DF 4 - where does the energy come from Flashcards
specification reference - (e)
what factor is the most important thing to consider for a fuel
the energy density
what is the energy density of a fuel
how much energy you get per kg of a fuel
how can we work out the energy density of a fuel
this can be worked out from the enthalpy change of combustion using the relative molecular mass.
what do all chemical reactions involve
breaking and making chemical bonds
what must happen for a chemical reaction to occur
bonds have to break in the reactants so that new bonds can form in the products.
where do the energy changes in chemical reactions come from
the energy changes that happen when bonds are broken and made
what is a chemical bond
its basically electrical attraxtion between atoms or ions.
what does breaking a bond involve
breaking a bond involves overcoming these attractive forces.
what is theoretically needed for a bond to break completely
the atoms or ions need to be an infinite distance apart
what holds two hydrogen atoms together
two hydrogen atoms are bonded by a shared pair of electrons. both nuclei are attracted to the same shared pair of electrons
what is bond enthalpy
the quantity of energy needed to break a particular bond in a molecule
what do bond enthalpies indicate
how strong bonds are. the stronger the bond, the more energy is needed to break it and the higher its bond enthalpy
why do atoms move together when bonds form
there are attractive forces between the nuclei and electrons
what other forces are present between the nuclei of the two bonded atoms
repulsive forces
when do the repulsive forces between bonded atoms get bigger
as the atoms approach more closer together. they eventually stop moving
what is the equilibrium bond length
the distance between two atoms when they stop moving closer together
the shorter the bond length…
the stronger the attraction between the atoms
what is an average bond enthalpy
the average quantity of energy needed to break a particular bond
what does the exact value of a bond enthalpy actually depend on
on the particular compound in which the bond is found
which have higher bond enthalpies - single or double bonds
double bonds
- triple bonds have a higher bond enthalpy and so on
the higher the bond enthalpy…
… the shorter the bond
why do shorter bonds have a higher bond enthalpy
there are more electrons between the atoms being attracted to the positive nuclei. more attraction makes shorter bonds.
what makes it difficult to measure bond enthalpies
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.
how are bond enthalpies measured indirectly
using enthalpy cycles
CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g).
in this reaction which bonds are being broken?
the C-H bonds in methane and the bond between oxygen in two oxygen molecules
what is required for bonds to be broken
energy
what can happen once the bonds are broken
atoms can join together to form new bonds
CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g).
in this reaction which bonds are being formed
two carbon-oxygen double bonds
four oxygen-hydrogen bonds
what do bond enthalpies always refer to
breaking bonds in the gaseous compound. this is to make comparisons between different bonds
why dont you have to break all the old bonds before you start making new ones
new ones start forming as soon as the first of the old bonds are broken
is bond breaking endothermic or exothermic
endothermic
is bond making endothermic or exothermic
exothermic
when is the reaction exothermic
if the energy taken in during the bond breaking steps is less than the energy given out during the bond making steps
when is the reaction endothermic
if the energy taken in during the bond breaking steps is more than the energy given out during the bond making steps
why is bond breaking endothermic
energy needs to be gained from the surroundings to break the bonds
why is bond making exothermic
energy is released to the surroundings when new bonds are made
why do many reactions need heating to get them started
as you need to break bonds before product molecules can begin to form. all reactions need energy initially to stretch and break bonds
why is the energy available from room temperature adequate for some reactions
some reactions only need a little bit of energy which is available in room temperature heat
why are bond enthalpies always positive
bond breaking is endothermic
what would happen if it was necessary for all the bonds to break before a reaction gets going
you would have to heat things to a very high temperature to get them to react
what can start happening once a few of the bonds break
new bonds can start to form and this usually gives enough energy to keep the reaction going
which reactions need continuous heating
slightly exothermic reactions
give the enthalpy cycle to show bond breaking and bond making in the combustion of methane
ΔH1
CH₄(g) + 2O₂(g) —> CO₂(g) + 2H₂O(g)
\ /
\ ΔH2 ΔH3 /
\ /
C(g) + 4H(g) + 4O(g)
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)
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
why is the value for enthalpy changes different from values given in different data sheets
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
why are bond enthalpies useful
they enable enthalpy changes to be measured when there is little specific data for a compound
what is the equation for enthalpy change
enthalpy = bonds - bonds
change broken made
