# Hess Law

Topics: Thermodynamics, Enthalpy, Observational error Pages: 7 (795 words) Published: September 30, 2010
IB CHEMISTRY LAB REPORT
PRACTICAL 13 : HESS’SLAW
PRACTICAL ASSESSMENT : DCP,CE

Collecting raw data:

Part 1

Weight of magnesium ribbon,Mg,m/g
(± 0.01)
Initial temperature of hydrocloric acid,HCl,T/C
(± 0.05)
Final tmperature of
HCl + Mg,T/C
(± 0.05)

0.31
26.00
57.50

Part 2

Weight of magnesium oxide,MgO,m/g
(± 0.01)
Initial temperature of hydrocloric acid,HCl,T/C
(± 0.05)
Final tmperature of
HCl + Mg),T/C
(± 0.05)

0.50
27.50
40.50

Processing Raw Data :

Part 1

Equation of the reaction;

Mg(s) + 2HCl(ℓ) → MgCl2(aq) + H2(g)

no of moles :

no of moles of Mg = mass/molar mass
= 0.31g___
24.31g/mol
= 0.01275 moles

no of moles of HCl = molarity x volume
= 1.0M x (25/1000)mL
= 0.025 moles

 heat of reaction will be 0.025 moles as HCL act as the
limiting reactant.

Enthalpy change of the reaction,H2

Volume of HCL solution = 25 mL

Assume the solution has the same density and specific heat capacity as water.

Volume of HCL solution = 25 mL

ρ = mass/volume
1.0g/cm³ = mass/ 25 cm³
m = 25g
= 0.025kg ± 0.00001

Temperature change, T = 57.50 – 26.00
= 31.5C ± 0.1

Heat evolved,Q = mHCLcwT
= 0.025kg x 4200Jkg-1K-1x 31.5C
= 3.3075 kJ

 H = -Q/1000n
= -3.3075kJ/0.025 mol
= -132.3 kJmol-1

Uncertainty of enthalpy change of reaction of H2

Part 1

The uncertainty for the reaction above is as follows:

Uncertainty of ∆H2=
= 0.01 + 0.1
25.00 31.5

= ± 0.004

Part 2

Equation of the reaction;

MgO(s) + 2HCl(ℓ) → MgCl2(aq) + H2O(g)

no of moles :

no of moles of MgO = mass/molar mass
= 0.50g___
40.31g/mol
= 0.01240 moles

no of moles of HCl = molarity x volume
= 1.0M x (25/1000)mL
= 0.025 moles

 heat of reaction will be 0.0025 moles as HCL act as the
limiting reactant.

Enthalpy change of the reaction,H3

Volume of HCL solution = 25 mL

Assume the solution has the same density and specific heat capacity as water.

Volume of HCL solution = 25 mL

ρ = mass/volume
1.0g/cm³ = mass/ 25 cm³
m = 25g
= 0.025kg ± 0.00001

Temperature change, T = 40.50 – 27.50
= 13.0C ± 0.1

Heat evolved,Q = mHCLcwT
= 0.025kg x 4.2Jkg-1K-1x 13.0C
= 1.365 kJ

 H = -Q/1000n
= -1.365kJ/0.025 mol
= -54.6 kJmol-1

Uncertainty of enthalpy change of reaction,H3

The uncertainty for the reaction above is as follows:

Uncertainty of ∆H3=
= ( 0.01 + 0.1) x546
25.00 13.0
= ± 0.008

 uncertainty H =(uncertainty H2 +uncertainty H3)
= ± ( 0.004 + 0.008)
= ± 0.012

Calculation of Enthalpy of Reaction

By using Hess’s Law, the enthalpy of reaction of

Mg(s) + H2O(ℓ) → MgO(aq) + H2(g) can be calculated

Mg(s) + 2HCl(ℓ) → MgCl2 (aq)+ H2(g) = -132.3 ± 0.004 kJ mol-1
MgCl2(aq) + H2O (ℓ) → MgO(s) + 2HCl(ℓ) = 54.6 ± 0.008 kJ mol-1
Mg(s) + H2O(ℓ) → MgO(aq) + H2(g) = -77.7 ± 0.012kJ mol-1

Therefore, the ∆rxn for the reaction is = -77.7 ± 0.012 kJ mol-1

Degree of Accuracy

Experimental error = Experimental value-Real value ×100% real value...