# Hess's Law

Topics: Thermodynamics, Energy, Enthalpy Pages: 3 (873 words) Published: April 11, 2013
Lab report
ABSTRACT
This experiment, the aim is to determine the heat of combustion of Magnesium. Whit the aid of Hess law, the heat of combustion of magnesium is computed based on the known heat values of the following reactions: •MgO(s)+2 HCl(aq) MgCl2(aq)+H2O(l) (1) •Mg(s)+2 HCl(aq) MgCl2(aq)+H2(g) (2) •H2(g)+1/2O2(g) H2O(l) (3) By reversing the first reaction and combining those reactions equations together, the outcome is the reaction equation of Magnesium combustion which is Mg(s) + ½ O2 MgO¬(s). In this experiment, heats of reaction for equation (1) and (2) were determined and heat of reaction for reaction (3) has assumed to be known as – 285.8kJ. Both equation (1) and (2) were found to be exothermic with respectively values of -129.11KJ and ΔH¬2=-398.94KJ and the ΔH4 of magnesium combustion= -555.6KJ but this value is different to the correct value in the book because the experiment was carried out in an open system; therefore heat was released from the system to environment. INTRODUCTION

Hess' Law states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. Hess's law also states that energy changes are state functions. The amount of energy depends only on the states of the reactants and the state of the products, but not on the intermediate steps. Energy (enthalpy) changes in chemical reactions are the same, regardless whether the reactions occur in one or several steps. The total energy change in a chemical reaction is the sum of the energy changes in its many steps leading to the overall reaction. Hess's law helps us to calculate enthalpies of reactions that are difficult to measure like the combustion of magnesium. PRECAUSTION

Hydrochloric acid can cause painful burns if it contacts with skin; therefore, it must...