C4 Flashcards
enthalpy change
heat energy change at constant pressure
temperature
average kinetic energy of particles
heat
total energy of all particles present
standard molar enthalpy of formation
Enthalpy change when 1 mole of compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states.
standard molar enthalpy of combustion
Enthalpy change when 1 mole of substance is completely burnt in oxygen under standard conditions, all reactants and products in their standard states.
specific heat capacity
the amount of heat energy required to raise the temperature of 1g of substance by 1K with no change of state.
hess’s law
the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
bond dissociation enthalpy
the enthalpy change required to break a covalent bond with all species in the gaseous state.
mean bond enthalpy
the mean amount of energy required to break one mole of a specified type of covalent bond in a gaseous species.
standard conditions measuring enthalpy change
100kPa . 298K
enthalpy change equation
q= mc deltaT
q= enthalpy change J
m= mass of substance being heated g
c = specific heat capacity J-1Kg-1
delta T= temperature change (K)
to measure enthalpy change
heat needs to be transferred to a particular mass of a substance, often water
need to know 3 things:
- mass of substance being heated/cooled
- temperature change
- specific heat capacity of the substance
why enthalpy of combustion developed experimentally is less exothermic than that calculated using enthalpies of formation
heat loss e.g. poor insulation
incomplete combustion
why is O=O bond enthalpy NOT a mean value
O2 is the only substance with an O=O bond
when using a calorimeter…
insulate sides
wind shield
to reduce heat loss
reducing % uncertainty
increase concentration/ mass of reactants
why is experimental bond enthalpy values different from those quoted in a data book?
data book value= derived from many different compounds, not just the compound in experimet
–> it is a mean value
standard enthalpy of formation for an element in its standard state
0
energy must be ___ to break bonds and ___ when bonds are formed
put in, given out
improving an enthalpy change experiment (not a flame calorimeter)
- pipette instead of measuring cylinder
- use polystyrene cup not beaker- better insulation
- re-weigh watch glass after adding solid- subtract from value weighed out
- record the temp of the solution initially for a few mins before addition of solid
- measure and record temp after addition at regular intervals for 8+ minutes
- plot a graph of temp/time
- extrapolate to point of addition
—> determine deltaT at this point
to reduce errors due to heat loss in flame calorimeter
- reduce distance between flame and beaker
- sleeve around flame to protect from drafts
law of conservation of energy
energy can never be created or destroyed
if one substance is of lower energy than other, it is…
energetically more stable
the standard state of carbon
graphite (s, graphite)
