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IB Chemistry HL > 9 redox > Flashcards

9 redox Flashcards

1
Q

what is corrosion called in the case of iron and steel

A

rusting

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2
Q

what is rust chemical formula

A

hydrated iron(III) oxide (Fe2O3.xH2O)

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3
Q

what is essential for iron to rust

A

water and oxygen

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4
Q

what increases rate of corrosion

A

salt and acid rain

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5
Q

what is sacrifical protection

A

an alternative method of rust prevention in which blocks of a more reactive metal are attatched to the iron surgace. these mtals are more reactive than iron and will be corroded in preference to it

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6
Q

what is galvanising

A

a layer of more reactive metal coats the object. It has the advantage over other plating methods in that the protection still works even if the zinc layer is badly scratched. The zinc layer can be applied by several different methods. These include electroplating or dipping the object into molten zinc. The bodies of cars are dipped into a bath of molten zinc to form a protective layer.

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7
Q

what type of reaction is a corrosion

A

redox, involving both oxidation and reduction

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8
Q

oxidation is

A

the loss of electrons and increase in oxidation state

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9
Q

reduction is

A

the gain of electrons and decrease in oxidation state

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10
Q

oxidising agent

A

is a species that oxidises another species

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11
Q

a reducing agent is

A

a species that redudces another species

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12
Q

a positive oxidation state means

A

it has lost electron control, and is consequerntly oxidised

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13
Q

a negative oxidation state means

A

it has gained control over electrons, and is consequently reduced

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14
Q

oxidation state is

A

a number which, with its sign, indicates the gain or loss of electron control of an atom during a reaction.

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15
Q

an oxidation number is

A

the numerical part of the oxidation state

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16
Q

Free elements such as O2, Cl2, N2, are assigned an oxidation state of

A

0

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17
Q

the sum of the oxidation states of all the atoms in a compound must be equal to

A

the net charge on the compound

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18
Q

The alkali metals (Li, Na, K, Rb and Cs) in compounds are always assigned an oxidation state of

A

+1.

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19
Q

Fluorine in compounds is always assigned an oxidation state of

A

−1.

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20
Q

The alkaline earth metals (Be, Mg, Ca, Sr, Ba and Ra) and Zn in compounds are always assigned an oxidation state of

A

+2

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21
Q

Hydrogen in compounds is assigned an oxidation state of X except in certain metal hydrides (e.g. NaH), where it is Y

A 
X = +1
Y = -1
22
Q

Oxygen in compounds is assigned an oxidation state of X, except in peroxides (e.g. H2O2), where it is Y, or when it is combined with fluorine.

A 
X = -2
Y = -1
23
Q

Chlorine in compounds is assigned an oxidation state of WHAT unless

A

-1, unless it is combined with oxygen or fluorine.

24
Q

The charge on a metal ion is the same as

A

its oxidation state, e.g. Zn2+ has an oxidation state of +2.

25
Q

The sum of the oxidation states in a polyatomic ion (such as CO32−) must add up to

A

the charge on the ion

26
Q

what is the average oxidation state

A

the average of the oxidation states of the same atom in a compound.

27
Q

what do roman numerals signify

A

the oxidation state.

For example, in copper(I) oxide, the copper has an oxidation number of +1, whereas in copper(II) oxide, it is +2.

28
Q

when does disporportionation occur

A

when the same species is oxidised and reduced simultaneously

29
Q

example of disproportionation

A

catalytic decomposition of hydrogen peroxide

2H2O2 (aq) → 2H2O (l) + O2 (g)

30
Q

what is the winkler method

A

uses a series of redox reactions to determine the amount of dissolved oxygen in water

31
Q

what is BOD

A

biological oxygen demand

32
Q

describe what BOD is

A

a measure of the dissolved oxygen (ppm) required to decompose the organic matter in was over a set period of time, usually five days

33
Q

how is BOD measured

A

a measured sample of water is then incubated in the dark at a fixed temperature for five days while microorganisms in the water oxidise the organic material. the oxygen content of the water sample is fixed by adding an excess of manganese sulfate

34
Q

first step of the winkler method

A

an excess of manganese salt is added of the sample under alkaline conditions

35
Q

second step of the winkler method, following adding an excess of manganese salt

A

potassium iodide is then added which is oxidised by the manganese (IV) oxide to form iodine

36
Q

what is the ratio between O2 and S2O3 2-

A

1:4

37
Q

what do you use to test for iodine

A

starch

38
Q

what is ppm

A 
parts per million
g/1000000g water
g/1000dmcubed
mg/dmcubed
so ppm is just mg/dmcubed
39
Q

why can metals conduct electricity

A

cations in a sea of delocalised electrons that are free to move around the structure and carry charge

40
Q

why can ionic substnaces only someitmes conduct electricity

A

when molten or liquid the ions dissociate so the ions are free to move and carry charge about the structure

41
Q

what is the negative electrode

A

anode

where oxidation happens

42
Q

what is the postive electrode

A

cathode

where reduction happens

43
Q

describe electroysis

A

ionic solid heated in water. two metal rods (electrodes) inserted
the cations attract to the anode and the anions attract to the cathode

44
Q

what is BOD

A

biological oxygen demand

45
Q

what does a BOD mean

A

measure of dissolved O2 in ppm required to decompose the organic matter in water over a set period of time

46
Q

polluted water with a high bOD means

A

it cannot sustain aquatic life

47
Q

what does a high bod lead to

A

eutrophication

48
Q

how do you calcuate BOD

A

sample of water saturated with O2 so iniatl conc of dissolved O2 is known

incubated in the dark at a fixed temp for five days while microorganisms in the water oxidise the organic material

O2 content is fixed with excess of MgS

  • excess maganese salt added
  • potassium iodide which is oxideised by the manganese oxide to form iodine
  • iodine released is titrated with the standard sodium thiosulfate
49
Q

mole ratio of winkler method

A

1 mol O2 –> 2 mol I2 –> 4 mol S2O3 2-

50
Q

outline reasosn taht BOD may be different along downstream

A

oxygen demanding wastes broken down downstream decreasing BOD
as oxygen demanding wastes are higha t the form BOD is high
oxygen levels dcrease as oxygen is used up in the decomposition of plants

51
Q

what happens to O2 in step 1 of the winkler mthod

A

gets reduced as it gains electrons

IB Chemistry HL (12 decks)

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