therefore there will be more electrons and so the electrode potential decreases
What is the oxidation states of an element (uncombined)?
0
What does oxidation state show you?
The total number of electrons it has donated or accepted
Whats the half equation for the reaction that occurs at the cathode in a lead - acid cell?
PbO₂(s) + SO₄²⁻ (aq)+ 4H⁺(aq) + 2e⁻ ⇋ PbSO₄(s) + 2H₂O(l)
If a reaction has more moles on the right hand side what happens to the position of equilibrium is the pressure is decreased?
The system seeks to oppose the decrease in pressure by favouring the side with the most moles, favouring the forwards reaction, shifting the position of equilbirum to the right and producing more products
Whats the half equation for the reaction occurng at the cathode of a dry cell alkaline battery?
2MnO₂(s) + 2NH₄⁺(aq) + 2e⁻ → Mn₂O₃(s) + 2NH₃(aq) + H₂O(l)
In an electrochemical cell diagram which direction to the electrons flow?
From the left to the right
Draw what a hydrogen - oxygen fuel cell looks like.
In a lead acid cell what forms the anode, cathode and electrolyte?
Anode - lead
Cathode - lead(IV) dioxide
Electrolyte - Sulphuric acid
Does the cell with the more negative electrode potential go on the right or left hand side or the electrochemical cell diagram?
Left
What is the oxidation state of hydrogen in a compound with metals?
-1
What is the purpose of the salt bridge between the two solutions in an electrochemical cell?
To allow ions to flow through it and balance out the charges which completes the circuit
What does the cell diagram look like for a hydrogen - oxygen fuel cell?
Pt(s) | H2(g) | H+(aq) || O2(g) |H+(aq) | H2O(l) | Pt(s)
If a reaction is exothermic what happens to the position of equilibrium if the temperature is decreased?
The system seeks to oppose the decrease in temperature by favouring the exothermic direction which is the forward reaction, shifting the position of equilibirum to the right and producing more products
What is a reducing agent in terms of electrons?
Electron donor
Why are standard conditions used in electrochemical cells?
So you get the same result each time and you can compare values for different cells
What are three cons of rechargebale batteries?
- ) More expensive to buy
- ) Last a shortert time than a rechargebale battery
- ) Contain more toxic materails compared to non rechargeables - lead and cadmium
If a reaction has more moles on the right hand side what happens to the position of equilibrium is the pressure is increased?
The system seeks to oppose the increase in pressure by favouring the side with the fewer moles, favouring the backwards reaction, shifting the position of equilbirum to the left and producing more reactants
Do more reactive metals have larger or smaller electrode potentials? And why?
Smaller because they want to lose electrons more easily and are more easily oxidised
In an electrochemical cell do electrons flow from the most to the least reactive metals or from the least to the most reactive metals? And why?
From the most to the least - more reactive metals give up electrons more easily
Which sides to oxidation and reduction occur on in a electrochemical cell diagram?
Left - oxidation
Right - reduction
Even though you can get the reactions to run in reverse in a dry cell alkaline battery why is it not practical (two reasons) ?
- ) The zinc anode forms the casing of the battery so becomes thinner as the zinc oxidises, this could make the battery leak or explode
- ) The ammonia ions would product hydrogen gas which would escape from the battery
What does E⦵cell show?
The voltage between the two half cells
Describe what happens in a zinc/copper cell in 3 steps.
- ) Zinc atoms are oxidised to form Zn2+ ions and release 2 eletroms
- ) The electrons flow from the zinc anode through the external circuit to the copper cathode
- ) Cu2+ ions accept two electrons and are reduced to form copper atoms
What does oxidation mean in terms of electrons?
Loss of electrons
What is the electrode potential of the standard hydrogen electrode?
0.00V by defintion
Is the copper electrode the cathode or anode in the zinc/copper cell?
Cathode
What are four pros of rechargebale batteries?
- ) Cheaper in the long run as you don’t have to replace the battery
- ) You can recharge and re use them
- ) They supply more power
- ) Less batteries end up in landfill, less waste
Where is an electrochemical cell a redox process?
