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
