energetics Flashcards
def: enthalpy change
the change in heat energy at constant pressure
in order for a reaction to happen…
all the reactants need to break all their bonds,
all bonds in the products need to form
breaking bonds is an …. process
endothermic
(requires energy)
making bonds is an …. process
exothermic
releases energy
def: activation energy
the minimum amount of energy needed to start a reaction,
(amount of energy required to break the reactant bonds)
enthalpy diagram: exothermic
reactants
products
difference is -ve
enthalpy diagram: endothermic
products
reactants
difference is +ve
def: mean bond enthalpy
the energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds
the more positive the bond enthalpy…
the larger the amount of energy needed to break the bond,
so the stronger the bond
state why the enthalpy of formation of Na(s) is zero
Na is an element
state why the enthalpy of formation of liquid Na is not zero
Na(l) is not the standard state of Na
what substances cannot be combusted
water, CO2, most other oxides,
have a zero enthalpy of combustion
why may the enthalpy of combustion of a compound be difficult to?
incomplete combustion can occur + highly exothermic so difficult to monitor
why are some types of enthalpy change very difficult to measure experimentally?
reaction may have a very high activation energy,
reaction rate may be too slow,
possibility of competing reactions
give one reason why the bond enthalpy that you calculated above is different from the mean bond enthalpy quoted in a data book
the data book value is averaged over a range of compounds.
what do we use calorimetry for?
to measure the change in temperature of the surroundings
Q=mc delta t
Q = energy released or taken in during a reaction (J)
m = masss of solution or liquid that is heated up (g)
c = specific heat capacity
delta t = change in temp (K)
enthalpy change equation:
Q (kJ) / n (mol) = deltaH (kJmol-1)
assumption made in calorimetry:
the energy transferred to the water from the reacting chemicals is equal to the energy released by the reaction
sources of error in combustion calorimetry:
- heat lost to surroundings
- incomplete combustion of the fuel
- heat energy transferred to the metal calorimeter
- some fuel evaporates
improvments to minimise sources of error in combustion calorimetry:
- add a lid - reduces heat loss
- insulate sides of calorimeter - reduces heat loss
- reduce distance between flame and beaker - reduces heat loss
- put sleeve around the flame to protect it from draughts
sources of error in solution calorimetry:
- heat loss to surroundings
improvments to minimise sources of error in solution calorimetry:
- add a lid - reduces heat loss
- insulate sides of calorimeter - reduces heat loss
steps to measure an enthalpy change using a cooling curve:
- record the temperature for a suitable time (3 mins) before adding reactants together
- to establish an accurate initial temperature
- mix reactants at minute 4 then record the temperature every minute until a trend is seen (for at least 8-10 mins)
- plot a graph of temperature against time
- extrapolate the cooling curve back to the point of addition
- to establish the theoretical temperature change accounting for heat loss
- to reduce heat loss, use a polystyrene cup with a lid
a 50g sample of water was used in this experiment.
explain how you could measure out this mass of water without using a balance
water has a known density of 1.0gcm-3
a volume of 50cm3 could be measured out
suggest reasons why using powders increases the rate of reaction:
increases the surface area of reactants,
increases collision frequency/ rate of collisions
increase contact between particles
suggest reasons why extracting strontium is expensive (by heating a mixture of powdered oxide and powdered aluminium):
- reaction/process requires heat
- it is endothermic
