Saturday, July 20, 2019
Experiment to Compare the Enthalpy Changes of Combustion of Different Alcohols :: GCSE Chemistry Coursework Investigation
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. Background 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. Exothermic reactions give out energy to the surroundings. Endothermic reactions take energy from the surroundings. 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.