energetics Flashcards
system
the chemicals being studied (reactants +products)
- we measure the energy change of the system
surrounding
everything around th esystem (container, air, water)
- we cant measure the temp of the system - so we measure the surrounding temp
exothermic
energy of system decreases
- heat lost to surroundings
temp of surroundings increases
-ΔH
endothermic
energy of system increases
- heat taken in from surroundings
temp of surroundings decreases
+ΔH
energy level diagram
energy on y axis
reaction progress on x
reactants +/-ΔH then products
reaction profile diagram
energy level diagram with activation evergy
why reaction cant react when acid is solid
acids need to be aq to react so ions can move and can release H+ ions
why use steel wool not a nail
larger suraface area increases rate of reaction
what factors affect temperature change measured
- heat loss to surroundings
- stir - removes hot spots
- distance between flame and calorimeter
- vol of solution - less = faster = less heat loss
- initial water temp - hot = more heat loss
- incomplete combustion
calorimeter
any apparatus used to measure ΔT
Q (heat energy transferred )equation
Q = mcΔT
(mass) x (specific heat capacity) x (temp chnage)
specific heat capacity
4.18
amount of heat energy needed to raise the temp of 1g of a substance by 1degree
assumptions in calorimetry
- all heat is absorbed/released by the water present
- any aq mixture has a c = to water
- any aq mixture has a density = to water (vol = mass)
mass in calorimetry
vol = mass
add mass of 2 solutions
ignore mass of solids
enthalpy change
molar energy change (ΔH)
ΔH equation
ΔH = (Q/1000)/n
(energy transfered KJ)/ (amount of LIMITING reactant)
ΔH sign
-ΔH = exo
+ΔH = endo
%error between experimental and true values
= (difference between experimental +true /true) x 100
calculated vs true values
ΔH was less exo/endothermic than true values - lots of heat loss to surroundings/not transfered to calorimeter
sources of error in experiment
- incomplete combustion - not as much energy release lower ΔT
- heat loss to surroundings lower ΔT
- lid not put on quickly - fuel evaporates - larger mass and n so less exo ΔH
- assumption all heat is absorbed by water not equiptment - lower ΔT
modifications to make final values more acurate
- add lid - insulation
- polestyrene cup - insulation
- insulation to sides /draught sheild
- stir - minimise hotspots
- make flame closer to water
- add O2 supply to base to encourage complete combustion
neutralisation reaction
EXOTHERMIC
reaction in solution
eg HCl +NaOH -> NaCl + H2O
- record intial temp of both solutions
- add base
- add acid
- stir and record max temp reached
displacment reaction
EXOTHERMIC
reaction in solution
eg Zn + CuSO4 -> ZnSO4 +Cu
- add CuSO4 solution to polestyrene cup
- record intial temp
- add Zn
- stir and record max temp reached
combustion reaction
EXOTHERMIC - ethanol
- put water in calorimeter and record initial temp
- record initial mass of fuel
- light fuel under water
- record max temp reached
- record final mass of fuel
salts dissolving in water
- record initial temp of water and salt
- put water in cup
- put salt in water
- record max temp reached
see pics for calorimeters, energy level diagrams
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bond breaking is
endothermic - takes in energy from surroundings
bond making is
exothermic - gives out energy to surroundings
so if bond breaking energy is more then bond making energy
if the energy taken in to break bonds is more than the energy given out when making bonds it is endothermic
so if bond breaking energy is less then bond making energy
if the energy taken in to break bonds is less than the energy given out when making bonds it is exothermic
activation energy
the minimum energy needed to start a reaction
used to break bonds so a reaction can happen
change in H in terms of bond making/ breaking
bonds broken - bonds made
mean bond energy
the average amount of energy needed to break 1 mole of a particular type of covalent bond
why is mean bond energy sometimes inaccurate
bond energy is an average from a range of compounds - strength of bonds can vary depending on atoms in the molecule
