Electrode potentials and cells Flashcards by Angel Ssemanda

What is a half cell

when a piece of metal is dipped into a solution of its metal ions an equilibrium is set up that represents a half equation

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Outline what happens when a Zinc rod is immersed in a solution of Zinc ions

The Zn atoms on the rod can deposit two electrons on the rod and move into the solution as Zn^2+

Zn > Zn2+ + 2e-

alternatively the Zn^2+ ions in solution could accept two electrons from the rod and move onto the rod and become Zn atoms

Zn2+ + 2e- > Zn

Equilibrium reaction : Zn2+ + 2e- ⇌ Zn

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What is the equilibrium equation for a Copper rod in a solution containing Cu2+ ions

Cu2+ + 2e- ⇌ Cu

in this half cell the equilibrium could lie further to the right so fewer electrons wld be deposited onto the rod

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What happens if two different electrodes are connected

the potential difference between the two electrodes will cause a current to flow between them

an electromotive force is established and the system can generate electrical energy

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What are the features of a cell

Voltmeter
Wire
Electrodes
Salt bridge

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What is the function of a Voltmeter

measures the potential pushing power of electrons through the circuit but keeps the current flow at zero

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What is the function of the wire?

allows movement of electrons

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What is the function of the electrodes

Where half equations are taking place and are referred to as half cells

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What is the function of the salt bridge?

completes the circuit and allows the movement of ions to compensate for changes in conc in each half cell

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What is a salt bridge

usually a filter paper soaked in a solution of KNO3

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Why is KNO3 a suitable solution to use in a salt bridge

its inert

it doesn’t react with any of the ions in solution so it doesn’t interfere with the redox reaction

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Why is KCl not used as a salt bridge solution for a cell that contains Ag+ ions

white ppt of AgCl- will form

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What are the 3 types of electrodes

Metal
Gas
Redox

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What is a metal electrode?

Consist of a metal surrounded by a solution of its ions

e.g Zn(s)|Zn2+(aq)

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What is a redox electrode

for two different ions of the same element (e.g Fe2+ and Fe3+)

where two types of ions are present in solution with an inert metal electrode to allow the flow of electrons

Pt(s) | Fe2+(aq), Fe3+ (aq)

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What is a gas electrode?

a gas and a solution of its ions

an inert metal is the actual electrode to allow the flow of electrons

Pt(s) | H2(g) | H+ (aq)

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What effect does the surface area of the electrode have

SA has no effect on the electromotive force

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Why is Platinum used in Redox and Gas electrodes

It is unreactive (inert) and conducts electricity

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When do you use a Pt electrode

when there is no solid metal rod

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Describe the standard hydrogen electrode (SHE)

The primary standard that is used and assigned the potential of 0 volts

Hydrogen gas is bubbled into a solution of H+ ions, and a Pt electrode is used

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What is the half equation for a Standard Hydrogen electrode

H+(aq) + e- ⇌ 1/2H2(g)
E∅ = 0.00

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What are the conditions and components of The SHE

Standard conditions:
- 1.00 moldm-3 HCl
- 100kPa
- 298K
- 0.50 moldm-3 H2SO4

Components:
- 1.00 moldm-3 HCl
- H2 gas
- Pt electrode

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How do you draw a diagram of a cell

solution must be in contact with the salt bridge and electrode
Check if a Pt electordde is needed
Label all parts of the diagram and give concentrations and state symbols

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What reagents and concentrations are needed for a Fe2+ and Fe3+ electrode

1.00 moldm-3
FeCl3

1.00 moldm-3 FeCl2

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What does a negative electrode potential mean

Good at giving away electrons, more likely to shift left On the RHS and are the best reducing agents -ve terminal

What does a positive electrode potential mean

Good at attracting electrons, more likely to shift right On the LHS and are the best oxidising agents +ve terminal

How can you predict if a reaction is feasible

The redox equation of the more -ve electrode must be reversed to show oxidation - The E∅ OF A/B (+ve) is greater than the E∅ of C/D (-ve) So A is reduced to B and D is oxidised to C

How do you write a conventional cell diagram

D|C || A|B left = -ve electrode right = +ve electrode

In a conventional cell what do the single lines represent

a change of physical state

When is a comma used in a conventional cell

if the phases of the two components are in the same phase eg. (s),(s) (aq),(l) (aq),(aq)

