Chemistry Lesson 3.2 Energy & Chemistry PDF

Lesson 3.2 Energy & Chemistry 1 Chapter Objectives Define ΔHfo and write formation reactions for compounds. Explain Hess’s law in your own words. Calculate ΔHo for chemical reactions from tabulated data. 2...

Lesson 3.2 Energy & Chemistry 1 Chapter Objectives Define ΔHfo and write formation reactions for compounds. Explain Hess’s law in your own words. Calculate ΔHo for chemical reactions from tabulated data. 2 Defining Enthalpy The internal energy for a reaction equals the sum of the heat flow and the work. During an expansion, w = –PΔV. Under constant volume conditions, ΔV = 0, and ΔE = qv. 3 Defining Enthalpy Enthalpy is the heat flow under conditions of constant pressure. The enthalpy change can be expressed as 4 Defining Enthalpy The conditions under which heat flow, q, occurs will have an impact on the measurement that is made. Combustion of octane releases 5.45 x 103 kJ under constant volume conditions, represented as qv. Combustion of octane releases 5.48 x 103 kJ under constant pressure conditions, represented as qp. 5 Defining Enthalpy When a system releases heat, the process is said to be exothermic. The value of ΔH is less than zero; the sign on ΔH is negative. When a system absorbs heat, the process is said to be endothermic. The value of ΔH is greater than zero; the sign on ΔH is positive. 6 ΔH of Phase Changes Phase changes occur under constant pressure conditions. The heat flow during a phase change is an enthalpy change. 7 ΔH of Phase Changes The heat required to convert a liquid to a gas is the heat of vaporization, ΔHvap. ΔHvap is endothermic with a positive value. The heat released to convert a gas to a liquid is the heat of condensation, ΔHcond. ΔHcond is exothermic with a negative value. ΔHcond = –ΔHvap The values of enthalpy changes in opposite directions have equal numeric values and differ only in their signs. The magnitude of enthalpy change depends on the substance involved. 8 ΔH of Phase Changes Standard molar enthalpies and temperatures for phase changes of water. 9 ΔH of Phase Changes The value of ΔH for a phase change is compound specific and has units of kJ/mol. The heat flow can be calculated using the number of moles of substance, n, and the value of the enthalpy change. 10 Example Problem 1. Calculate the enthalpy change when 245 g of ice melts. Solution: use 11 Vaporization and Electricity Production The enthalpy change for the conversion of ice to liquid and then to steam can be calculated. A heat curve breaks the calculation down into specific heat calculations (sections of the heat curve where temperature changes) and phase change enthalpy calculations (sections of the heat curve where temperature does not change). 12 ΔH of Phase Changes Heat curve for the heating of 500 g ice from -50°C to 200°C. 13 Vaporization and Electricity Production Schematic diagram of the important elements of a standard electric power plant. 14 Vaporization and Electricity Production The large amount of energy required to convert water from a liquid to a gas is exploited in converting chemical energy into electricity. The goal of the power plant is to convert as much chemical energy as possible into electricity. The large heat of vaporization for water is ideal for “trapping” the heat energy given off in the combustion reaction. 15 Heat of Reaction Enthalpy changes can be calculated for chemical reactions, in addition to temperature changes and phase transitions. The enthalpy change is commonly referred to as the heat of reaction. 16 Bonds and Energy The enthalpy change for a reaction can be estimated using bond energies. During a chemical reaction, reactant bonds are broken and product bonds are made. Breaking bonds requires energy. Making bonds releases energy. If the amount of energy released making product bonds is greater than the amount of energy required to break reactant bonds, the reaction is exothermic. If the energy released is less than the energy required, the reaction is endothermic. 17 Bonds and Energy The combustion of methane breaks 4 C-H bonds and 2 O=O bonds. 2 C=O bonds and 4 O-H bonds are made. 18 Bonds and Energy The accuracy of enthalpy changes calculated from tabulated bond energies is not very good. The bond energies used are averages. Bond energy method used to estimate enthalpy changes for reactions involving compounds with no available thermochemical data. A thermochemical equation summarizes the overall energetics for a chemical reaction. The sign on the ΔH indicates whether the reaction is endothermic or exothermic 19 Bonds and Energy The combustion of methane is an exothermic reaction and releases 890.4 kJ of heat energy when 1 mole of methane reacts with 2 moles of oxygen. For thermochemical equations, if the stoichiometric coefficients are multiplied by some factor, the heat of reaction must also be multiplied by the same factor. 20 Heats of Reaction for Some Specific Reactions Some classes of chemical reactions are given their own labels for heats of reactions. Heat of combustion, ΔHcomb Heat of neutralization, ΔHneut Heat of formation, ΔHf, is the heat of reaction for formation of substances. Fractional coefficients are allowed for formation reactions because only one mole of product can be formed. 