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chemistry sem 2 > Redox & Electrochemistry > Flashcards

Redox & Electrochemistry Flashcards

1
Q

what does oxidation mean in terms of:
- electrons
- oxidation number

A
  • loss of electrons
  • increase in oxidation number
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2
Q

what does reduction mean in terms of:
- electrons
- oxidation number

A
  • gain of electrons
  • decrease in oxidation number
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3
Q

what are examples of oxidation?

A

metals lose electrons to for positive ions

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4
Q

what are examples of reduction?

A

non-metals gain electrons to form negative ions

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5
Q

what is an oxidising agent?

A

causes other substances to lose electrons. The oxidising agent is reduced

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6
Q

what is a reducing agent?

A

causes other substances to gain electrons. The reducing agent itself is oxidised.

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7
Q

what are redox reactions?

A

redox reactions are reactions in which both oxidation and reduction takes place simultaneously

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8
Q

what are the rules to find out oxidation numbers? step 1

A

The oxidation state of atoms in uncombined elements is 0

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9
Q

what are the rules to find out oxidation numbers? step 2

A

In neutral compounds, the sum of the oxidation states is 0

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10
Q

what are the rules to find out oxidation numbers? step 3

A

In ions, the sum of the oxidation states equals the charge on the ion.

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11
Q

what are the rules to find out oxidation numbers? step 4

A

In any substance, the more electronegative atom has the negative oxidation state, the less electronegative atom has the positive oxidation state.

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12
Q

what are the rules to find out oxidation numbers? step 5

A

Fluorine in compounds always has oxidation number of –1

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13
Q

what are the rules to find out oxidation numbers? step 6

A

Hydrogen in compounds always has oxidation number of +1 (except with metals e.g. hydrides)

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14
Q

what are the rules to find out oxidation numbers? step 7

A

Oxygen in compounds almost always has oxidation number of –2 (except in peroxides –1, or with fluorine +2)

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15
Q

what are the rules to find out oxidation numbers? step 8

A

Group I metals in compounds are always +1

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16
Q

what are the rules to find out oxidation numbers? step 9

A

Group VII (halogens) in compounds are usually –1 (except with oxygen or fluorine)

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17
Q

what is the usual oxidation number of an element?

A

always zero

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18
Q

what is the usual oxidation number of group 1 metals?

A

always +1

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19
Q

what is the usual oxidation number of group 2 metals?

A

always +2

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20
Q

what is the usual oxidation number of oxygen and some exceptions?

A

usually -2

  • peroxides (-1)
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21
Q

what is the usual oxidation number of hydrogen and exceptions?

A

usually +1

  • metal hydroxides (-1)
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22
Q

what is the usual oxidation number of group 7 and any exceptions?

A

usually -1

  • compounds with O&F
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23
Q

what is the usual oxidation number of fluorine?

A

always -1

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24
Q

What is the first step in identifying if a reaction is a redox reaction?