What is the electromotive force (E∅ cell)

The potential difference across the two electrodes

How do you calculate E∅ cell

E∅ cell = E∅ (reduction) - E∅ (oxidation)

What does a positive E∅ mean

The reaction is feasible

What happens to E∅ if equilibrium shifts to the right

E∅ will become more +ve

What happens to E∅ if equilibrium shifts to the left

E∅ will beocme more -ve

What happens to equilibrium if the concentration of a reactant is increased

Equilibrium shifts to the right to oppose the change This inceases E∅

What happens to equilibrium if the concentration of a product is increased

Equilibrium shifts to the left to oppose the change This deceases E∅

How can you increase the E∅ when given two half cell equations

Make the -ve E∅ MORE -ve so it shifts to the right then make the +ve E∅ MORE +ve so it shifts to the right - this will increase the value E∅ ,cell

Describe a non rechargeable cell

Chemicals are used up over time, and the EMF drops - when chemicals have been completely used up, the cell is flat and the EMF is 0 - can't be recharged bc the casing is a reactant

Describe the Daniell cell (Zn/Cu cell)

Non rechargeable - bc the Zn electrode runs out overtime, the equation can't be reversed, also the solutions make it not practical for transport

What is the purpose of a porous cup in a Daniell call

acts as a salt bridge - allows movement of ions

Describe a Zinc/Carbon cell

Zn behaves as the -ve electrode, acts as a case, and has a paste electrolyte solution. C acts as the +ve electrode and is located as a rod down the centre of the canister. It behaves like Pt

Why may a cell leak after being used for a long time

Zn is oxidised to Zn2+ causing the case to wear away

What are advantages of alkaline batteries

last longer but have a higher cost

Describe rechargeable cells

The **current can be forced back in the opposite direction** by an applied external current (plugging them in) so they can work again - electrodes are designed so they don't decompose or wear down quickly - small light and portable - the half-cell equations are reversed when it is recharged

Describe a lithium ion battery

Lithium is a light-weight metal and gives for a light battery (cell) The **electrolyse phase (liquid) is a polymer, meaning the battery wont leak** - cell generates 4V

What is the equation of the two half cells in a lithium battery

Li+ + CoO22 + e- ⇌ LiCoO2 Li+ + e- ⇌ Li Overall equation CoO2 + Li > LiCoO2

What is a fuel cell?

Have a continuous supply of the chemicals into the cell so the chemicals don't run out or need recharging however they do need to have a constant supply

What is the most common fuel cell

Hydrogen Oxygen fuel cell

Describe a hydrogen oxygen fuel cell

consists of **two platinum electrodes separated by a polymer electrolyte** which allows ions to pass through - runs in acidic and alkaline conditions but the overall emf is the same

What are the two half cells for a hydrogen-oxygen fuel cell in **acidic** conditions

H2(g) > 2H+(aq) + 2e- E∅=0.00 4H+(aq) + O2(g) + 4e- > 2H2O-(l) E∅ = 1.23 E∅ cell = 1.23

What are the two half cells for a hydrogen-oxygen fuel cell in **alkaline** conditions

H2(g) + 2OH- > 2H2O(aq) + 2e- E∅ = -0.83 2H2O(aq) + O2(g) + 4e- > 4OH-(l) E∅=0.40 E∅ cell = 1.23

What is the overall equation for a hydrogen-oxygen fuel cell

2H2(g) + O2(g) > 2H2O(l)

What powers a motor

the movement of electrons powers a motor

How does the EMF of a fuel cell remain the same

- there's a continuous supply of the fuel - concentrations remain the same

What are benefits and risks of using cells

B: Portable source of energy R: waste issues

What are benefits and risks of using non rechargeable cells

B: cheap R: waste issues

What are benefits and risks of using rechargeable cells

B: less waste, cheap, low environmental impact R: some waste issues

What are benefits of using hydrogen fuel cells

- only waste product is water - doesnt need recharging - **very efficient**

What are risks of using hydrogen fuel cells

- need a constant supply of fuel - Hydrogen is flammable and explosive - hydrogen is usually made using fossil fuels - high cost of fuel cells

What does a emf vs time graph of a fuel cell look like

Straight line across emf remains constant

What happens to the EMF when the voltmeter is removed or replaced with a bulb

a current flows EMF decreases to 0 as reactants are used up