Standard Enthalpy of Formation Reaction The standard enthalpy of formation (Delt

Question

Standard Enthalpy of Formation Reaction The standard enthalpy of formation (Delta Hfdegree ) is the enthalpy change that occurs when exactly 1 mol of a compound is formed from its constituent elements under standard conditions. The standard conditions are 1 atm pressure, a temperature of 25 Degree C , and all the species present at a concentration of 1 M . A "standard enthalpies of formation table" containing Delta Hf degree values might look something like this: What is the balanced chemical equation for the reaction used to calculate Delta Hf degree of MgCOs (s)? If fractional coefficients are required, enter them as a fraction (i.e. 1/3). Indicate the physical states using the abbreviation (s), (1), or (g) for solid, liquid, or gas, respectively without indicating allotropes. Use (aq) for aqueous solution. Express your answer as a chemical equation.

Explanation / Answer

Part A

Mg(s) + C(s) + 3/2 O2(g) ----> MgCO3(s)

Part A

Ca(OH)2(S) + CO2(g) -----> CaCO3(s) + H2O(l)

DHrxn0 = (DHf0 H2O + DHf0caCO3)-(dhf0 Ca(OH)2 + DHf0 C + 3/2DHf0O2)

     = (-285.8-1207)-(-393.5-986.1)

    = -113.2 kj/mol

Part B

Ca(OH)2(S) -----> CaO(s) + H2O(l)

DHf = (DHf0 CaO + DHf0 H2O) -(DHf0 Ca(OH)2)

65.2 = (DHf0 CaO+ (-285.8))-(-986.1)

DHf0 CaO = -635.1 kj/mol

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