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developing fuels 1 (df1-df4) (Hess's law (the law (the total enthalpy…
developing fuels 1 (df1-df4)
gas volumes
using a gas syringe allows you to measure gas volume, measures TOTAL volume (not separate volumes for each gas produced)
pV=nRT
for any temp and pressure
T= temperature (K)
R= gas constant (8.314 Jk-1 mol-1)
n= number of moles
V=volume (m3)
p= pressure (Pa)
number of moles= volume (dm3) / 24
equation to use when pressure and temp are constant (25 degrees/ 298K and 100kPa
enthalpy changes
bond enthalpy is related to the length of the bond
the stronger the attraction between the atoms, the higher the bond enthalpy, the shorter the bond length
the distance between the two nuclei is the distance where the attractive and repulsive forces balance. this distance is the bond length
and the positively charged nuclei also repel eachother and so do the negatively charged electrons
in covalent molecules, the positive nuclei are attracted to the negative electrons
average bond enthalpies are not exact
its the average of a wide range of molecules, not necessarily correct for one specific bond
enthalpy change can be calculated using average bond enthalpies
stronger bonds have higher bond enthalpies
enthalpy change of reaction= total energy absorbed to break bonds - total energy released in making bonds
the enthalpy change for the overall reaction is the overall effect of these changes
energy is released when bonds are FORMED making it exothermic
energy is need to BREAK bonds so is endothermic
exo or endo
endothermic= absorb energy- enthalpy change is positive
exothermic= gives out energy - enthalpy change is negative
chemical reactions have enthalpy changes
standard conditions= 298K and 100kPa and standard state of element
caused by bond breaking and bond making
enthalpy change= the heat transferred in a reaction at a constant pressure units are kJ mol-1
more on enthalpy changes
q= mct
t= change in temp of water/ solution (K)
c= specific heat capacity of water (4.18Jg-1 K-1)
m= mass of water
q= heat lost of gained
calorimetry
to reduce heat loss to surroundings use a BOMB CALORIMETER
neutralization
2) add a known volume of alkali and record the temp rise
1) add a known volume of acid to an insulated container and measure the temp
combsution
3) ideally all the heat given out would be absorbed by the water but some heat is usually lost to the surroundings
2) as the fuel burns it heats the water
1) to find the ENTHALPY OF COMBUSTION of a flammable liquid you burn it
= you can find out how much heat is given out by a reaction by measuring the temperature change of water
enthalpy level diagrams
endo
products at a higher level than reactants
exo
reactants at a higher level than products
the less enthalpy a substance has the more stable it is
activation energy= the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
shows how the enthalpy changes in a reaction
Hess's law
standard enthalpy change of neutralization
= the enthalpy change when an acid and an alkali react together under standard conditions to form one mole of water
the law
the total enthalpy change of a reaction is always the same no matter which route is taken
standard enthalpy change of combustion
= the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions
standard enthalpy change of formation
= the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
standard enthalpy change of reaction
= the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions
combustion arrows point downwards, formation arrows point upwards