All chemical reactions involve changes in energy. Typically this energy comes in the form of heat.

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1 Topic: Thermochemistry Essential Question: How does energy flow in chemical reactions? Name: Class: Date: / / Period: All chemical reactions involve changes in energy. Typically this energy comes in the form of heat THE FLOW OF ENERGY 1. Differentiate between potential & kinetic energy, and between heat & temperature. 2. Convert between calories and joules. 3. Calculate heat flow between objects using changes in temperature. Energy the ability to do work or produce heat Thermochemistry - the study of energy changes that occur during chemical reactions and changes in state. Potential energy stored Kinetic energy motion Law of Conservation of Energy Energy is neither created nor lost in a chemical reaction. Heat v. Temperature Heat (q) = energy in the process of flowing from a higher kinetic energy to an area of lower kinetic energy. Temperature (T) = average kinetic energy in a sample of matter Calculating Heat calorie (cal) = q required to raise 1g H2O by 1 o C Calorie (kcal) = 1000 calories ( a nutritional Calorie ) Joule (J) = SI unit of energy (4.184 J = 1 cal) Endothermic and Exothermic Processes Chemical reactions and changes in physical state generally involve either the absorption or release of heat. System = the part of the universe on which you focus your attention.

2 Surroundings = everything else in the universe. If the energy of the system increases during that process, the energy of the surroundings must decrease by the same amount. If the energy of the system decreases during that process, the energy of the surroundings must increase by the same amount. Endothermic process = heat is absorbed from the surroundings (system is gaining heat; q is positive). Exothermic process = releases heat into its surroundings (system is losing heat; q is negative). Specific Heat Calculating Heat Changes Heat capacity = amount of heat required to raise the temperature of a substance 1 o C. The heat capacity of an object depends on both its mass and its chemical composition. Specific heat = amount of heat required to raise the temperature of 1 gram of a substance 1 o C. Specific heat of water = J/(g o C) or 1 cal/(g o C) Measuring Heat Finding Specific Heat using Water The specific heat of substance may be found using calorimetry with water as the surrounding media. Find the heat gained by the water using qwater = mwater cwater Twater. Set the heat lost by the substance equal to qwater qsubstance = -qwater. Using the temperature change of the substance and its mass, calculate c substance. csubstance = qsubstance /(msubstance Tsubstance) MEASURING AND EXPRESSING ENERGY CHANGES 1. Calculate the specific heat of a substance. 2. Define enthalpy. 3. Describe enthalpy changes in endothermic and exothermic reactions.

3 Calorimetry study of heat flow Heat flow can t be measured directly, it must be calculated by observing changes in temperature Basics of calorimetry o Heat change from a reaction is equal, but opposite in sign to heat change in surroundings qreaction = -qsurrounding. o One can determine the heat absorbed by the surroundings by using the formula q = m c T. where: m = mass of surrounding media c = specific heat of the surrounding media T = change in temperature of surrounding media (T = T final T initial ) o o Usually water is used as the surrounding media because it is easy to work with and has a well known specific heat. qwater = mwater 4.184J/(g o C) T Once the energy change of the water is known, the energy change of the sample is calculated by changing the sign ( + ) or ( + ) of q. Thermochemistry The study of heat changes in chemical reactions Enthalpy (H)- heat content of a system at constant pressure Exothermic Reactions o Reactions that release energy Heat is a product of the reaction Hreactants > Hproducts C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) kJ

4 Endothermic Reactions o Reactions that absorb energy Heat is a reactant in the reaction Hreactants < Hproducts C(s) + H2O(g) + 113kJ CO(g) + H2(g) Heats of Reaction and Heats of Combustion These are enthalpy changes. Heat of Reaction = enthalpy change for the chemical equation exactly as written. Heats of reaction are reported as H. The physical state of reactants and products must be given, Heat of Combustion =heat of reaction for the complete burning of one mole of a substance. Enthalpy Change (H) Hrxn = Hproducts Hreactants Endothermic Reactions o Since Hproducts > Hreactants, Hrxn will be positive (system adds heat)

5 Exothermic Reactions o Since Hproducts < Hreactants, Hrxn will be negative (system loses heat) The H for a reaction is typically listed at the end of a thermochemical equation. C(s) + H2O(g) CO(g) + H2(g) ; H = +113kJ Standard Enthalpy Changes (H 0 ) change in enthalpy when a reaction occurs with reactants and products in their standard states. Standard enthalpy state is the most stable pure form of the element at 1 atm & and 25 o C Enthalpy of elements is arbitrarily set to zero 17.3 HEAT IN CHANGES OF STATE 1. Calculate enthalpy changes for a chemical reaction. 2. Calculate enthalpy changes for a change of phase. Using Thermochemical Equations Calculating Enthalpy Changes In a chemical reaction, the enthalpy change applies proportionally to the amounts of reactants & products. For example: C(s) + H2O(g) CO(g) + H2(g) ; H = +113kJ If 2 moles of C are used and react completely, the energy absorbed (because H is positive) would be 226kJ. H can be set equivalent to any formula in a balanced equation and calculated for a reaction using stoichiometry. Heat Flow in Changes of State Heat flow also occurs in phase changes. Molar Enthalpy of Vaporization (aka, Heat of Vaporization) o Heat required to vaporize 1 mol of a liquid H2O(l) H2O(g) ; Hvap = 40.7 kj/mol Molar Enthalpy of Fusion (aka, Heat of Fusion) o Heat required to melt 1 mol of a solid H2O(s) H2O(l) ; Hfus = 6.01 kj/mol Molar Enthalpy of Solidification (aka, Heat of Solidification) o Heat lost when 1 mol of a liquid solidifies

6 H2O(l) H2O(s) ; Hsolid = kj/mol Molar Enthalpy of Condensation (aka, Heat of Condensation) o Heat released when 1 mol of a vapor condenses H2O(g) H2O(l) ; Hcond = kj/mol Molar Enthalpy of Solution (aka, Heat of Solution) o Heat required to melt 1 mol of a solid CaCl2(s) Ca 2+ (aq) + 2Cl - (aq) ; Hsoln = kj/mol 17.4 CALCULATING HEATS OF REACTION 1. Describe the basis for Hess s Law Hess s Law If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction. Hess s law allows you to determine the heat of reaction indirectly by using the known heats of reaction of two or more thermochemical equations. Hnet = H1 + H2

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