All Flashcards
What is the standard enthalpy of formation (ΔHf)?
The enthalpy change when one mole of a compound is formed from its elements in their most stable state at standard conditions (25°C and 1 atm).
What are standard conditions for enthalpy of formation?
25°C (298 K) and 1 atm.
What is meant by 'most stable state'?
The form of an element that is most stable at standard conditions (e.g., O2(g), C(s, graphite)).
What is the ΔHf of an element in its standard state?
Zero.
Define ΔHrxn.
The enthalpy change for a chemical reaction.
How do you calculate ΔHrxn using ΔHf values?
ΔHrxn = ΣnΔHf(products) - ΣmΔHf(reactants), where n and m are stoichiometric coefficients.
What is the first step to calculate ΔHrxn?
Write out the balanced chemical equation.
What is the second step to calculate ΔHrxn?
Identify the ΔHf values for each reactant and product.
What is the third step to calculate ΔHrxn?
Apply the formula: ΔHrxn = ΣnΔHf(products) - ΣmΔHf(reactants)
What is the fourth step to calculate ΔHrxn?
Make sure to pay attention to the stoichiometric coefficients in the balanced equation when applying the formula.
What is the effect of using the wrong state symbol (e.g., H2O(g) instead of H2O(l))?
It will lead to an incorrect ΔHrxn calculation.
What is the effect of not balancing the chemical equation correctly?
Incorrect stoichiometric coefficients will lead to an incorrect ΔHrxn calculation.
What is the effect of a catalyst on ΔH°?
The value of ΔH° would not be affected because a catalyst does not change the enthalpy of the reaction, it only changes the rate.