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What is the difference between H<sub>2</sub>O(l) and H<sub>2</sub>O(g) when calculating ΔH<sub>rxn</sub>?
They have different ΔH<sub>f</sub> values. Always use the value corresponding to the correct state.
How do you calculate ΔH<sub>rxn</sub> using ΔH<sub>f</sub> values?
ΔH<sub>rxn</sub> = ΣnΔH<sub>f</sub>(products) - ΣmΔH<sub>f</sub>(reactants), where n and m are stoichiometric coefficients.
What is the first step to calculate ΔH<sub>rxn</sub>?
Write out the balanced chemical equation.
What is the second step to calculate ΔH<sub>rxn</sub>?
Identify the ΔH<sub>f</sub> values for each reactant and product.
What is the third step to calculate ΔH<sub>rxn</sub>?
Apply the formula: ΔH<sub>rxn</sub> = ΣnΔH<sub>f</sub>(products) - ΣmΔH<sub>f</sub>(reactants)
What is the fourth step to calculate ΔH<sub>rxn</sub>?
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., H<sub>2</sub>O(g) instead of H<sub>2</sub>O(l))?
It will lead to an incorrect ΔH<sub>rxn</sub> calculation.
What is the effect of not balancing the chemical equation correctly?
Incorrect stoichiometric coefficients will lead to an incorrect ΔH<sub>rxn</sub> 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.