Electrochemical Cells Flashcards by Tania Ahmed

What is the function of a cell

Convert chemical energy into electrical energy

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What is a half cell

A metal electrode dipped into a solution of its own ions

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What is an electrode

A solid (metal) conductor of electricity

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What is an electrochemical cell

2 half cells connected together by a salt bridge

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Why is a high resistance voltmeter used in a cell

ensures 0 current conditions

- allows the measure of maximum potential difference b/t 2 cells

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How is a salt bridge made?

By dipping a piece of filter paper into a saturated solution of potassium nitrate (KNO3) or potassium chloride (KCL)

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What are the functions of a salt bridge?

(a) to allow ions to move freely between the half cells

(b) to maintain electrical connection or to complete the circuit

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Can a metal be used as the salt bridge? Why?

No
-the salt bridge must not interfere with the reaction (must not react with the solution or electrodes)

-if a metal is used, it may generate an additional metal ion potential which will interfere with the reaction

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What is used as the electrode if a metal is not part of the reaction? Why is that used?

Platinum

-it is an inherent/ unreactive metal and will not interfere with the reaction

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In a cell diagram, where does each half equation go?

OXIDATION ; left

REDUCTION ; right

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What is the Standard Hydrogen Electrode (SHE)

And where is it found

The primary standard that is used to determine the electrode potential of half- cells

It is always the left hand electrode

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What is the electrode potential of the SHE

By convention/ definition

It is 0.00V

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What are the conditions of the SHE

Pressure: 100KPa/ 1 bar

Concentration of solutions: 1.0 mol dm^-3

Temperature: 298K

Current: 0 amps

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In the SHE what is added onto the electrode

In the SHE a platinum electrode is used, and platinum black is added to the end of the electrode to act as a catalyst
It increases the rate at which equilibrium is attained between H2 and H+

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What is the main disadvantage of the SHE

How is it overcome

It is awkward/ cumbersome to set up

A secondary standard is used to overcome this

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What is a secondary standard

An electrode that has been calibrated against the SHE

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What is the E0

The maximum potential difference measured relative to the SHE, under 0 current conditions

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What is an electrochemical series

A list of E0 values arranged in ascending/ descending order

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What are the uses of an electrochemical series

(a) used to predict which reaction will occur when 2 half cells are connected
(b) It can be used to calculate the EMF of p.d. of a cell

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What is the EMF

The Electrodemotive force

Is the maximum potential difference between 2 half cells

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What is the equation used to calculate the emf

E0cell

EMF) = E0(R) - E0(L

What must the emf be for a spontaneous reaction

Emf/ E0 > or = 0

Must be POSITIVE

Even if the emf is positive, a reaction may not always occur. Why?

the activation energy is too high
Rate is too slow
emf only applies to reactions under standard conditions

How do you find the strongest oxidising agent in an electrochemical series

it is on the left hand side

- AND has the most positive emf

How do you find the weakest oxidising agent in an electrochemical series

- it is on the left hand side | - AND has the least positive emf

How do you find the strongest reducing agent in an electrochemical series

- it is on the right hand side | - AND has the least positive emf

How do you find the weakest reducing agent in an electrochemical series

- it is on the right hand side | - AND has the most positive emf

What factors affect emf

- Temperature - Pressure - [ conc ] of solutions - Current

What is the effect of increasing temperature on emf

- Most spontaneous reactions are exothermic - if temp is increased - equilibrium will shift to the left - emf will decrease

What is the effect of increasing the concentration of one of the solutions

If the concentration of a solution is increased The equilibrium will shift away from the solution Depending on which electrode (left or right) is made more positive The emf will increase or decrease

What effect would current have on the emf

If the cell is allowed to draw current Emf will decrease and become 0 When the reaction reaches equilibrium

What effect would a catalyst have on the emf

NO EFFECT ON THE EMF VALUE but, can increase the rate at which equilibrium is reached

What can the electrode potential tell you about the direction of a redox reaction?

The more negative the electrode potential the more it oxidises; Reaction occurs BACKWARDS The more positive the electrode potential the more it reduces; Reaction occurs FORWARDS

Are primary cells rechargeable? | Why?

NOT rechargeable; | The reactions are not reversible

When a cell is discharging what does that tell you about the emf

Emf is greater than or equal to 0 | Occurs spontaneously

State one essential property of the non reactive porous separator

Allows ions to flow/ move freely

State the function of the carbon rod

Conduct electricity

State one environmental advantage of rechargeable cells compared to non rechargeable ones

- metal compounds can be reused - less mining - less use of landfill - less use of energy

Are secondary cells rechargeable? | Why?

Yes; | The reactions are reversible

When a cell is recharging what does that tell you about the emf

Emf < 0 | Needs electrical current to occur

What does the graph of a secondary cell look like & explain the shape

Voltage/ emf remains constant for some time before dropping suddenly: Voltage drops at this point because the reactants get used up

State the half equations & overall equation of a hydrogen- fuel cell in ACIDIC conditions

OXIDATION: H2—> 2H+ + 2e- REDUCTION: O2 +4H+ + 4e- —> 2H2O Overall: 2H2 + O2 —> 2H2O

State the half equations & the overall equation of a hydrogen- fuel cell under ALKALINE conditions

OXIDATION: H2 + 2OH- —> 2H2O + 2e- REDUCTION: O2 + 2H2O + 4e- —> 4OH- Overall: 2H2 + O2 —> 2H2O

What is the emf of the cell in acidic conditions? What is the emf of the cell in alkaline conditions? Why?

They’re the same; | The overall equations are the same

Explain the shape of the graph of a fuel cell

It’s is a straight horizontal line; | Voltage is constant because fuel is consistently being supplied to the cell; the concentration remains constant

Name some advantages of fuel cells

- only waste is water | - very efficient (converts more available energy into kinetic energy)

Name some disadvantages of fuel cells

- limited life time - expensive - hydrogen is highly flammable - fossil fuels burnt to obtain hydrogen

Are hydrogen fuel cells carbon neutral? Why?

Not entirely; Depends on how the hydrogen obtained; H2 obtained from methane is not carbon neutral (methane is a carbon source)

What are the half cell equations & overall equation of ethanol fuel cells

OXYGEN ELECTRODE: 4e- + 4H+ + O2 —> 2H2O ETHANOL ELECTRODE: C2H5OH + 3H2O —> 2CO2 + 12H+ + 12e- Overall: C2H5OH + 3O2 —> 2CO2 + 3H2O

Name some advantages of the ethanol fuel cells

- made from renewable sources in a carbon neutral way - abundant raw materials (to produce ethanol) - less explosive/ easier to store than H2