Enthalpy Theory
Define enthalpy in terms of heat. When is the change in enthalpy equal to the heat flow?
Enthalpy is the heat content of a system. H =q under conditions of constant pressure.
Write the summation formula used to calculate the standard enthalpy of a reaction using enthalpies of formation.
H rxn = Sum of n H (products) - Sum of n H (reactants)
If you reverse a chemical equation, what must you do to the sign of its H value?
Reverse the sign
If a reaction has a negative H, is the reaction exothermic or endothermic? Does the temperature of the surroundings increase or decrease?
Exothermic; the temperature of the surroundings increases as heat is released.
Calculate H for: 2H2 + O2 -> 2H2O
H H2O = -286 kJ/mol
-572 kJ
If you multiply all coefficients in a balanced equation by 2, how does the H value change
x H by 2
What is the standard enthalpy of formation for any element in its most stable form, such as O2 or C?
0 kJ/mol
H rxn = -890 kJ
H CO2 = -394 kJ/mol
H H2O= -286 kJ/mol
-76 kJ/mol
Given:
A -> B H = +50 kJ
B -> C H = -30 kJ
Calculate the H for the reaction A-> C
+20 kJ
State Hess’s Law in your own words.
If a reaction is carried out in a series of steps, the total H for the overall reaction is the sum of the enthalpy changes for the individual steps.
Why must you multiply the H value of a substance by its coefficient in a balanced chemical equation?
Because H is an extensive property (it depends on the amount of substance present, usually given per 1 mole).
Use the following data to find H rxn for 2NO + O2 -> 2NO2
1. N2 + O2 -> 2NO H= +180 kJ
2. N2 + 2O2 -> 2NO2 H = +66 kJ
-114 kJ
Explain why Enthalpy is considered a State Function.
Its value depends only on the current state of the system (reactants and products), not on the path taken to get there.
Calculate the heat released when 4.0 grams of H2 reacts with excess O2 to form H2O.
H rxn = -572 kJ for 2 mols of H2
572 kJ
Find the H for C (diam) -> C (graph) given:
1. C(diam) + O2 -> CO2 H= -395.4 kJ
2. C(graph) + O2 -> CO2 H = -393.5 kJ
-1.9 kJ