1.4 energetics Flashcards
what is enthalpy change?
heat energy transferred in a reaction at constant pressure.
overall effect of bond making/ breaking
units of enthalpy change
KJmol-1
what are standard conditions?
100KP (1 atm) pressure and 298K/25c
exothermic reactions
give energy to surroundings
temp up
products less energy than reactant
- enthalpy change
exo reaction examples
oxidation, respiration, neutralisation
endothermic reactions
take in energy from surroundings
temp down
products more energy than reactant
+ enthalpy change
endo reaction examples
photosynthesis, decomposition, sodium carbonate+ ethanoic acid
what are bond enthalpies?
energy needed to break a bond
atoms in molecules held by covalent bonds so take energy to break and release to make
is bond breaking exo or endo?
endo
is bond making exo or endo?
exo
what is a mean bond enthalpy?
average energy needed to break certain type of bond over range of compounds
why do we have MEAN bond enthalpies?
because energy needed depends on where bond is
eg c-c bond needs diff amount of energy in methane than ethane
How do you calculate enthalpy change?
total energy absorbed-total energy released
use average bond enthalpies to work these out
what are the different types of enthalpy change?
standard enthalpy change of formation
standard enthalpy change of combustion
standard enthalpy change of reaction
what is standard enthalpy change of formation?
1 mol of a compound formed from elements in standard states and conditions
what is standard enthalpy change of combustion?
1 mol of substances completely burned in oxygen under standard conditions and products and reactants in standard states
what is standard enthalpy change of reaction?
when a reaction occurs in molar quantities shown in chemical equation under standard conditions with reactants and products in standard states
what do you need to know to work out enthalpy change?
moles of reactant
temperature change
mass of stuff youre heating
what is a calorimetry experiment?
experiment measuring heat given out
calorimetry and combustion reactions
measure temp change as reaction burns to see energy given out
to find enthalpy of combustion of liquid, burn in calorimeter
fuel burns which heats water. can find energy absorbed if know mass, temp change, specific heat cap of water
why may calorimetry of combustion reactions be inaccurate?
not all energy is absorbed by water- some heats surroundings
some fuel may evaporate
combustion may be incomplete (less energy given off)
how can you calculate an accurate temperature change?
- record temp at reg intervals starting couple mins before start of reaction
- plot graph
- draw 2 lines of best fit- 1 before reaction, 1 during
- extend the lines- distance between them at start of reaction is temp change.
how do you calculate standard enthalpy change of combustion?
- q=m x c x temp change
- convert q from J to KJ
- calculate mol of fuel from mass reacted
- enthalpy change=q/n
Hess’ law
The total enthalpy change for a reaction is independent of the route taken
When is Hess’ law useful?
working out enthalpy change you cant find directly by doing experiment
eg if you know enthalpy change for route 2, you can calculate enthalpy change for route 1
Why is the value for the enthalpy change of formation of elements 0?
elements are being formed from the element so there is no change in enthalpy
alreadyin its standard state
steps for using enthalpies of formation
write balanced equation
write elements balanced and in standard states underneath
draw arrows from elements to equation work out enthalpy change of formation for reactants and products. reverse the arrow going to reactants and change sign then find total
route 2 = route 1
steps for using enthalpies of combustion
write balanced equation
write combustion products underneath
draw arrows pointing down and work out total enthalpies for reactants and products
reverse second arrow and sign of the value then find total
route 2 = route 1
why do arrows point upwards when using enthalpies of formation?
enthalpy of formation tells you enthalpy change from elements to compounds
Why can we say that the volume of liquid used in calorimetry is equal to the mass?
we can assume that the density is 0
practicals in this topic
measuring enthalpy change