A

write the oxidation number under each element

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25
What is the second step in identifying if a reaction is a redox reaction?
identify which element, if any, has changed oxidation number
26
What is the third step in identifying if a reaction is a redox reaction?
- increase in oxidation number = oxidation - decrease in reduction number = reduction
27
when can you write a ionic half equation?
for those elements that have changed oxidation number, you can use these to show the loss and gain of electrons
28
what is the first step in writing an ionic half equation?
write the two species for each element that changes oxidation number
29
what is the second step in writing an ionic half equation?
balance the equations by adding electrons
30
what does an electrochemical cell control?
control electron transfer to convert chemical energy to electrical energy.
31
how are electrochemical cells made?
by connecting two half cells
32
what is each half cell made up of?
the chemical species present in a redox half equation. one half will be the reduction half cell and the other will be the oxidation half cell.
33
what happens when the two half cells are connected together?
they make a complete circuit, where electrons can flow and voltage can be measured.
34
what are electron potential values a measure of?
the tendency to gain electrons
35
what will each half cell have and why?
an electrode potential value, so we can compare values and deduce which half-cell will gain electrons.
36
what is the golden rule in terms of electron potentials?
the more positive the electrode potential value, the easier it is for the species to gain electrons
37
what are the electrode potential values given as?
standard electrode potentials
38
what is the hydrogen half-cell used for?
measuring against all other half cells. the standard hydrogen electrode. Its a reference point
39
what is a definition of the standard electrode potential?
the voltage measured under standard conditions (temp 298K/ all case 1 atm/ 101pa pressure, all solutions 1moldm-3) when an electrochemical half-cell is combined with the standard hydrogen electrode.
40
what happens to the standard electrode potential if the conditions change?
the voltage would be different
41
what is the hydrogen electrode made of?
platinum and allows good contact between hydrogen gas and hydrogen ions in solution
42
why is platinum used as the electrode?
The half-cell needs an electrode & Platinum is unreactive.
43
if the standard electrode potential of the half-cell is greater than 0...
then the half-cell is more easily reduced than hydrogen
44
if the standard electrode potential of the half-cell is less than 0...
then the half-cell is more easily oxidised than hydrogen
45
When we compare the E θ values of two half cells...
we can think of the half reaction with the more positive value as ‘going forward’. i.e the half-cell with the more positive Eθ value will be the reduction half-cell.
46
how do you measure a standard electrode potential?
- the half-cell is connected to a standard hydrogen electrode. - The two electrodes are connected by a wire to allow a controlled flow of electrons. A high resistance voltmeter is used to measure the standard electrode potential. -  The two solutions are connected with a salt bridge which completes the circuit and allows ions to flow. The salt bridge contains a saturated solution of a salt soaked onto filter paper (usually potassium nitrate).
47
when the metal with the half cell is more positive Eө...
value will be the positive electrode.
47
what does the single line represent?
The single line | represents a phase change
48
what does a double line represent?
The double line || represents the salt bridge.
49
The positive electrode is?
the electrode of the half-cell with the most positive Eθ
50
where do electrons flow in the Half-cell?
towards the half-cell with the most positive Eθ
51
How is the electrochemical series built up?
The electrochemical series is built up by arranging various redox equilibria in order of their standard electrode potentials (redox potentials).
52
Where are the most negative Eθ values placed?
At the top of the electrochemical series
53
Where are the most positive Eθ values placed?
at the bottom of the electrochemical series
54
how are the equations for Eθ values written?
The equations for Eθ values are ALWAYS written as an equilibrium reaction, with the forward reaction being reduction.
55
The more -ve the Eθ Reverse reaction occurs
- the species on the right will be oxidised/lose electrons - Best reducing agent - Negative electrode
56
what happens if the forward reaction occurs?
- the species on the left will be reduced / gain electrons - Best oxidising agent - Positive electrode
57
what can the standard electrode potentials predict?
The standard electrode potentials can be used to predict how a reaction will occur, or whether a reaction is feasible, and also to predict the voltage produced by a cell made up of any pair of half cells.
58
the more positive the electrode potential...
The more positive the electrode potential is, the more likely it will go forward and be reduced (and vice-versa).
59
How do you want to know the voltage produced by a cell between two ions?
If we want to know the voltage produced by a cell between two ions, we take the more positive voltage and subtract the less positive one. - Emf (Eθcell) = (more +ve Eθ ) – (least +ve Eθ) OR - Eθcell = Eθred - Eθox (if given an overall equation)
60
if the non-metal is a gas what will the electrode be?
If the non-metal is a gas – the electrode is platinum like the hydrogen half-cell, with the gas pumped in at the top of the glass tube under standard conditions.
61
how can the feasibility of a reaction be predicted?
The feasibility of a reaction can be predicted using the half equations and their electrode potential values.

chemistry sem 2 (10 decks)

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