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Chapter 6 (Essentials of General Chemistry, 2nd Edition) (Ebbing and Gammon) Thermochemistry Thermodymanics thermodynamics - the science of the relationships between heat and other forms of energy thermochemistry - one area of thermodynamics - study of the quantity of heat absorbed or evolved by chemical reactions - heat measurements provide data needed to determine whether a particular chemical reaction occurs and, if so, to what extent Karen Hattenhauer (Fall 2007) 2 Energy and Its Units energy - the potential or capacity to move matter - exists in different forms that can be interconverted Types of energy: i.) kinetic energy ii.) potential energy iii.) internal energy Karen Hattenhauer (Fall 2007) 3 1 Kinetic Energy kinetic energy - the energy associated with an object by virtue of its motion - an object of mass, m, and speed or velocity, v, has kinetic energy, EK, equal to EK = ½ mv2 with units of kg·m2/s2 or J - a joule is an extremely small unit (often use kJ) calorie (cal) - the amount of energy required to raise the temperature of one gram of water by one degree Celsius - a non-SI unit of energy commonly used by chemists - defined in terms of a joule: 1 cal = 4.184 J Karen Hattenhauer (Fall 2007) 4 Potential Energy potential energy - the energy an object has by virtue of its position in a field of force - Consider water of a given mass m at the top of a dam is at a relatively high position h in the gravitational field g of the earth. - The potential energy would be given by Ep = mgh where Ep = potential energy of a quantity of water at the top of the dam m = the mass of the water g = the constant acceleration of gravity h = the height of the water measured from some standard level Karen Hattenhauer (Fall 2007) 5 Note: - the choice of this standard level is arbitrary since only differences of potential are important in any physical situation - convenient to choose the standard level to be the surface of the earth - potential energy of water at top of dam is converted to kinetic energy when water falls to a lower level Karen Hattenhauer (Fall 2007) 6 2 Internal Energy internal energy, U - the sum of the kinetic and potential energies of the particles making up a substance total energy, Etot, of a quantity of water - the sum of its kinetic energy and potential energies as a whole (EK + Ep) plus its internal energy Etot = EK + Ep + U - when study in lab, substance is at rest in vessel, which makes its kinetic energy as a whole equal to zero - potential energy as whole is constant and can be taken to be zero - therefore, the total energy of the substance equals its internal energy, U Karen Hattenhauer (Fall 2007) 7 Law of Conservation of Energy Law of Conservation of Energy - energy may be converted from one form to another, but the total quantity of energy remains constant Karen Hattenhauer (Fall 2007) 8 Heat of Reaction thermodynamic system (or system) - the substance or mixture of substances under study in which a change occurs surroundings - everything in the vicinity of the thermodynamic system Karen Hattenhauer (Fall 2007) 9 3 Heat Heat (q) - the energy that flows into or out of a system because of a difference in temperature between the thermodynamic system and its surroundings - as long as system and surroundings are in thermal contact, energy flows between them to establish temperature equality or thermal equilibrium - heat flows from region of higher temperature to one of lower temperature; once temperature becomes equal, heat flow stops Note: by convention: q is positive (q = +) if heat is absorbed by system q is negative (q = -) if heat is evolved by system Karen Hattenhauer (Fall 2007) 10 Heat of Reaction heat of reaction - the value of q required to return a system to the given temperature at the completion of the reaction Classification of physical changes and chemical reactions: i.) exothermic process ii.) endothermic process chemical reaction or - chemical reaction or physical change in physical change in which heat is evolved which heat is absorbed Karen Hattenhauer (Fall 2007) 11 Enthalpy and Enthalpy Change - heat absorbed or evolved by reaction depends on conditions under which reaction occurs - reaction usually takes place in vessel open to the atmosphere and therefore at constant pressure of atmosphere - in this case, write heat of reaction as qp (p indicates process occurs at constant pressure) enthalpy (H) - an extensive property of a substance that can be used to obtain the heat absorbed or evolved in a chemical reaction - a property of a substance that is related to heat of reaction, qp - is a state function Karen Hattenhauer (Fall 2007) 12 4 State function state function a property of a system that depends only on its present state, which is determined by variables such as temperature and pressure and is independent of any previous history of the system - that is, a change in enthalpy does not depend on how the change was made, but only on the initial state and final state of the system Karen Hattenhauer (Fall 2007) 13 Enthalpy of Reaction enthalpy