21 Heats of Reaction for Some Specific Reactions A formation reaction is the chemical reaction by which one mole of a compound is formed from its elements in their standard states. The standard state is the most stable form of an element at room temperature, 25°C, and pressure, 1 atm, indicated with a superscript °. ΔHf° = 0 for an element in its standard state. 22 Hess’s Law and Heats of Reaction Direct calorimetric determinations of some reactions may be too difficult or dangerous to perform. An indirect method is needed to obtain heats of reaction. Hess’s law: the enthalpy change for any process is independent of the particular way the process is carried out. Enthalpy is a state function. A state function is a variable whose value depends only on the state of the system and not its history. 23 Hess’s Law and Heats of Reaction Hess’s Law, also known as "Hess's Law of Constant Heat Summation," states that the total enthalpy of a chemical reaction is the sum of the enthalpy changes for the steps of the reaction. Therefore, you can find enthalpy change by breaking a reaction into component steps that have known enthalpy values. Hess's Law says the total enthalpy change does not rely on the path taken from beginning to end. Enthalpy can be calculated in one grand step or multiple smaller steps. 24 Hess’s Law Steps To solve this type of problem, organize the given chemical reactions where the total effect yields the reaction needed. There are a few rules that you must follow when manipulating a reaction. The reaction can be reversed. This will change the sign of ΔHf. The reaction can be multiplied by a constant. The value of ΔHf must be multiplied by the same constant. Any combination of the first two rules may be used. 25 Hess’s Law Conceptual diagram representing Hess’s law. Enthalpy is a state function, so any convenient path can be used to calculate the enthalpy change. 26 Hess’s Law Enthalpy diagram for the combustion of methane. The CH4 is converted to CO, then the CO is converted to CO2. The ΔH for each step is used to calculate the ΔH for the overall reaction. The ΔH will be the same for both paths. 27 Example 1 One origin of SO3 is the combustion of sulfur, which is present in small quantities in coal, according to the following equation. 1. Given the thermochemical information below, determine the heat of reaction for this final reaction. Answer: 28 Example 2 Find: Given: 29 Example 3 Find: Given: 30 Example 4 Answer: The change in enthalpy for the reaction is -1075.0 kJ/mol 31 Formation Reactions and Hess’s Law The enthalpy change for a reaction can be calculated using Hess’s law and heats of formation. 32 Example Problem 9.8 Propane, C3H8, is a hydrocarbon that is commonly used as fuel. The heat of combustion of propane is -2,219.2 KJ. Calculate the heat of formation of propane given that ΔHf°of water = -285.8 KJ/mol and ΔHf° of CO2 = -393.5 KJ/mol. 33 Energy and Stoichiometry A thermochemical equation allows for the stoichiometric treatment of energy. For an exothermic reaction, energy is treated as a product. For an endothermic reaction, energy is treated as a reactant. The thermochemical equation is used to convert between the number of moles of a reactant or product and the amount of energy released or absorbed. The stated value of ΔH for a thermochemical equation corresponds to the reaction taken place exactly as written, with the indicated numbers of moles of each substance reacting. 34 Energy and Stoichiometry Flow chart detailing the sequence of steps needed to calculate the amount of energy released or absorbed when a chemical reaction is carried out using a given amount of material. 35 Example Problem 9.10 An engine generates 15.7 g of nitric oxide gas during a laboratory test. How much heat was absorbed in producing this NO? 36 Energy Density and Fuels When deciding the economic merits of a fuel, several factors must be considered: The technology available to extract the fuel. The amount of pollution released by its combustion. The fuel’s relative safety. The ease of transporting the fuel. The fuel’s energy density. 37 Energy Density and Fuels Energy density is the amount of energy that can be released per gram of fuel burned. The higher the energy density of a fuel, the less fuel that must be transported to the customer. 38 Power Distribution and the Electrical Grid Electrical grid: the complicated, multipart system designed to balance the demand for electricity with its production Demand varies in complex ways Season of year/weather Time of day Operating conditions of power plants 39 Power Distribution and the Electrical Grid The main components of the electrical grid system 40 Power Distribution and the Electrical Grid Key elements of electrical grid Generating station: where electricity is generated (e.g., burning of fossil fuels) Transmission substation: increases voltage to levels on the order of 105 V to reduce loss over long distance Distribution substation: steps down voltage then distributes electricity to consumers over relatively short distances 41