# experiment Hess Law

Name                : Mohd Haziq Al-Hakim Bin Hamirruddin

Class                : M08F

Date                : 9 July 2009

Practical         : 16

Data Collection and Processing

Part A:-

Mass of magnesium ribbon        = 0.2836 ± 0.0001g

Volume of 0.5 M of HCl                 = 50.0 ± 0.5 cm3

Table 1 shows the initial and highest temperature for reaction between Mg and HCl

To calculate the change in temperature, the following equation will be used:-

Table 2 shows the change in temperature for reaction between Mg and HCl

Equation for part A:-

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)

Mole of magnesium        Mole of hydrochloric acid

= mass                        = MV

RMM                      1000

= 0.2836 g                = 50.0x0.5

24.3                      1000

= 0.0118 mol                = 0.025 mol

Thus, the limiting reagent is magnesium

Heat released in the reaction can be calculated by the formula:

Q = mc∆T

Whereby;

m                = Mass of the solution

c                = Specific heat of water (4.2 Jmol-1°C-1)

∆T                = Change in temperature

Enthalpy change of the reaction can be determined by the formula:-

H = Amount of heat released/absorbed, Q

Number of moles of reactant, n

Calculation:

Heat change, Q                = mc∆T

= 50g × 4.2 Jmol-1°C-1 × 13.5 °C

= 2835 J

Hrxn        = -2835_

0.0118

= -240.25 kJmol-1

Uncertainties:

Mass                =                =        1.0 %

Temperature        =                =        7.4 %

No ...