Results Table:
Therefore,
- The average change in temperature for Mg is (9+10)/2=9.5°C or 9.5K.
- The average change in temperature for MgO is (15+15.2)/2=15.1°C or 15.1K.
Calculations:
For Mg=
Energy change= mass x specific heat capacity x change in temperature.
➔mass of water= mass of acid (ASSUMING)
➔specific heat capacity of water= 4.2kJ/kgK
➔change in temperature= 9.5 K
= 0.05kg x 4200 J/kgK x 9.5 K
= 1995J/mol
= 1.995 kJ/mol
This is the amount of energy which is given out for 0.24g.
If we convert this to mol, we will get = (mass/molar mass) = (0.24g/24gmol-1) = 0.01mol.
Therefore, for one mol the energy given out would be:
1.995 kJ/mol x 100 = 199.5 kJ/mol.
For MgO=
Energy change= mass x specific heat capacity x change in temperature.
➔mass of water= mass of acid (ASSUMING)
➔specific heat capacity of water= 4.2kJ/kgK
➔change in temperature= 15.1 K
= 0.025kg x 4200 J/kgK x 15.1 K
= 1585.5 J/mol
= 1.5855 kJ/mol
This is the amount of energy which is given out for 0.1g of MgO.
If we convert this to mol, we’ll get = (mass/molar mass) = (0.1g/ 40.3gmol-1) = 0.00248mol.
Therefore, for one mol the energy given out would be:
0.00248 kJ/mol x 100 = 0.248 kJ/mol.
Therefore by Hess’ Law, we can calculate the heat of formation of the reaction because:
Enthalpy change of a reaction is the same regardless of what pathway is taken to achieve the products.
Therefore,
Heat of reaction= heat of formation of reactants + heat of formation of products.
Heat of reaction= 199.5 kJ/mol + 0.248 kJ/mol
Heat of reaction= 199.748 kJ/mol
Conclusion and Evaluation:
RANDOM ERRORS:
-
Volume acid used for Mg = +0.00005g [balance]
➔ (+0.00005g/50) x 100= 0.0001%
-
Volume acid used for MgO= +0.00005g [balance]
➔ (+0.00005g/25) x 100=0.0002%
-
Thermometer (+0.5°C) [x 8(initial and final for 2 trials for Mg and MgO)]= 4°C
➔(4°C/237.2°C)= 0.01686%
Total error= 0.0001 + 0.0002 + 0.01686 = 0.01716% of 0.248 kJ/mol.
➔ (0.01716/100) x 0.248 = 4.2565x10-5
Hence we have been able to calculate the change in energy of a reaction, that wasn’t possible by a direct method, using an indirect method. This was done by using Hess’ Law.
Since we do not know the expected value, we cannot compare the answer we got to the actual answer but some the errors which could have occurred are:
- Reusing the polystyrene cup can lead us to inaccurate results as the second reaction could be contaminated if the cup was not washed properly with distilled water.
- The lid on the polystyrene cup was placed on it only after both the substances were put into the cup and the reaction started occurring. This would give us inaccurate results as the temperature was not measure justly as a lot of initial heat given off in the reaction was not taken into account.
- The thermometers could have been influenced by the temperature outside of the cup as the experiment was being carried out next to a window.
- During a couple of the trials, the lid was not able to be fixed properly on the cup.
- One of the major errors was the fact that we assumed that the hydrochloric acid was equal to the amount of water. We can’t really change this considering that 100% pure HCl is in gaseous state. It is an error that we must take account of though.
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