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developing fuels > DF 4 - where does the energy come from > Flashcards

DF 4 - where does the energy come from Flashcards

specification reference - (e)

1
Q

what factor is the most important thing to consider for a fuel

A

the energy density

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2
Q

what is the energy density of a fuel

A

how much energy you get per kg of a fuel

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3
Q

how can we work out the energy density of a fuel

A

this can be worked out from the enthalpy change of combustion using the relative molecular mass.

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4
Q

what do all chemical reactions involve

A

breaking and making chemical bonds

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5
Q

what must happen for a chemical reaction to occur

A

bonds have to break in the reactants so that new bonds can form in the products.

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6
Q

where do the energy changes in chemical reactions come from

A

the energy changes that happen when bonds are broken and made

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7
Q

what is a chemical bond

A

its basically electrical attraxtion between atoms or ions.

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8
Q

what does breaking a bond involve

A

breaking a bond involves overcoming these attractive forces.

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9
Q

what is theoretically needed for a bond to break completely

A

the atoms or ions need to be an infinite distance apart

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10
Q

what holds two hydrogen atoms together

A

two hydrogen atoms are bonded by a shared pair of electrons. both nuclei are attracted to the same shared pair of electrons

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11
Q

what is bond enthalpy

A

the quantity of energy needed to break a particular bond in a molecule

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12
Q

what do bond enthalpies indicate

A

how strong bonds are. the stronger the bond, the more energy is needed to break it and the higher its bond enthalpy

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13
Q

why do atoms move together when bonds form

A

there are attractive forces between the nuclei and electrons

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14
Q

what other forces are present between the nuclei of the two bonded atoms

A

repulsive forces

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15
Q

when do the repulsive forces between bonded atoms get bigger

A

as the atoms approach more closer together. they eventually stop moving

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16
Q

what is the equilibrium bond length

A

the distance between two atoms when they stop moving closer together

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17
Q

the shorter the bond length…

A

the stronger the attraction between the atoms

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18
Q

what is an average bond enthalpy

A

the average quantity of energy needed to break a particular bond

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19
Q

what does the exact value of a bond enthalpy actually depend on

A

on the particular compound in which the bond is found

20
Q

which have higher bond enthalpies - single or double bonds

A

double bonds
- triple bonds have a higher bond enthalpy and so on

21
Q

the higher the bond enthalpy…

A

… the shorter the bond

22
Q

why do shorter bonds have a higher bond enthalpy

A

there are more electrons between the atoms being attracted to the positive nuclei. more attraction makes shorter bonds.

23
Q

what makes it difficult to measure bond enthalpies

A

the fact that there is often more than one type of bond in a compound. its also very difficult to make measurements when everything is in the gaseous state.

24
Q

how are bond enthalpies measured indirectly

A

using enthalpy cycles

25
Q

CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g).
in this reaction which bonds are being broken?

A

the C-H bonds in methane and the bond between oxygen in two oxygen molecules

26
Q

what is required for bonds to be broken

A

energy

27
Q

what can happen once the bonds are broken

A

atoms can join together to form new bonds

28
Q

CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g).
in this reaction which bonds are being formed

A

two carbon-oxygen double bonds
four oxygen-hydrogen bonds

29
Q

what do bond enthalpies always refer to

A

breaking bonds in the gaseous compound. this is to make comparisons between different bonds

30
Q

why dont you have to break all the old bonds before you start making new ones

A

new ones start forming as soon as the first of the old bonds are broken

31
Q

is bond breaking endothermic or exothermic

A

endothermic

32
Q

is bond making endothermic or exothermic

A

exothermic

33
Q

when is the reaction exothermic

A

if the energy taken in during the bond breaking steps is less than the energy given out during the bond making steps

34
Q

when is the reaction endothermic

A

if the energy taken in during the bond breaking steps is more than the energy given out during the bond making steps

35
Q

why is bond breaking endothermic

A

energy needs to be gained from the surroundings to break the bonds

36
Q

why is bond making exothermic

A

energy is released to the surroundings when new bonds are made

37
Q

why do many reactions need heating to get them started

A

as you need to break bonds before product molecules can begin to form. all reactions need energy initially to stretch and break bonds

38
Q

why is the energy available from room temperature adequate for some reactions

A

some reactions only need a little bit of energy which is available in room temperature heat

39
Q

why are bond enthalpies always positive

A

bond breaking is endothermic

40
Q

what would happen if it was necessary for all the bonds to break before a reaction gets going

A

you would have to heat things to a very high temperature to get them to react

41
Q

what can start happening once a few of the bonds break

A

new bonds can start to form and this usually gives enough energy to keep the reaction going

42
Q

which reactions need continuous heating

A

slightly exothermic reactions

43
Q

give the enthalpy cycle to show bond breaking and bond making in the combustion of methane

A

ΔH1
CH₄(g) + 2O₂(g) —> CO₂(g) + 2H₂O(g)
\ /
\ ΔH2 ΔH3 /
\ /
C(g) + 4H(g) + 4O(g)

44
Q

use this cycle to work out a value for the enthalpy change of combustion for methane
ΔH1
CH₄(g) + 2O₂(g) —> CO₂(g) + 2H₂O(g)
\ /
\ ΔH2 ΔH3 /
\ /
C(g) + 4H(g) + 4O(g)

A

1) enthalpy change when bonds are broken
4 x (C-H) + 2 x (0=0)
= +2648kJmol-1
—–> +ve enthalpy (endo)
2) enthalpy changes when bonds are made
2 x (C=O) + 4 x (O-H)
= -3466kJmol-1
——-> -ve enthalpy (exo)
3) ΔH1 = ΔH2 + ΔH3
+2648kJmol-1 + (-3466kJmol-1)
= - 818kJ mol-1

45
Q

why is the value for enthalpy changes different from values given in different data sheets

A

1) the value of ΔH1 is not the standard value. the water product is gaseous, H₂O is gaseous not liquid as it should be when under standard conditions. H₂O is used because when using bond enthalpies you have to work in the gasesous state
2) bond enthalpies are averages from several compounds so results from such calculations aren’t always precise

46
Q

why are bond enthalpies useful

A

they enable enthalpy changes to be measured when there is little specific data for a compound

47
Q

what is the equation for enthalpy change

A

enthalpy = bonds - bonds
change broken made

developing fuels (11 decks)

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