Because oxidation occurs at one electrode and reduction happened at another
What are three cons of hydrogen - oxygen fuel cells?
- ) Energy is required to produce hydrogen and oxygen from the electrolysis of water
- ) Not carbon neutral - energy for electolysis of water comes from fossil fuels
- ) hydorgen is highly flammable so has to be stored and transported carefully
What does the electrochemical cell diagram for a dry cell alkaline battery look like?
Zn(s)| Zn²⁺(aq) || MnO₂(s), Mn₂O₃(s)
If you cant use an electrode of the same metal as in the solution what can you use? And two reasons why?
Platinium electrode - because it is inert and conducts electricity so allow the transfer of electrons
What does the diargam for zinc/copper cell look like?
Zn(s) | Zn2+(aq) || Cu2+(aq)| Cu(s)
Name a common fuel cell?
A hydrogen - oxygen fuel cell
What does a oxidising agent do and what happens to itself?
It causes something to be oxidised and is reduced itself
If the position of equilibirum moves to the left, is the forward or backward reaction faster? And do the amount of reactants or produts increase?
Backward - reactants
Describe what is happening in a hydrogen-oxygen fuel cell in three steps?
- ) hydrogen is fed to the anode, where it is oxidised to H+ ions and electrons
- ) electrons flow through the external circuit to the cathode, the H+ ions flow through the ion-exhange memtbane to the cathode
- ) oxygen is fed to the cathode, where it is accepts electrons an H+, and is reduced to water
Draw what the standard hydrogen electrode look like.
What is the oxidation state of group 1 metals in a compound?
+1
What are three other ways of saying E⦵cell?
The cell potential, e.m.f or the electromotive force
What is the oxidation state of oxygen in a compound (most compound)?
-2
What do the lines represent in electrochemical cell diagrams like this: Zn(s) | Zn2+(aq) || Cu2+(aq)| Cu(s)?
|| - salt bridge
- phase boundary
What are four steps within creating harder half equations?
1.) Balance the atom that is either by reduced or oxidised 2.) Balance the O’s by adding water to either side 3.) Balance the H’s by adding H+ to the other side 4.) Then balance out the charge on either side by adding electrons
What is the main way of making an electrochemical cell?
Two different metals dipped in salt solution of their own ions connected by an external circuit, with a salt bridge joining the two salt solutions
Name a three common type of rechargeable battery?
Lead - acid cell, NiCad(nickel-cadmium) cell and the L ion(lithium ion) cell
What does reduction mean in terms of electrons?
Gain of electrons
What happens in a half cell which has a platinium electrode and a solution containing Fe2+ and Fe3+ if the other half cell is made out of a more reactive metal?
Fe3+ ions accept electrons and are reduced to Fe2+ ions
What metal is used for the electrode in the standard hydrogen electrode?
Platinium
What is the electrochemical series?
A list of all the standard electrode potentials
Do more non reactive metals have larger or smaller electrode potentials? And why?
Larger because they more easily gain electrons and so are more easily reduced
Name a common electrochemical cell?
Zinc/Copper cell
What type of battery used reversible reactions and what uses irrevserible?
Reversible - rechargeable
Irreversible - non rechargeable
Where are lead-acid cells commonly used?
Car batteries
If an electrochemical cell uses a platinium electrode how do you show it in an electrochemcial cell diagram?
Place on it on either side of the diagram, separated by |
If a reaction is exothermic what happens to the position of equilibrium if the temperature is increased?
The system seeks to oppose the increase in temperature by favouring the endothermic direction which is the backward reaction, shifting the position of equilibirum to the left and producing more reactants
What electrode potentials do the best reducing and oxidsing agent have?
Best reducing agent - most negative
Best oxidising agent - more positive
Are E⦵ values affected by change in conditions?
Yes - the half cell reactions are reversible, as the conditions change so does the position of equilirbium and so the E⦵ value changes
Which side of the equation do electrons appear on in oxidation half equations?
Right hand side
Name a common type of non rechargeable battery.
