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Chemistry 100. Enthalpy and Hess’s Law. Energy Changes in Chemical Reactions. Let’s take a typical reaction CH 4 (g) + O 2 (g) CO 2 (g) + 2 H 2 O (l) This is an example of ‘burning a fossil fuel.’ This reaction releases energy! What about this reaction? KCl (s) KCl (aq)
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Chemistry 100 Enthalpy and Hess’s Law
Energy Changes in Chemical Reactions • Let’s take a typical reaction CH4 (g) + O2 (g) CO2 (g) + 2 H2O (l) • This is an example of ‘burning a fossil fuel.’ This reaction releases energy! • What about this reaction? KCl (s) KCl (aq) • This reaction requires energy!
Energy Changes System energy surroundings Almost all chemical and physical processes are accompanied by the loss or gain of energy from the system.
Enthalpy Enthalpy H º the heat content of a substance. Enthalpy change - DH thermal energy (heat) evolved or absorbed under constant pressure conditions.
Exothermic vs. Endothermic Processes • Exothermic process ® heat content of system decreases, the system enthalpy decreases (i.e., DH < 0) • e.g., the combustion of methane CH4 (g) + O2 (g) CO2 (g) + 2 H2O (l) • Endothermic process ® energy of the system is increased (i.e. DH > 0). • e.g., dissolving potassium chloride in water. KCl (s) KCl (aq)
The Enthalpy Change in a Chemical Reaction • We want to know how much energy is released when we burn ethane! C2H6 (g) + 7/2 O2 (g) 2 CO2 (g) + 3 H2O (l) • We wish to know the difference in the heat content of the reactants vs. the products. • H npH (products) - nr H (reactants) • np and nr represent the number of moles of products and reactants, respectively.
The Enthalpy Change (cont’d) • For the ethane combustion reaction 1C2H6 (g) + 7/2 O2 (g) 2 CO2 (g) + 3 H2O (l) H npH (products) - nr H (reactants) = 3 H[H2O (l)] + 2 H[CO2 (g)] - (7/2 H[O2(g)] + 1 H[C2H6 (g)] )
Conventions for Writing Thermochemical Equations • For exothermic reactions, H <0; for endothermic reactions, H > 0. • H values are given for processes occurring at 25C and 1 atm pressure • STTP - standard thermodynamic temperature and pressure.
Conventions (cont’d) • The physical state of each substance participating in the process must be stated • s solid • l liquid • g gas • aq aqueous solutions
Conventions (cont’d) Fractional stoichiometric coefficients are permitted in thermochemical equations. The stoichiometric coefficients = the number of moles of each substance involved in the transformation
Conventions (cont’d) • Multiply or divide a chemical equation by a factor, the H value must also be multiplied or divided by that factor • Enthalpy is an extensive property. • When an equation is reversed, the sign of the H value is changed but its magnitude stays the same.
DH Values and Hess’s Law • Calculating enthalpy changes for physical and chemical transformations. • tabulated values of reaction enthalpies (largely measured with a calorimeter). • indirect Method – Hess’s Law.
Hess’s Law • Hess’s Law - enthalpy changes for sequences of reactions. • Enthalpy change accompanying a reaction is the same whether the reaction occurs in a single step or in many steps.