1 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Section 6 Thermochemistry

2 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Thermochemistry Thermochemistry is the study of the quantity of heat absorbed or evolved by chemical reactions.

3 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Energy Energy is defined as the capacity to move matter. Energy can be in many forms: –Radiant Energy -Electromagnetic radiation. –Thermal Energy - Associated with random motion of a molecule or atom. –Chemical Energy - Energy stored within the structural limits of a molecule or atom.

4 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Energy There are three broad concepts of energy: –Kinetic Energy is the energy associated with an object by virtue of its motion. –Potential Energy is the energy an object has by virtue of its position or composition. –Internal Energy is the sum of the kinetic and potential energies of the particles making up a substance.

5 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Energy Internal Energy is the energy of the particles making up a substance. The total energy of a system is the sum of its kinetic energy, potential energy, and internal energy, U.

6 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Energy The Law of Conservation of Energy: Energy may be converted from one form to another, but the total quantities of energy remain constant.

7 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Heat of Reaction In chemical reactions, heat is often transferred from the “system” to its “surroundings,” or vice versa. The substance or mixture of substances under study in which a change occurs is called the thermodynamic system (or simply system.) The surroundings are everything in the vicinity of the thermodynamic system.

8 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Heat of Reaction Heat is defined as the energy that flows into or out of a system because of a difference in temperature between the system and its surroundings. Heat flows from a region of higher temperature to one of lower temperature; once the temperatures become equal, heat flow stops.

9 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Heat of Reaction Heat is denoted by the symbol q. –The sign of q is positive if heat is absorbed by the system from surroundings (endothermic process) –The sign of q is negative if heat is evolved by the system (exothermic process). –Heat of Reaction is the value of q required to return a system to the given temperature at the completion of the reaction. HW 50

10 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Enthalpy and Enthalpy Change The heat absorbed or evolved by a reaction depends on the conditions under which it occurs. Usually, a reaction takes place in an open vessel, and therefore at the constant pressure of the atmosphere. The heat of this type of reaction is denoted q p, the heat content at constant pressure. This is also referred to as enthalpy, H.

11 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. –An extensive property is one that depends on the quantity of substance. –Enthalpy is a state function, a property of a system that depends only on its present state and is independent of any previous history of the system. Enthalpy and Enthalpy Change Enthalpy, denoted H, is an extensive property of a substance that can be used to obtain the heat absorbed or evolved in a chemical reaction at constant pressure.

12 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Enthalpy and Enthalpy Change The change in enthalpy for a reaction at a given temperature and pressure (called the enthalpy of reaction) is obtained by subtracting the enthalpy of the reactants from the enthalpy of the products.

13 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. The change in enthalpy is equal to the heat of reaction at constant pressure. This represents the entire change in internal energy (  U) minus any expansion “work” done by the system. Therefore, we can define enthalpy and internal work by the First Law of Thermodynamics which states in any process, the total change in energy of the system,  U, is equal to the sum of the heat absorbed, q, and the work, w, done by the system. Enthalpy and Enthalpy Change

14 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. –In chemical systems, work is defined as a change in volume at a given pressure, that is: Enthalpy and Internal Energy Enthalpy and Enthalpy Change

15 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. This is the 1 st Law of Thermo. Note: system doing work “-” surroundings doing work on system “+” heat absorbed “+” heat released or evolved “-” watch units Since the heat at constant pressure, q p, represents  H, then Enthalpy and Enthalpy Change HW 51

16 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Thermochemical Equations A thermochemical equation is 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.

17 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. In a thermochemical equation it is important to note phase labels because the enthalpy change,  H, depends on the phase of the substances. Thermodynamic Standard State – 25 o C (298K) and 1 atm. Thermochemical Equations

18 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. The following are three important rules for manipulating thermochemical equations: –When a thermochemical equation is multiplied by any factor, the value of  H for the new equation is obtained by multiplying the  H in the original equation by that same factor. –When a chemical equation is reversed, the value of  H is reversed in sign.

19 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. –The magnitude of  H is directly proportional to the amount of reactants or products.

20 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Applying Stoichiometry and Heats of Reactions Consider the reaction of methane, CH 4, burning in the presence of oxygen at constant pressure. Given the following equation, how much heat could be obtained by the combustion of 10.0 g CH 4 ? 10.0 g O 2 ? 10.0 g of each? HW 52

21 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. –The heat capacity, C, of a sample of substance is the quantity of heat required to raise the temperature of the sample of substance one degree Celsius. –Changing the temperature of the sample requires heat equal to: Heat Capacity and Specific Heat Measuring Heats of Reaction

22 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. A Problem to Consider Suppose a piece of iron has heat capacity of 6.70 J/ o C. The quantity of heat required to raise the temperature of the piece of iron from 25.0 o C to 35.0 o C is:

23 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Measuring Heats of Reaction Heat capacities are also compared for one gram amounts of substances. The specific heat capacity, s, (or “specific heat”) is the heat required to raise the temperature of one gram of a substance by one degree Celsius. –To find the heat required you must multiply the specific heat, s, of the substance times its mass in grams, m, and the temperature change,  T.

24 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. A Problem to Consider Calculate the heat when the temperature of 15.0 grams of water is raised from 20.0 o C to 50.0 o C. (The specific heat of water is J/g.o C.) HW 53

25 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Heats of Reaction: Calorimetry A calorimeter is a device used to measure the heat absorbed or evolved during a physical or chemical change. –The heat absorbed by the calorimeter and its contents is the negative of the heat of reaction.

26 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. A Problem to Consider When 23.6 grams of calcium chloride, CaCl 2, was dissolved in water in a calorimeter, the temperature rose from 25.0 o C to 38.7 o C. If the heat capacity of the solution and the calorimeter is 1258 J/ o C, what is the enthalpy change per mole of calcium chloride? HW 54

27 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. 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 is the sum of the enthalpy changes for the individual steps. Ex 1 Hess’s Law

28 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. 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 is the sum of the enthalpy changes for the individual steps. Ex 2 Hess’s Law

29 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Hess’s Law use these data to obtain the enthalpy change for the following reaction?

30 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Ex. Hess’s Law HW 55

31 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. The standard enthalpy of formation of a substance, denoted  H f o, is the enthalpy change for the formation of one mole of a substance in its standard state from its component elements in their standard state (298K and 1 atm). –Note that the standard enthalpy of formation for a pure element in its standard state and H + is zero. Standard Enthalpies of Formation

32 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. The law of summation of heats of formation states that the enthalpy of a reaction is equal to the total formation energy of the products minus that of the reactants..  is the mathematical symbol meaning “the sum of”, and m and n are the coefficients of the substances in the chemical equation. Standard Enthalpies of Formation

33 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Ex. Generic Law of Summation aA + bB  cC + dD

34 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. A Problem to Consider –What is the standard enthalpy change for this reaction? HW 56

35 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline.