limitations of enthalpy experiments

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Determining Enthalpy Change EXPERIMENT 1

Introduction: When zinc reacts with copper(II) sulphate solution in a displacement reaction it causes an enthalpy change. The heat this change produces can be calculated by an experiment in which known amounts of the 2 substances are mixed in a vessel and the change in temperature recorded.

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The purpose of the experiment was to investigate the enthalpy changes in several reaction and relationship of three exothermic reaction with hess’law.

Background Information-The reaction of Zn and HCl is exothermic as heat energy is released during the reaction which makes the Change in enthalpy (ΔH) negative. The reaction is also a single displacement as the reactants are Zn +HCl and the products are ZnCl2+H2. In single displacement reactions one atom from the reactants displaces the other from its compound and forms a new compound. In this case Zn + HCl = H2 + ZnCl2 as we can notice that the hydrogen atom has been displaced from its compound by zinc which is more electronegative than hydrogen. Because of this reaction hydrogen gas is produced as bubbles during the experiment. In this reaction, the zinc metal begins the process as a pure metal with an ionic charge of zero and loses an electron as it reacts with the acid to become a positively charged zinc ion. [1] At the same time, hydrogen undergoes reduction by gaining an electron to become a neutral molecule and it basically forms a covalent bond with 2 atoms of itself.

Metallurgical Transactions A, 1993

Elaboration of a basic calorimeter applied in the determination of the enthalpy of chemical reactions (Atena Editora), 2022

In thermodynamics, calorimetry studies the energies between the energy content between the products and reactant of an enthalpy reaction, which can occur with both compounds in liquid, solid and gaseous states. For the accomplishment of this work, products called acids and bases were used in their liquid form, resulting in a neutralization reaction. In this article, experimental data will be presented in order to demonstrate the enthalpy variations of an acid-base reaction from the construction of a low cost calorimeter, built with easily accessible materials, aiming to concentrate this energy, so that there was the lowest possible dissipation of it, seeking to reach the most plausible possible results compared to the parameters already pre-established for each reaction. We used an expanded polystyrene container (styrofoam), a conventional thermometer, the products and reagent, and at the end thermochemical equations were applied, such as the determination of the heat capacity of the measurement object and finally the appropriate transformations of the neutralization enthalpy calculations. , taking into account the possible interventions of the environment and the parameters not efficiently and accurately of the materials.

Objectives • Measure the enthalpy of reaction for the decomposition of hydrogen peroxide • Measure the heat capacity of a Styrofoam cup calorimeter using the heat of neutralization of a strong acid with a strong base • Graph your temperature vs time data to find temperature change when solutions are mixed Pre-­‐Laboratory Requirements • Read chapter 6 in Silberberg • Pre-lab questions (if required by your instructor) • Laboratory notebook—prepared before lab (if required by your instructor) Safety Notes • Eye protection must be worn at all times. • Hydrochloric acid and sodium hydroxide are caustic and should not come in contact with your skin or clothing. Wear gloves when handling these chemicals. A lab coat or lab apron is recommended. • The hydrogen peroxide used in this experiment is often used as an antiseptic. However, avoid unnecessary exposure to your skin and clothing. Do not splash hydrogen peroxide in your eyes. Discussion When a chemical reaction occurs, bonds are broken and new bonds are formed. The breaking of bonds requires energy while the formation of new bonds releases energy. For most spontaneous chemical reactions, the energy released by the new bond formation is greater than that required to break the old bonds. In this process, energy is released (usually as heat) into the environment. These reactions are called exothermic and a minus sign is put in front of the quantity of energy released. Some reactions require energy from the environment to proceed; these are called endothermic reactions. Because energy must flow from the environment into the reactants for the reaction to occur, a plus sign is put in front of the amount of energy absorbed. The amount of energy released or absorbed in the reaction is called the Heat of Reaction, H rxn. If ΔH rxn =-145.3 kJ/mole for a specific reaction, this reaction is exothermic, the energy released by new bond formation exceeds that required to break the old bonds, and for each mole of reaction 145.3 kJ are released. If a reaction gives off heat (or takes in heat), the heat has to go (or come from) somewhere—remember the law of conservation of energy. Adding heat to a system will increase the kinetic energy of the system. As a result of the added energy, the atoms will move faster and the temperature will increase. Temperature is a measure of how fast the atoms and molecules are moving, and can be changed by adding or removing heat energy. Every substance has its own unique response to added heat energy. For some materials, even a small amount of added heat will cause a large increase in temperature. For others materials, it takes a great deal of heat energy to raise the temperature even slightly. The ratio between the heat energy change and the temperature change is called the heat capacity.

1. To be able to determine the heating value of a solid pure substance such as naphthalene using the bomb calorimeter. II. Theory Combustion is a chemical reaction between substances including oxygen and is usually accompanied by the generation of heat and light in the form of flame. Reactants combine in a high rate or speed partly because of the nature of the chemical reaction itself and also because more energy is generated than the energy that can escape into the surrounding medium. This results in the rise of the reactants' temperature causing the reaction to accelerate even more (Enclyclopedia Brittanica, 2016). Calorimetry is an important field of analytical chemistry which deals accurately with measuring heats of reaction and finds application in fields ranging from nutritional analysis to explosive yield tests (Melville, 2014). It is the science of measuring the heat produced in a combustion process and commonly used in determining the calorific value or the heating value of substances using an adiabatic bomb calorimeter. The heating value is defined as the quantity of heat released by the complete combustion of a unit mass of fuel with oxygen at a constant volume process. In other words, it denotes the heat liberated by the combustion of all carbon and hydrogen with oxygen to form carbon dioxide and water, including the heat liberated by the oxidation of other elements such as sulfur which may be present in the sample. It is expressed as a unit of energy per unit mass or volume of the substance such as kcal/kg, kJ/kg, J/mol and Btu/m 3 (Parr Instrument Company, 2007). Aside from carbon monoxide and carbon dioxide, the combustion process generates water vapor and certain techniques may be used to recover the quantity of heat contained in this water vapor by condensing it. The Higher Heating Value (HHV), also known as the Gross Calorific Value or Higher Calorific Value, is determined when the water of combustion is entirely condensed and that the heat contained in the water vapor is recovered. It is the amount of heat produced by the complete combustion of a unit quantity of fuel. The HHV is obtained when all products of combustion are cooled down to the pre-combustion temperature and when the water vapor produced from combustion is condensed (The Engineering Toolbox, n.d). Meanwhile, the Lower Heating Value (LHV) or also known as Net Calorific Value or Lower Calorific Value, is determined when water vapor is contained in the products of combustion and that the heat in the water vapor is not recovered. To calculate for the LHV, the formed water vapor's latent heat of vaporization or the energy required to change the state of water from liquid to vapor at constant temperature, is subtracted by the combustion from the HHV (The Engineering Toolbox, n.d). Perry and Green (2008) shows the calculation of LHV where W represents the ratio of weight of water in the combustion products and weight of the fuel burned. The factor K is the latent heat of vaporization at the partial pressure of the vapor in the gas at standard temperature. This is shown in equation 1 below: Net ΔHc = Gross ΔHc – KW (1) Heating values of substances can be determined using a bomb calorimeter.

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