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Chemistry Unit 3/4 > U3AOS1 - Galvanic Cells as a source of energy > Flashcards

U3AOS1 - Galvanic Cells as a source of energy Flashcards

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1
Q

Redox Reactions

A

Transfer of electrons, from one species to another, in two half reactions (oxidation and reduction)

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2
Q

Oxidation Number

A

number of electrons lost or gained by an atom to form a chemical bond with another species

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3
Q

Examples of Redox Reactions

A

Combustion, Photosynthesis

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4
Q

How to determine if a redox reaction has occured

A

If the Oxidation Number of the molecules has changed

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5
Q

What is the oxidation number of a free element (e.g. O2)?

A

Zero

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6
Q

What is the Oxidation of a simple ion (e.g. Na+)?

A

Equal to the charge of the ion

(so Na+ is +1)

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7
Q

What is the sum of the oxidation numbers of a compound (e.g. KCl or SO42-)

A

KCl -> ON of 0
SO42- -> ON of -2

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8
Q

What is Hydrogens Oxidation Number (in a compound - including special rule)

A

Normally +1

Except in metal hydrides, where its -1 (e.g. CaH2)

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9
Q

What is Oxygens Oxidation Number (in a compound - including special rule)

A

Normally -2

Except in peroxides where its -1 (e.g. H2O2)

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10
Q

Conjugate Reducing & Oxidizing Agents

A

An electron donator and its corresponding electron reciever

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11
Q

Spectator Ions

A

Ions that are not involved in the redox reaction - will have no change in oxidation number

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12
Q

Do Noble Gases react in a redox reaction

A

No - they can have no change in electrons - as they have a full shell

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13
Q

Oxidation (electron, ON, sample reaction)

A

Reactant loses an Electron
Increase of ON
Reactant = Product + e-

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14
Q

Reduction (electron, ON, sample reaction)

A

Reactant gains an electron
Decrease in ON
Reactant + e- = Product

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15
Q

Reduction Agent (reductants)

A

substance that causes the other substance present to undergo reduction

it will itself, undergo oxidation

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16
Q

Oxidation Agents (oxidants)

A

substance that causes the other substance present to undergo oxidation

it will itself, undergo reduction

17
Q

How to balance a half equation

A

K - key elements
O - balance oxygen by adding H2O
H - balance hydrogens by adding H+
E - balance electrons
S - include states

18
Q

How to balance a basic (alkaline) half equation

A

use KOHES,

Then, for every H+ present, add OH- to BOTH sides (should cancel out, creating waters instead)

19
Q

What is a Galvanic Cell (+basic structural features)

A

Electrochemical Cell in which chemical energy from a - spontaneous redox reaction - is converted into - electrical energy -

Each cell contains two half cells and a salt bridge

20
Q

Common Design Feature of Galvanic Cells - Electrodes

A

material that is able to conduct electricity

Often a metal (which can react) or a pure carbon rod (inert)

21
Q

Common Design Feature of Galvanic Cells - Anode

A

Negatively Charged Electrode

Oxidation occurs at the anode (will lose electrons)

Label as ‘Anode (-ve)’

22
Q

Common Design Feature of Galvanic Cells - Cathode

A

Positively Charged Electrode

Reduction occurs at the cathode (gains electrons)

Label as ‘Cathode (+ve)’

23
Q

Common Design Feature of Galvanic Cells - Salt Bridge

A

connection between two half cells - allows the flow of ions into the two half cells

purpose is to maintain electrical neutrality, and complete the circuit (allowing electrons to flow through the connecting wire) by preventing polarity

24
Q

Common Design Feature of Galvanic Cells - Salt Bridge - how it balances the anode half cell

A

Over time, this cell becomes positively charged (as there is a loss of electrons - oxidation)

therefore, the salt bridge will provide a flow of negatively charged substances to maintain electrical neutrality

25
Q

Common Design Feature of Galvanic Cells - Salt Bridge - how it balances the cathode half cell

A

Over time, this cell becomes negatively charged (as there is a gain of electrons - reduction)

therefore, the salt bridge will provide a flow of positively charged substances to maintain electrical neutrality

26
Q

Common Design Feature of Galvanic Cells - Salt Bridge - composition requirements

A

Must be:
- soluble in water (doesn’t form an insoluble compound - precipitate - as this will prevent a circuit from flowing as electrons cannot flow through solid ionic substances)
- nonreactive

27
Q

Chemical Processes of Galvanic Cells - Electron Ion Flows

A

Electrons will flow form the anode to the cathode

and charged substances will flow from the salt bridge

28
Q

Common Design Feature of Galvanic Cells - Separation of Half-Cells

A

allows for electrical current to be captured through the connecting wire

29
Q

Common Design Feature of Galvanic Cells - Potential Difference

A

Ecell = Ereduction - Eoxidation

30
Q

What is a Spontaneous Redox Reaction

A

reactions that can occur naturally (spontaneous), and do not need to be forced

31
Q

What is a direct, spontaneous redox reaction

A

Occurs when the reacting species are in the same vessel - with chemical energy being released as heat

e.g. combustion

32
Q

What is an indirect, spontaneous redox reaction

A

Occurs when the reacting species are kept separately in different vessels

chemical energy is released predominantly as electrical energy (although not 100% efficient, some energy will be released as heat)

e.g. galvanic cells

33
Q

Types of Half Cells

A

Solid/Aqueous Cell: The electrode will participate in the reaction

Double Aqueous Cell: Inert Electrode will be used

Gas/Aqueous Cell: Inert Electrode will be used - which is often covered in a fine powdery - which increases the surface area of electrode - increasing reaction rate (for platinum electrodes - its called platinum black)

34
Q

Designing Galvanic Cells - how to influence voltage and current

A

Voltage - depends on how much energy is being released in the reaction - can be determined by looking at the electrochemical series

Current - depends on the rate of reaction - with a higher rate of reaction yielding a higher current because the transfer of electrons is occurring at a faster rate

Chemistry Unit 3/4 (16 decks)

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