DF 4 - where does the energy come from

Question 1

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

Answer 1

the energy density

Question 2

what is the energy density of a fuel

Answer 2

how much energy you get per kg of a fuel

Question 3

how can we work out the energy density of a fuel

Answer 3

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

Question 4

what do all chemical reactions involve

Answer 4

breaking and making chemical bonds

Question 5

what must happen for a chemical reaction to occur

Answer 5

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

Question 6

where do the energy changes in chemical reactions come from

Answer 6

the energy changes that happen when bonds are broken and made

Question 7

what is a chemical bond

Answer 7

its basically electrical attraxtion between atoms or ions.

Question 8

what does breaking a bond involve

Answer 8

breaking a bond involves overcoming these attractive forces.

Question 9

what is theoretically needed for a bond to break completely

Answer 9

the atoms or ions need to be an infinite distance apart

Question 10

what holds two hydrogen atoms together

Answer 10

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

Question 11

what is bond enthalpy

Answer 11

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

Question 12

what do bond enthalpies indicate

Answer 12

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

Question 13

why do atoms move together when bonds form

Answer 13

there are attractive forces between the nuclei and electrons

Question 14

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

Answer 14

repulsive forces

Question 15

when do the repulsive forces between bonded atoms get bigger

Answer 15

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

Question 16

what is the equilibrium bond length

Answer 16

the distance between two atoms when they stop moving closer together

Question 17

the shorter the bond length…

Answer 17

the stronger the attraction between the atoms

Question 18

what is an average bond enthalpy

Answer 18

the average quantity of energy needed to break a particular bond

Question 19

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

Answer 19

on the particular compound in which the bond is found

Question 20

which have higher bond enthalpies - single or double bonds

Answer 20

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

Question 21

the higher the bond enthalpy…

Answer 21

… the shorter the bond

Question 22

why do shorter bonds have a higher bond enthalpy

Answer 22

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

Question 23

what makes it difficult to measure bond enthalpies

Answer 23

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.

Question 24

how are bond enthalpies measured indirectly

Answer 24

using enthalpy cycles

Question 25

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

Answer 25

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

Question 26

what is required for bonds to be broken

Answer 26

energy

Question 27

what can happen once the bonds are broken

Answer 27

atoms can join together to form new bonds

Question 28

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

Answer 28

two carbon-oxygen double bonds four oxygen-hydrogen bonds

Question 29

what do bond enthalpies always refer to

Answer 29

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

Question 30

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

Answer 30

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

Question 31

is bond breaking endothermic or exothermic

Answer 31

endothermic

Question 32

is bond making endothermic or exothermic

Answer 32

exothermic

Question 33

when is the reaction exothermic

Answer 33

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

Question 34

when is the reaction endothermic

Answer 34

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

Question 35

why is bond breaking endothermic

Answer 35

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

Question 36

why is bond making exothermic

Answer 36

energy is released to the surroundings when new bonds are made

Question 37

why do many reactions need heating to get them started

Answer 37

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

Question 38

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

Answer 38

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

Question 39

why are bond enthalpies always positive

Answer 39

bond breaking is endothermic

Question 40

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

Answer 40

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

Question 41

what can start happening once a few of the bonds break

Answer 41

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

Question 42

which reactions need continuous heating

Answer 42

slightly exothermic reactions

Question 43

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

Answer 43

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

Question 44

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)

Answer 44

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

Question 45

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

Answer 45

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

Question 46

why are bond enthalpies useful

Answer 46

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

Question 47

what is the equation for enthalpy change

Answer 47

enthalpy = bonds - bonds change broken made