1.11 Electrochemical Cells
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What is an electrochemical cell?
A metal is placed into a solution containing its ions An equilibrium is established One of these is called a half cell
What are two half cells called?
A Daniell cell Two half cells combined by a salt bridge With a voltmeter in circuit An electric current flows so the electrons pass from the more reactive metal to the less reactive metal = equilibrium’s can be different
What is a salt bridge?
A piece of filter soaked in a solution of potassium nitrate KNO3 Containing electrolyte which doesn’t react It completes the circuit so ions can move in and out of it It is unreactive with half cells
What is significant about the voltmeter?
A high resistance voltmeter will not draw current from the circuit
What is the standard hydrogen electrode?
It is used to compare how easily metals lose electrons It is the standard that we measure the emf values against
In the standard hydrogen electrode what is the general set up?
H2 gas is bubbled in H+ are in aqueous solution provided by and acid eg H2SO4 or HCl Platinum electrode It goes on the LEFT ALWAYS!
What does the standard hydrogen electrode include?
A platinum electrode Which is platinum foil covered in porous platinum to increase surface area It is unreactive and a good conductor of electricity
What are the conditions for the standard hydrogen electrode?
100kPa 298K 1.0 moldm-3
What is the standard electrode potential? E°
The potential difference between any electrode under standard conditions and the standard hydrogen electrode
How do we determine if something is a strong/weak reducing/oxidising agent?
More negative E° = stronger reducing agent More positive E° = stronger oxidising agent
How do you workout the overall E° cell?
E° right hand side - E° left hand side Or E° comparatively more positive - E° comparatively more negative
What does an overall more positive E° value mean?
The reaction will happen spontaneously
In a Daniell Cell set up whig side does each half cell go on?
The more negative E° value goes on the left hand side
What is included in cell representation?
|| = salt bridge (goes between the two half cells) , = separates different substances in the same phase = phase boundary (only between different states of matter)
How do you know where the half cells go in cell representation?
The form of the substance with the highest oxidation state is put next to the salt bridge Eg 0, 2+, 2+, 0
What can we deduce from cell representation?
Zn(s)| Zn2+(aq) || Cu2+(aq) | Cu(s) Reactant, product, reactant, product And so we can easily form half equations in the order we read them Zn -> Zn2+ + 2e-
In cell representation what happens if there is no solid surface?
No solid surface to conduct electricity A platinum electrode is used like in the SHE (standard hydrogen electrode) Eg Pt(s) | Fe2+(aq), Fe3+(aq) || Cl2(g) | Cl-(aq) | Pt(s) Only put on an end of a half cell with no (s)
What happens to the E° if concentration is changed?
Increase concentration = E° increases Decrease concentration = E° decreases
How can temperature change an E° cell?
Most cells are exothermic in the spontaneous direction So Increase in temperature moves the equilibrium to the left hand side = decrease E° cell
How can activation energy affect an E° cell?
If the reaction in the cell has a high activation energy There’s a chance the reaction won’t happen regardless of a positive E° cell value
What are the types of chemical cells?
Primary cells - irreversible and must not be recharged Secondary cells - reversible and can be recharged by an electrical current Fuel cells - generated electricity by the continual oxidation of an external source of fuel
Give an example of a primary cell?
Zinc-Carbon dry cell Positive electrode: Graphite rod(carbon) Negative electrode: Zinc container Surrounded by a paste of manganese dioxide and ammonium chloride (MnO2 + NH4Cl)
What is the zinc-carbon dry cell overall equation?
Zn2+ + 2NH4+ ⇌ 2NH3 + H2 + Zn
Why is Zinc-carbon dry cell a primary cell?
You don’t recharge it as the zinc container will wear away Therefore not safe