Electrochemistry Flashcards
What is oxidation?
The loss of electrons
What is reduction?
The gain of electrons
What are electrochemical cells?
Cells that use the transfer of electrons during redox reactions to produce electrical energy
What are electrochemical cells made of?
Two half cells that are connected via an external circuit like a conducting wire. Electrons flow accross the wire
A salt bridge, where filter paper is soaked in potassium chloride or potassium nitride, allowing ions to flow accross.
Under standard conditions
What are the standard conditions that electrochemical cells are under?
All solitions are 1moldm3
100kpa
298K
Describe a standard copper zinc cell
Tye copper electrode is the positive electrode and on the right, the zinc electrode is the negative and on the left. Electrons flow from zinc to copper
Descrobe the flow lf electrons in an electrochemical cell, and where does oxidation and reeuction occur?
Electrons flow from the negative to positive electrode. Oxidation occurs at the negative electrode. Reeuctipn at the positive
What is a half cell?
A cell that contains two species of the same element, with different oxidation states.
What is the standard electrode potential?
The e.m.f of a half cell compared to a standard hydrogen half cell, under standard contitions
What is the primary standard ekectrode potential?
The standard hydrogen electrode potential (0.00V) and is used to determine the potential lf other half cells
Describe a hydrogen half cell
Contains 100kpa H2 gass
1 moldm3 H+ ions (HCl)
Pt electrode
Describe the conventional representation of an electrochemical cell
A double verticle line indicates a salt bridge
A single,e verticle line indicates a phase boundary (change in state)
The species with the highest oxidation state for each half cell is written next to the salt bridge
Positive electrode is on the right and negative on the left (unless hydrigen is used in which case hydrogen is always on the left)
What happens in an electrochemical cell if a half cell contains species whoch are both aqueous solutions?
A Pt electrode is used to ensure there is both an oxidised and reeuces species in the half cell
In the electrochemical series, which species are the best reducing agents?
Species with the most negative potential on the right hand side
In the electrochemical series, which are the best oxidising agents?
The species with the highest potential on the left hand side
How do you calculate an electrochemical cells potential when hydrogen is not used?
Potential of the right species- potential of the left species
When reading an electrochemical series, how do you know is two element will react with eachother?
Pne must be the best oxidising reagent, and another must be a best reducing reagent.
A species must be more negative than another to be able to reduce it
Or a species must be more positive than another to oxidise it
How does a change in mass of electrodes predict redox reactions?
If the mass decreases, oxidation, electrode looses electrons so looses mass
If mass increases, reduction, electrode gains electrons so gains mass
Does the concentration of a solution affect e.m.f?
Yes, of not standard conditions (1moldm-3) it will increase or decrease the emf
What happens when the conc. at the positive electrode is > 1moldm-3?
Equilibrium shifts to the right
E becomes more positive because the forward reaction gains more electrons
Emf increases because the difference in E of both electrons increases
What happens when the conc. at the positive electrode is < 1moldm-3?
Equilibrium shifts to the left
E of electrode is less positive because the forward reaction gains less electroms
Emf decreases because the difference between the E of both electrodes decreases
What happens when the conc. at the negative electrode is > 1moldm-3?
Equilibrium shifts to the right
E of electrode is less negative because forward reaction looses fewer electrons
Emf decreases because difference of both electrodes E decreases
What happens when the conc. at the negative electrode is <1moldm-3?
Equilibrium shifts to the left
E of electrode is more negative because the forward reaction looses more electrons
Emf increases because the difference in E of both electrodes is greater
What are limitations for using electrode potentials for predicting redox reactions?
Predictions only tell us about the equilibrium, not how slow the rate is or how high the activation energy may be
Many lab reactions dont use standard conditions, so the electrode potential values are affected
State three types of electrochemical cells
Non-rechargeable cells
Rechargeable cells
Fuel cells
What are non-rechargeable cells?
Cells that can only be used once. The reactions cannot be reversed so eventually the chemicals are used up and the emf falls fo zero
Give examples of non- rechargeable cells
Alkaline based zinc and manganese dioxide cells
In smoke detectors and clock batteries
What are rechargeable cells?
Cells than can be used more than once because the reactions can be revrsed so the chemicals in the reaction can be recharged
Give examples of three rechargeable cells and where theyre used
Lithium ion cells in phones and laptops
Lead-acid car batteries
Ni-Cd cells in torches
Why are lithium ion cells good to use in phones and laptops?
Because lithium is the least dense metal, so is very light
What are fuel cells?
Electrochemical cells that use energy from the reaction between a duel and oxygen to create a voltage
Descrobe the how a alkaline hydrogen- oxygen fuel cell works
Uses two poroud platinum electrodes, separated by a partially permeable membrane.
Uses the electrolyte sodium hydroxide solution
Hydrogen enters at the negative electrode: H2 + 2OH- —> 2e- + 2H2O
Electrons flow to the positive electrode
Oxygen enters at the positive electrode: 4e- + O2 + 2H2O —> 4OH-
Overall eqation: 2H2 + O2 —> 2H2O
What are the two half equations of an acid hydrogen oxygen fuel cell?
H2 —> 2H+ + 2e-
O2 + 4H+ + 4e- —> 2H2O
Why do alkaline and acid hydrogen hydrogen oxygen duel cells both have the same emf?
Because they have the same overall eauation
2H2 + O2 —> 2H2O
What are three advantages of fuel cells?
They are the most efficient, as they convert more of their available energy into kinetic energy
They do not produce Carbon dioxide
They do not need to be recharged and produce a continuous supply of energy
What are three disadvantages of fuel cells?
Hydrogen must be produced, whoch is done via the electrolysis of water which uses electricity from fossil fuel
Hydrogen is highly flammable
There is no technology to provode hydrogen fuel cells for cars
