4 Energetics Flashcards
what is thermochemsitry
the study of heat changes during chemical reactions
during an endothermic reaction what happens to the heat energy
heat energy is absorbed from the surroundings
during an exothermic reaction what happens to the heat energy
heat energy is released to the surroundings
what is another phrase for enthalpy change
heat change
what are the standard conditions when meausirng enthalpy change
pressure of 100kPa
temperature of 298K
is ∆H negative or positive for exothermic reactions and why
the products end up with less heat energy than the starting materials because they have lost energy when they heated up their surroundings therefore ∆H id negative
is ∆H negative or positive for endothermic reactions and why
the produvts end up with more energy than the starting materials thefore is ∆H is negative
why is it imporntant to have a standard pressure for measuing enthalpy changes
pressure affects the amount of heat energy given out by reactions that involve gases.If a gas is given out some energy is required topush away the atmosphere the greater the atmospheric pressure the more energy is used for this.This kmeans less nergy remains to be given out as heat by the reaction
define standard enthalpy of combustion (∆cHƟ)
the enthalpy chnage when one mole of a substance is nurnt completely in oxygen under standard conditions
define standard enthalpy of formation (∆fHƟ).
the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions
define standard enthalpy of neutralisation (∆nHƟ).
is the enthalpy chnage when one mole of water is fomred in the reaction between an acid and a base
what si the equation used to energy transferred in a reaction
energy transferred
=mass of solution x specific heat capacity of water x temp change
what is the equation used to calculate enthalpy change
enthalpy change=energy transferred/number of moles
=mass of solution x specific heat capacity of water x temp change /number of moles
what assumptions do we make during calorimeter practicals
we assume complete combustion of the fuel
assume no heat is transferred from burner to surroundings
assume heat is evenly distributed to the water in the can
what improvements can be made in calorimeter practicals
metal can be used instead of a glass beaker as its a better conductor of heat
if glass beaker is used make sure it is covered using mineral wool as mineral wool isnt flammable
what does Hesses law state
states that the energy chnage for a chemical reaction is always the same regardless of the route taken to get from the reactants to the products
when do we use the combustion cycle
when given enthalpy combustion data
what does the combustion cycle show
arrows pointing down to combustion elements CO2 + H2O
how do you calculate ∆rHƟ in the combustion cycle
you flip the direction of the arrow from products combsution element so the arrow points upwards towards the products
when do we use the formation cycle
when given enthalpy of formation data
what does the formation cycle show
arrows pointing up from the elements to the reactants and products
how do you calculate ∆rHƟ in the formation cycle
you flip the direction of the arrow going from elements to reactants so the arrow points from the reactands down towards the elements at the bottom
what are the rules in combustion and formation cycles
the main equation is balanced
changind direction of arrow=chnaging the sign
you need the arrows to be rearranged so they follow the reactants to the products to find ∆rHƟ
enthalpy changes of an element is zero
define bond dissocaiton enthalpy
wnthalpy chnage required to break a covalent bond with all species in their gaseous state
define mean bond enthalpy
enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules
what do bond enthalpies do
they give a measure of the strength of bonds and can help predict which bond in in a molecule is most likely to break
explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law.
the mean bond enthalpies are averages whereas the answer obtained from hesss cyle are more accurate as the enthalpy of formation values have been obtained from the actual compunds involved.
explain why using bond dissociation energies only give you an approximate value for the overall energy change
bond energies are average values and the average value may be different from the bond energy in that particular environment
is endothemric reactions breaking or making bonds
breaking bonds
is exothermic reactions breaking or making bonds
making bonds
