enthalpy ( year 12 )

Question 1

green box : spirit burner practical ( for determination of enthalpy change of combustion )

Answer 1

1. pour 150 cm3 of water into measuring cylcinder and then pour into beaker. record intitial temp of water to nearest 05. degrees celc
2. • methanol–> spirit burner. weigh spirit burner containing methanol
3. place spirit burner under beaker. light burner nad burn methanol whilst stirring water w/ thermometer
4. after 3 minutes, extinguish the flame . record the max temperature reached by the water
5. reweigh spirit burner containing methanol. ASSUMPTION: WICK HASNT BEEN BURNT!!!!!

Question 2

factors that affect the accuracy of experiment enthalpy of combustion value?

Answer 2

.heat loss to surroundings other than water, heat lsos mainly to the air surrounding it

. incomplete combustion of methanl- Carbon monoxide produced instead of carbon dioxide. carbon layer would be black soot

.evaporateion of methanol from wick (COVER SPIRIT BURNER TO REDUCE ERROR! ) ( burner MUST be weighed AS SOON AS methanol evaporates from wick !!! )

.non standard conditons

all but last of these reasons lead to enthalpy of combustion being less than expected

Question 3

the limitations of using average bond enthalpies to calculate enthalpy change

Answer 3

actual energy involved in breaking bonds and making bonds are slightly different/ mean bond energies ARE NOT EXACT ! ( bonds may be in different environments )

Question 4

Why do reactions with small activation energies takep alce very rapidly ?

Answer 4

the energy needed to break bonds is readily available from the surrroundings

Question 5

enthalpy of formation of elements

Answer 5

its 0

Question 6

what about hte value of enthalpy of neutralisation

Answer 6

ITS THE SAME FOR ALL REACTIONS !

Question 7

DETERMINATION OF ENTHALPY CHANGE OF NEUTRALISATIOM (PAG 3)

Answer 7

1. measure 25 cm3 of HCL with measuring cylinder and add to polystyrene cup
2. place cup in 250 cm3 glass beaker
3. make table to record temperature of acid at minute intervals up to 10 minutes
4. measure 25 cm3 NaOH into measuring cylinder
5. start timer and record temp of HCL in cup
6. continue recording temp for each minute for three minutes
7. fouth minute: add NaOH. Dont recordt emp
8. 5th minute: continue recordign temp up to 10 minutes . STIR solution in cup EACH time temperature is recorded

.CALCULATONS:
plot graph of temperature against time, draw line of best fit through points before addition of sodium hydroxide and then draw second second line of best fit through the points after the addition of odium hydroxide. Extrapolate line before the reactants were added together . calculate energy absorbed by q=mc (delta T ). calculate amount of HCL used, calculate the enthalpy of neutralisation

Question 8

GREEN BOX COOLING CURVE

Answer 8

1. pipette 25 cm3 of 1 moldm3 of cuso4 in polystyrene cup. weigh out excess zinc powder!
2. start stop clock and take temperature of solution every 30 seconds until temeperature stays constant
3. add zinc to solution and stir and record temp every 30 seconds until temperature has fallen for several mintues
4. plot graph of temperature agaisnt time

correct for cooling by extrapolating cooling curve section of graph back to ttime when zinc was added
. draw vertical line from time that solutions were mixed to the extrapolated cooling curve