of reaction - the change in enthalpy for a reaction at a given temperature and pressure - obtained by subtracting the enthalpy of the reactants from the enthalpy of the products H = Hfinal Hinitial - since you start from reactants and end with products, enthalpy of reaction is H = H(products) H(reactants) - is a state function - key relation: enthalpy and heat of reaction H = qp - the enthalpy of reaction equals the heat of reaction at constant pressure Karen Hattenhauer (Fall 2007) 14 Enthalpy Diagram 2 Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g) Figure 6.6: Enthalpy Diagram Karen Hattenhauer (Fall 2007) 15 5 Enthalpy and Internal Energy - enthalpy, H, defined precisely as the internal energy, U, plus pressure, P, times volume, V H = U + PV - if we label the initial quantities (those for reactants) with subscript i and final quantities (those for products) with subscript f, then H = Hf Hi = (Uf + PVf) - (Ui + PVi) - collecting internal-energy terms and the pressurevolume terms, you can rewrite H = (Uf - Ui ) + P(Vf - Vi) = U + P V - rearranging, gives U= H-P V 16 Karen Hattenhauer (Fall 2007) Thermochemical Equations thermochemical equation - the chemical equation for a reaction (including phase labels) in which the equation is given a molar interpretation, and the enthalpy of reaction for these molar amounts is written directly after the equation - phase labels are very important (must be included) eg. Reaction of sodium and water 2 Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g) H = -368.6 kJ Karen Hattenhauer (Fall 2007) 17 Two important rules for manipulating thermochemical equations: 1.) When a thermochemical equation is multiplied by any factor, the value of H for the new equation is obtained by multiplying the value of H in the original equation by that same factor. 2.) When a chemical equation is reversed, the value of H is reversed in sign. Karen Hattenhauer (Fall 2007) 18 6 Stoichiometry and Heats of Reaction Consider the reaction of methane, CH4, burning in oxygen at constant pressure. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l) H = -890.3 kJ How much heat could you obtain from 10.0 g of methane? grams of CH4 moles of CH4 kilojoules of heat 19 Karen Hattenhauer (Fall 2007) Measuring Heats of Reaction Heat capacity © of a sample of substance - the quantity of heat needed to raise the temperature of the sample of substance one degree Celsius (or one Kelvin) - changing the temperature of the sample from an initial temperature ti to a final temperature tf requires heat equal to q=C t where t = tf ti - heat capacity is directly proportional to amount of substance Karen Hattenhauer (Fall 2007) 20 molar heat capacity of a substance - the heat capacity for one mole of substance - that is, the quantity of heat needed to raise the temperature of one mole of a substance one degree Celsius specific heat capacity (or specific heat) - the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius (or one kelvin) at constant pressure - to find heat required to raise the temperature of a sample: q =s x m x t where s = specific heat of substance m = mass in grams t = temperature change Karen Hattenhauer (Fall 2007) 21 7 Table 6.1 Karen Hattenhauer (Fall 2007) 22 Measurement of Heat of Reaction Figure 6.10 A Simple Coffee-cup Calorimeter Figure 6.11 Bomb Calorimeter Karen Hattenhauer (Fall 2007) 23 Using Heats of Reaction Hess s Law - enthalpy change for chemical reaction is independent of path by which products are obtained Hess s Law of Heat Summation - states that for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps Karen Hattenhauer (Fall 2007) 24 8 Enthalpy Diagram Illustrating Hess s Law Figure 6.12 25 Karen Hattenhauer (Fall 2007) Standard Enthalpies of Formation standard state - the standard thermodynamic conditions chosen for substances when listing or comparing thermodynamic data: 1 atm pressure the specified temperature (usually 25oC) - standard conditions are indicated by a superscript degree sign (o) eg. The enthalpy change for a reaction in which reactants in their standard states yield products in their standard states is denoted Ho 26 Karen Hattenhauer (Fall 2007) allotrope - one of two or more distinct forms of an element in the same physical state reference form - the stablest form (physical state and allotrope) of the element under standard conditions Figure 6.13 Allotropes of Sulfur Karen Hattenhauer (Fall 2007) 27 9 Standard enthalpy of formation, Hof (standard heat of formation) - the enthalpy change for the formation of one mole of the substance in its standard state from its elements in their reference form and in their standard states - values of standard enthalpies of formation are listed (Table 6.2) - standard enthalpy of formation of an element is dependent on the form of the element Using standard enthalpies of formation to find the standard enthalpy of a reaction: Ho = n Hof (products) m Hof (reactants) Karen Hattenhauer (Fall 2007) 28 10 This document was created with Win2PDF available at http://www.daneprairie.com. 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