Experiment to Compare the Enthalpy Changes of Combustion of Different Alcohols

Luke Smith 12MPC

Experiment to Compare the Enthalpy Changes of Combustion of Different

Alcohols

Introduction:  This plan will try to outline how the experiment of comparing changes of combustion of different alcohols will be conducted and what results are expected.

When chemical reactions take place they are often accompanied by energy changes.

Chemical reactions most frequently occur in open vessels. That is, they take place at constant pressure. Enthalpy refers to energy at constant pressure (volume may vary).

Enthalpy:

An example is best to illustrate to show enthalpy works.  Methane - how much energy does its molecules contain? The first thing needed is the amount of methane present = 1 mole (16 g).  What ever its value, the total amount of energy in a given amount of a substance (sometimes called the Heat energy content) is known as the enthalpy, denoted H.

Methane is a fuel to get energy from it, react it with oxygen.

CH4(g) + 2O2(g)        CO2(g) + 2H2O(l)

The above chemical equation shows that 2 moles (64 g) of oxygen molecules are required to burn 1 mole of methane. Again, it is impossible to know the total enthalpy (heat energy content) of the oxygen. Likewise, we can't know the total heat energy content of 1 mole of CO2 and 2 moles of H2O (the products).

Enthalpy Change

H = (HCO2 + 2HH2O) - (HCH4 + 2HO2)

In general,

H = Hproducts - Hreactants

But remember, this is theoretical; it is not possible to determine the absolute value of the enthalpy of a chemical element or compound. However, H values for chemical reactions can be obtained. They can be measured experimentally, or calculated using Hess's Law (see later), or worked out in other ways.

Exothermic and Endothermic Reactions

When chemical reactions take place they are often accompanied by heat changes. The system (the reactants which form products) may give out heat to the surroundings, causing them to warm up. In this case the reactants have more stored energy (greater total enthalpy) than the products. Such chemical reactions are said to be exothermic. The system may take heat from the surroundings, causing them to cool down. In this case the reactants have less stored energy (less total enthalpy) than the products. Such chemical reactions are said to be endothermic.

Most reactions take place at constant pressure...

It is possible to measure changes in heat energy that accompany chemical reactions. Most reactions take place in vessels that are open to the atmosphere, that is, they take place at constant pressure (volume may vary). The special name given to a change in heat energy content measured at constant pressure is enthalpy change. This change in enthalpy is denoted by H. The value of H (often expressed in kJ, or kJ mol-1 when appropriate) is given a negative sign for exothermic reactions and a positive sign for endothermic reactions, indicating whether the system loses or gains energy as a result of the reaction.

The value of H is given a negative sign for an exothermic reaction. The value of H is given a positive sign for an endothermic reaction.

Precise thermochemical measurements are made in a closed vessel of fixed volume, such as a calorimeter. For a reaction involving a change in volume of gases there is a small but real difference in the measured heat change. You can read more about this .

Enthalpy Level Diagrams...

Enthalpy level diagrams can be used to illustrate overall exothermic and endothermic changes. They show the difference in total enthalpy of the reactants and products for a reaction:

For an exothermic reaction the total enthalpy of the products is less than that of reactants. For an endothermic reaction the total enthalpy of the reactants is less than that of the products. For each, the difference in these total enthalpies is equal to the overall enthalpy of the reaction, H.

Temperature and pressure matter...

As well as the amounts of substances reacting (molar amounts are taken), the precise value of H depends on both the temperature and pressure at which it is measured. For this reason H values are expressed at standard conditions (normally 298 K and 1 ...