Exp 1

EXPERIMENT 1

CALORIMETRY – HESS’S LAW

Name student & ID number : Muhammad Farizul Bin Md Ismail & 2013321763

Name partner & ID number : Amirnur bin Abd Masrik & 2013568627

Date of experiment : 27/3/2014

Date of submission : 3/4/2014

Name of lecturer : Pn Asiah binti Abdullah

Objective

To determine the standard enthalpy of formation of magnesium oxide, ΔH˚f

Introduction

Enthalpies of formation (ΔHf) are calculated from the specific bond energies within compounds. However, enthalpies of formation may also be determined by other means. Hess' Law of Reaction Enthalpies states that the heat evolved in a given process can be expressed as the sum of the heats of several processes that, when added, yield the reaction of interest. In order to use Hess' Law, the Heats (enthalpies) of reaction,ΔHrxn, must be calculated. Hess’s Law states, “Heats of reactions are additive in the same way as the reactions to which they pertain are additive”.

In this experiment, you will first determine the heat of reaction of magnesium oxide reacting with aqueous hydrochloric acid (reaction 1), and the heat of reaction of magnesium oxide reacting with aqueous hydrochloric acid (reaction 2). We also know the heat of formation (ΔH˚298) of water (reaction 3) and can then apply Hess’ Law and calculate the heat of formation of solid magnesium oxide:

Reaction 1:

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) ΔH(1) (kJ/mol)

Reaction 2:

MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) ΔH(2) (kJ/mol)

Reaction 3: formation reaction of H2O(l)

H2(g) + ½O2(g) → H2O(l) ΔH(3) = −286 (kJ/mol)

By rearranging above equations, we can calculate heat of formation of MgO:

Mg(s) + ½O2(g) → MgO(s) ΔH = ΔH(1) −ΔH(2) + ΔH(3)

The heat capacity of a calorimeter can be obtained by measured the temperature change when a known weight of hot water is added to a known amount of cold water in the calorimeter.

qhot = qcold + qcal

qhot =...