Molar mass of potassium hydrogen carbonate
Mr= 39.1+1.0+12.0+48
= 100g mol-1
Number of moles= m/Mr
= 4/100
= 0.04 mol
H for 1 mole= 1.3794KJ/0.04
= 34.49KJmol-1
Experiment 2:
Mass of weighing bottle and magnesium carbonate: 20.75g
Mass of weighing bottle: 8.75g
Mass of magnesium carbonate: 2.00g
Magnesium carbonate and hydrochloric acid:
MgCO3(S) + 2 HCl (aq) MgCl2 (aq) +H2O (l) + CO2 (g)
Rise in temperature for complete reaction:
24.5-30.0= (-5.5C)
H= -5.5C, Mass= 50g, C= 4.18Jg-1K-1
H2= MxCx T
= 50x4.18x (-5.5)
= -1149.51J/1000
= -1.1495KJ
Number of moles of acid = (50/100) x2 = 1 mole in excess
Molar mass of potassium hydrogen carbonate
Mr= 24+12+48
= 84g mol-1
Number of moles= m/Mr
= 2/84
= 0.02381 mol
H for 1 mole= -1.1495KJ/0.02381mol
= -48.3KJmol-1
H
MgCl2 (aq) +KHCO3 (g) +H2O (l) + CO2 (g) MgCO3(S) + 2KCl (aq) + 2H2O (l) + 2CO2 (g)
MgCO3(S) +2 KHCO3 (s) + 2 HCl (aq)
H3 = -2(H4) - H2+2(H1)
= -2(+121) - (-48.3) + (68.98)
= -242 – (-48.3) +68.98
= -124.72
Percentage Error
Temperature error = (05/11) x100+ (0.5/5.5) x100
= 4.55+9.09
= 13.64%
Stopwatch error = (0.01/14) x100
= 0.07%
Over all percentage error= Temperature error + stopwatch error
= 13.64% + 0.07%
Sources of error
-
The experimentally derived value for the enthalpy of it is slightly low because of the heat absorbed by the polystyrene cup and by the thermometer.
- A cm of the solution was assured to be equal to a gram of the solution.
- The specific heat capacity of water was taken as the specific heat capacity of the solution/ liquid.
- The heat lost to the surrounding was accounted for by extrapolating the graph of the temperature change against time.
- The accuracy of the burette was not accurate.
- The specific heat capacity of the cup was not accounted.
How the errors could be minimised?
- Use temperature sensors instead of the thermometer
- Use a vacuum flask rather than polystyrene cup and keep the lid shut. So the heat doesn’t escape.
