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Transcript
Thermochemistry Nearly every chemical reaction occurs with either the absorption or release of energy. The study of such energy transfers is thermochemistry. System refers to what is being studied. In chemistry, the balanced chemical equation indicates the chemical system. Everything else in the universe is the surroundings. Often the chemical system is separated from surroundings by a boundary, which prevents energy transfer and insulates the system (e.g. styrofoam). The flow of energy is controlled, allowing researchers to measure it. Energy Energy is defined as the ability to do work, is measured in joules (J). The first law of thermodynamics states that energy cannot be created or destroyed but only change forms (e.g. thermal energy can be converted to chemical energy). Chemical energy describes the potential energy that is contained within a compound. A more reactive substance contains greater chemical potential to undergo a reaction. A substance that is very stable contains little chemical potential energy. In any type of change (i.e. physical, chemical, or nuclear), the energy transfer can be determined by the amount of heat absorbed or released by the system. This was initially measured in calories (cal) where 1 calorie is the amount of energy required to raise 1 g of water 1oC. 1 cal = 4.184 J Measuring Energy Changes: Calorimetry Calorimetry is the technical process of measuring energy changes in a chemical system. During a chemical change where energy is given off, the surroundings will be warmed. If energy is absorbed, the surroundings will be cooled. the energy can be transferred to or absorbed from a particular amount of water causing its temperature to rise q = mcT q is the amount of heat transferred (J) m is the mass of substance heated (g) T is the temperature change (oC) c is the specific heat capacity (J/goC) Specific heat capacity is a property of a substance that describes the amount of heat required to raise the temperature of 1 g of the substance by 1oC. the specific heat capacity of water is 4.184 J/goC (arising from the value of 1 calorie) Enthalpy The chemical potential energy absorbed or released by a chemical system when it changes from reactants to products is the enthalpy change. The enthalpy of reaction is the change in potential energy of a chemical system (sometimes referred to as the “heat of reaction” or “change in heat”). In an exothermic reaction, the enthalpy change for the system has a negative value, as chemical potential energy is lost by the system. placed on products’ side of chemical equation In an endothermic reaction, the enthalpy change for the system has a positive value, as chemical potential energy is gained by the system. placed on reactants’ side of chemical equation Molar Enthalpy (Hx) The enthalpy change associated with one mole of a substance undergoing any change is called the molar enthalpy. expressed in kilojoules per mole (kJ/mol) If the enthalpy change is written with the balanced chemical equation, the value MUST correspond to the molar coefficients in the equation (i.e. the value needs to be converted to molar enthalpy). Hvap = 40.8 kJ/mol of H2O(l) Evaporation of water (physical change) 1H2O(l) 1H2O(g) H = 40.8 kJ Incorporated into equation (on reactant side) 1H2O(l) + 40.8 kJ 1H2O(g) 2H2O(l) + 2(40.8 kJ) 2H2O(g) Evaporation of 2 moles of water 2H2O(l) 2H2O(g) H = 81.6 kJ Condensation of water (reverse process) 2H2O(g) 2H2O(l) 1H2O(g) 1H2O(l) H = -81.6 kJ H = -40.8 kJ Incorporated into equation (on product side) 1H2O(g) 1H2O(l) + 40.8 kJ Combustion of methane gas 1CH4(g) + 2O2(g) 1CO2(g) + 2H2O(g) H = -890 kJ 1CH4(g) + 2O2(g) 1CO2(g) + 2H2O(g) + 890 kJ 1CH4(g) + 2O2(g) 1CO2(g) + 2H2O(g) H = -890 kJ (for 1 mol of CH4(g)) H = -890 kJ (for 2 mol of O2(g)) H = -890 kJ (for 1 mol of CO2(g)) H = -890 kJ (for 2 mol of H2O(g)) Combustion of 2 moles of methane gas 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) + 2(890 kJ) 