Dry cell alkaline battery
What happens to the position of equilibrium is the concentration of the products is increased?
The system seeks to oppose the increase in concentraion of the products by getting rid of the extra products, favouring the backward reaction, shifting the position of equilibrium to the left and so producing more reactants
What does a very positive electrode potential mean, in terms of electrons?
The metal is easily reduced, gaining electrons
In a dry cell alkaline battery what makes up the anode and cathode and the electrolyte?
Anode - zinc
Cathode - mixture of manganese dioxide and carbon
Electrolyte - Ammonium chloride
What are the electrodes separated by in a hydrogen-oxygen fuel cell? And why?
Ion exchange membrane, to allow H+ ions to pass but not electrons
Whats the half equation for the reaction occurng at the anode of a dry cell alkaline battery?
Zn(s) → Zn²⁺(aq) + 2e⁻
Is the zinc electrode the cathode or anode in the zinc/copper cell?
Anode
What is an oxidising agent in terms of electrons?
Electron acceptor
What can you use to measure how easily a metal is oxidised or reduced?
By using electrode potentials
If E⦵cell is negative what does it mean?
The reaction isnt feasible
What is a disproportionation reaction?
A reaction where one species is both oxidised or reduced in the same reaction
Are reactive metals easily reduced or oxidised?
Oxidised
Where are hydrogen - oxygen cells used?
To power electric vehicles
What happens in a half cell which has a platinium electrode and a solution containing Fe2+ and Fe3+ if the other half cell is made out of a less reactive metal?
Fe2+ ions are oxidised to Fe3+ ions, losing an electron
What is the oxidation state of group 2 metals in a compound?
+2
What does a very negative electrode potential mean, in terms of electrons?
The metal is easily oxidised so gives up electrons
What is the overall equation for what is happening in a hydrogen - oxygen fuel cell?
2H₂(g) + O₂(g) → 2H₂O(l)
Name two everyday objects that are electrochemical cells.
Batteries and mobile phones
How is a fuel cell diferent to normal cells?
The chemicals that are used to generatre the electricity are not contained in the electrodes or electrolyte but instead fed into the cell when electricity is required
What are two pros of hydrogen - oxygen fuel cells?
- ) They dont need electrical charging - as long as oxygen and hydrogen are supplied the cell will continue to produce electricity
- ) Only waste product is water
Whats the half equation for the reaction that occurs at the anode in a lead - acid cell?
Pb(s) + SO₄²⁻(aq) ⇋ PbSO₄(s) + 2e⁻
Draw what a zinc/copper cell looks like.
How do you find the E⦵cell from an electrochemcial cell diagram?
E⦵cell = E⦵right hand side - E⦵left hand side
What is the half equations for the reaction at the cathode of a hydrogen-oxygen fuel cell?
O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
What does the electrochemical cell diagram look like for a lead - acid cell?
Pb(s) | PbSO₄(s) || PbO₂(s)| PbSO₄(s)
How can you remember what oxidation and reduction mean?
OILRIG
In a hydrogen - oxygen fuel cell what do the platinium electrodes look like? And Why?
A porous ceramic material with a thin layer of platinum - cheaper and provides a larger surface area
What three factors affect the position of equilibrium?
Temperature, pressure and concentration of the reactants or products
How is a rechargeable battery made from the lead - acid cell recharged?
A current is supplied in the opposite direction to force the electrons to flow in the opposite direction around the circuit and reverse the reaction
What are all the standard electrode potentials measured by?
The standard hydrogen electrode
What is a redox reaction?
A reaction in which something is oxidised and something is reduced
Do oxidation states increases or decrease as electrons are gained?
Decrease
What is another name for the zinc/copper cell?
The Daniell Cell
What is the oxidation state of hydrogen in a compound with non-metals?
+1
Which side of the equation do electrons appear on in reduction half equations?
Left hand side
How are electrochemical cell diagrams different when the standard hydrogen electrode is used?
The standard hydrogen electrode is always on the left no matter whether the other half is more negative or not
