Multiple Choice
In a reaction energy diagram, which point or segment represents the enthalpy change (ΔH) of the reaction?
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8. Thermochemistry
Thermochemical Equations
Multiple Choice
Which of the following correctly describes how to calculate the enthalpy change (ΔH) of a reaction using bond energies?
A
ΔH = sum of bond energies of bonds broken + sum of bond energies of bonds formed
B
ΔH = sum of bond energies of all reactants and products
C
ΔH = sum of bond energies of bonds broken − sum of bond energies of bonds formed
D
ΔH = sum of bond energies of bonds formed − sum of bond energies of bonds broken
Verified step by step guidance1
Understand that bond energies represent the amount of energy required to break a bond between two atoms. Breaking bonds requires energy (endothermic), while forming bonds releases energy (exothermic).
Identify that the enthalpy change of a reaction (\( \Delta H \)) can be estimated by considering the energy needed to break all bonds in the reactants and the energy released when new bonds form in the products.
Write the general formula for enthalpy change using bond energies as:
\[ \Delta H = \sum \text{(bond energies of bonds broken)} - \sum \text{(bond energies of bonds formed)} \]
Note that the sum of bond energies of bonds broken corresponds to the energy input (positive), and the sum of bond energies of bonds formed corresponds to the energy released (negative), so subtracting the latter from the former gives the net energy change.
Avoid incorrect formulas such as adding bond energies of bonds broken and formed or summing all bond energies without distinction, as these do not correctly represent the energy changes during the reaction.
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