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STANDARD ENTHALPIES

STANDARD ENTHALPIES. Chapter 5.5. STANDARD HEATS OF REACTION. To compare the ΔH for different reactions, the same conditions must apply. Thus standard values for temperature and pressure are required. These have been set at 25°C and 1 atm .

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STANDARD ENTHALPIES

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  1. STANDARD ENTHALPIES Chapter 5.5

  2. STANDARD HEATS OF REACTION • To compare the ΔH for different reactions, the same conditions must apply. • Thus standard values for temperature and pressure are required. • These have been set at 25°C and 1 atm. • A “ ° ” symbol is added to ΔH to indicate that it refers to standard conditions. ΔH° is called the Standard Heat of Reaction. ΔH° ≡ Standard Heat of Reaction • It is not same as ΔH ΔH° ≡ ΔH at 25°C and 1 atm of pressure

  3. Standard Molar Enthalpy of formation (Hf): • The quantity of energy released or absorbed when one mole of a compound is formed directly from elements in their standard states (most stable form) H2(g)+ ½O2(g) H2O(liq)H°f = -285.8 kJ/mol • Hf of an element in its standard state is 0. • Hfis a molar enthalpy with respect to the product being formed

  4. WRITING FORMATION EQUATIONS • STEP 1: Write ONE MOLE of product in the state that has been specified • STEP 2: Write the reactant elements in their standard states • STEP 3: Choose equation coefficients for the reactants to give a balanced equation yielding ONE mole of PRODUCT. Answer Q#1(a) – (d) on page 332 of the textbook.

  5. Example Question • Determine the enthalpy of reaction using enthalpies of formation. Use the enthalpies of formation of each compound. The enthalpies are listed in appendix (pg#799-800).

  6. Example Question • enthalpies 1,2, and 3 must be multiplied by the number of moles for their corresponding compounds. • The enthalpy of reaction turns out to be the sum of nΔH for the products minus that of the reactants

  7. HOMEWORK • Answer • Q#2-5 on page 335 of the textbook • Q#7-10 on page 339 of the textbook • Q#1-5 on page 339 -340 of the textbook

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