As qp symbolized heat transfer at constant pressure

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Unformatted text preview: , by definition, Heat enthalpy enthalpy So, enthalpy (H) or change in enthalpy (∆H) is q So, (well the special case at constant pressure) which we have already been dealing with we State Functions State State function – changes in internal energy and enthalpy State that accompany chemical or physical changes do not depend on which path is chosen going from the initial state to the final state state – Examples Balloon – blow up to a certain amount – Or blow up higher and let air out Bank Account with $100 – – – – Deposit $100 Deposit $1 one hundred times Deposit $1,000, spend $900 Etc. Etc. Enthalpy (or change in enthalpy) is a state function Enthalpy Enthalpy Changes for Chemical Reactions Reactions All chemical reactions have enthalpy changes H2O(g) H2(g) + ½O2(g) ∆H=+241.8 kJ – +, so it is endothermic H2(g) + ½O2(g) H2O(g) O(g) ∆H=-241.8 kJ ∆H=-241.8 – -, so it is exothermic IMPORTANT: The quantity of heat transferred IMPORTANT: depends on the amounts of products formed or reactants used, reactants 2H2(g) + O2(g) 2H2O(g) ∆H=-483.6 kJ O(g) ∆H=-483.6 Enthalpy Changes for Chemical Reactions Reactions IMPORTANT: ∆H also depends on state IMPORTANT: H2(g) + ½O2(g) H2O(l) ∆H=-285.8 kJ O(l) ∆H=-285.8 More energy is released since 1 mol of vapor More condenses to 1 mol of liquid water condenses Enthalpy Changes for Chemical Reactions Reactions Enthalpy changes are specific to the reactants and Enthalpy products and their amounts. Both the identity of reactants and products and their states are important reactants ∆H has a negative value if heat is evolved (exothermic) has and a positive value if heat is required (endothermic) and Values of ∆H are numerically equal with a different sign if Values a chemical reaction is reversed chemical The enthalpy change depends on the molar amount of The reactants and products. The formation of 2 mol H2O(g) reactants O(g) from the elements results in an enthalpy change that is twice as large as the enthalpy change in forming 1 mol twice Enthalpy Changes for Chemical Reactions Reactions Enthalpies of reactions are expressed as Enthalpies energy per mole of reactant or per mole of product product Hess’s Law Hess’s Hess’s Law – if a reaction is the sum of Hess’s two or more reactions, ∆H for the overall process is the sum of the ∆H values of those reactions those Standard Enthalpies of Formation Standard Standard Molar enthalpies of formation, Standard ∆Hof – is the enthalpy change for the ∆H formation of 1 mol of a compound directly from its component elements in their standard states standard Standard state – most stable form of an Standard element or compound in the physical state that exists at 1 bar at a specified temperature, mostly 25oC temperature, Standard Enthalpies of Formation Standard Standard enthalpy of formation for an element in Standard its standard state is zero its Compounds in solution refer to the enthalpy Compounds change for the formation of a 1M solution of the compound from the elements + enthalpy change when the substance dissolves in water when Most ∆Hof values are negative, exothermic. Heat evolution when forming compounds is productevolution favored Enthalpy Change for a Reaction Enthalpy Enthalpy change for a reaction under standard Enthalpy conditions can be calculated if the molar enthalpies of formation are known for all reactants and products reactants ∆Hof = ∑[∆Hof (products)] - ∑[∆Hof (reactants)] Product or Reactant Favored Reactions and Thermochemistry Reactions When reaction goes left to right and most When reactants form products, product-favored reactants Most of the time, negative ∆Horrxn values xn are product-favored and positive ∆Horrxn are xn values are reactant favored. values...
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