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) H = -1780 kJ (for 2 mol of CH4(g)) Combustion of ½ mole of methane gas 1/2CH4(g) + 1O2(g) 1/2CO2(g) + 1H2O(g) H = -445 kJ (for 0.5 mol of CH4(g)) Incorporated into equation (on product side) 1CH4(g) + 2O2(g) 1CO2(g) + 2H2O(g) + 890 kJ 1CO2(g) + 2H2O(g) 1CH4(g) + 2O2(g) H = 890 kJ A visual representation of chemical energy changes can be used to portray a chemical reaction. potential energy diagrams Potential Energy Diagrams Potential Energy Diagrams products stored chemical energy is high Energy is released during the chemical change. products Time Energy Energy reactants stored chemical energy is low stored chemical energy is low stored chemical energy is high Energy is absorbed during the chemical change. reactants Time The value of the energy change shown in the diagram should reflect the molar coefficients of the balanced chemical equation. Standard Molar Enthalpy of Reaction the enthalpy change associated with the reaction of one mole of a substance at 100 kPa and a specified temperature of 25oC (SATP conditions) use the symbol Hxo for standard enthalpy H = nHxo and sinceH = q (opposite signs)* nHo= mcT *However, we MUST indicate the correct sign depending on whether the change is endothermic or exothermic. standard heat of formation (measured at standard conditions) A + B C + D + energy if endothermic 2A + 2B D + C 2C B H o = value of enthalpy change + energy if exothermic + 2D + A H o = 2(value of enthalpy change) H o = - (value of enthalpy change) Hess's Law of Heat Summation A state function is any physical property whose value does not depend on the system's history (e.g. temperature, pressure, volume). enthalpy change (H) is a state function Enthalpy change is determined only by the enthalpies of the initial and final states of the chemical system and not by the path taken. Consider, C(s) + O2(g) Ho = -393.5 kJ CO2(g) This could be written as two steps: C(s) + 0.5O 2(g) CO (g) Ho = -110.5 kJ CO (g)+ 0.5O 2(g) CO 2(g) Ho = -283.0 kJ (-110.5 kJ) + (-283.0 kJ) = -393.5 kJ Law of Heat Summation For any reaction that can be written in steps, the standard heat of reaction is the same as the sum of the standard heats for the steps. C(s) + O 2(g) Increasing H H o = -110.5 kJ CO (g)+ H o = 0.5O 2(g) -393.5 kJ H o = -283.0 kJ CO 2(g) Enthalpy diagram forC(s) + O 2(g) Total H o = (-110.5 kJ) + (-283.0 kJ) = -393.5 kJ CO 2(g) Standard Enthalpy of Formation The standard enthalpy of formation of a compound Hfo is the amount of heat absorbed or evolved when one mole of the compound is formed from its elements in their standard states. H2(g) + 0.5O2(g) H2O(l) Hfo = -285.9 kJ/mol The Ho for a reaction can be obtained by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products. nHof (products) Ho = aA + bB + ... Ho = [nHfo(N) + mHfo(M) + …] - nHof (reactants) nN + mM + ... - [aHfo(A) + bHfo(B) +…] Standard Enthalpy of Combustion The experimental determination of Hfo values is often difficult or impossible. Combustion of organic compounds involves the reaction with molecular oxygen to form gaseous carbon dioxide and liquid water, as seen in this example with sucrose: C12H22O 11(s)+ 12O 2(g) 12CO2(g)+ 11H2O(l) Hocombustion = -5640.9 kJ/mol The enthalpy change for the combustion of one mole of a compound under standard conditions is called the standard enthalpy of combustion. This value can be experimentally determined using a bomb calorimeter (q = Ct, where the heat capacity of the bomb, C, must be known). Because the Hfo for O2(g), CO2(g), and H2O(l) are known, the standard enthalpy of formation of sucrose can be found. (1mol)Hocomb = [(11 mol)Hfo(H2O) + (12 mol)Hfo(CO2)] - [(1 mol)Hfo(C12H22O11) + (12 mol)Hfo(O2)] (1 mol)(-5640.9 kJ/mol) = [(11 mol)(-285.8 kJ/mol) + (12 mol)(-393.5 kJ/mol)] - [(1 mol)Hfo(C12H22O11) + ( 12 mol)(0 kJ/mol)] -5640.9 kJ = [(-3144 kJ) +(-4722kJ)] – [(1 mol)Hfo(C12H22O11] (1 mol)Hfo(C12H22O11) = Hfo(C12H22O11) = -2225 kJ -2225 kJ/mol (units become “per mole” in the end)