state function revisited….state function revisited…. dependent only on a system’s state at a...
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Standard ConditionsStandard Conditions Represented by a “ ° “ beside Δ H or Δ E, etc. 25°C 1 atmTRANSCRIPT
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Hess’ Law and Enthalpy of Formation
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State Function revisited….
Dependent ONLY on a system’s state at a given moment in time. Only initial and final states Not based on the path to get to a given
condition
Ex. Energy, Enthalpy
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Standard Conditions
Represented by a “ ° “ beside ΔH or ΔE, etc.
25°C
1 atm
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Methods for determining ΔH
1) Calorimetry
2) Application of Hess’ Law
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Hess’ Law
Enthalpy change for a chemical reaction is the same whether it occurs in multiple steps or one step ΔHrxn = ΣΔHA+B+C (sum of ΔH for each step)
Allows us to break a chemical reaction down into multiple steps to calculate ΔH Add the enthalpies of the steps for the enthalpy for
the overall chemical reaction
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Guidelines for using Hess’ Law
Must use data and combine each step in a way that gives the chemical reaction with the unknown ΔH
Set up steps so chemical compounds not in the final reaction are cancelled
Reverse a reaction if necessary and change the sign on ΔH
Check for correct mole ratios
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Example 1:
H2O(l) H2O (g) ΔH° = ?
Based on the following: H2 + ½ O2 H2O(l) ΔH° = -285.83 kJ/mol
H2 + ½ O2 H2O(g) ΔH° = -241.82 kJ/mol
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Example 2:
C(s) + 4H2 C3H8 (g) ΔH° = ?
Based on the following: 2H2 + O2 2H2O ΔH° = -571.7 kJ/mol
C3H8 + 5 O2 3CO2 + 4H2O(g) ΔH°= -2220.1kJ/molC(s) + O2 CO2 (g) ΔH° = -393.5 kJ/mol
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Methods for determining ΔH
1) Calorimetry
2) Application of Hess’ Law
3) Enthalpies of Formation
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Enthalpy of Formation (ΔHf°)
Enthalpy for the reaction forming 1 mole of a chemical compound from its elements in a thermodynamically stable state.
Elements present in “most thermodynamically stable state” 25°C°, 1atm
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Example 3
MgO (s) + CO2 (g) MgCO3 (s)
MgO + CO2 Mg (s) + C (s) + 3/2 O2
Mg (s) + C (s) + 3/2 O2 MgCO3
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Apply Hess’ Law----
Really ΔHf (products) - ΔHf (reactants)
Calculate ΔHrxn based on enthalpy of formation (ΔHf)
aA + bB cC + dDΔH° =[c (ΔHf°)C + d(ΔHf°)D] - [a (ΔHf°)A + b (ΔHf°)B ]
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Example 4
Calculate the ΔH° for the combustion of 1 mole of pentaborane (B5H9) reacting with excess oxygen
2B5H9 + 12O2 5B2O3 + 9H2O ΔH° = ?
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Example 5
Isopropyl alcohol (rubbing alcohol) undergoes a combustion reaction
2(CH3)2CHOH + 9O2 6CO2 + 8H2O
ΔH° = -4011 kJ/mol
Calculate the standard enthalpy of formation for isopropyl alcohol.
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Homework
pp. 253 #65-67, 72