Enthalpy Of Reaction Worksheet
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1
2
3
4Enthalpy Of Reaction (∆
H)
rxn
CHEM1101 Worksheet 10: Enthalpy of Reaction (∆
H)
rxn
Model 1: Endothermic and Exothermic Processes
Breaking bonds requires energy to pull the atoms apart: bond breaking is endothermic (ΔH > 0). When bonds
are formed, energy is released – precisely the same amount of energy which would be required to break those
bonds: bond making is exothermic (ΔH < 0).
In most chemical reactions, bonds are broken and made. Whether a reaction is endothermic or exothermic
depends on the energy required to perform the changes in the bonding.
Critical thinking questions
1.
Are the following reactions exothermic or endothermic?
(a)
H
(g) 2H(g)
(b)
Cl
(g) 2Cl(g)
2
2
(c)
H(g) + Cl(g) HCl(g)
Model 2: Enthalpy of Atomization (∆
H) and Enthalpy of Atom Combination (∆
H)
atom
ac
When a mole of a compound is broken apart into
2 moles of H(g)
its constituent gas phase atoms, energy is
consumed and the energy change is called the
products
H
products
enthalpy of atomization (Δ
H):
atom
H (H
) = +435 kJ
atom
2
Δ
H = H(products) – H(reactants)
atom
1 mole of H
(g)
= H(atoms) – H(compound) (1)
2
reactants
H
reactant
When a mole of a compound is made from its
2 moles of H(g)
constituent gas phase atoms, energy is released
H
products
and the energy change is called the enthalpy of
products
atom combination (Δ
H):
ac
H (H
) = -435 kJ
ac
2
Δ
H = H(products) – H(reactants)
ac
1 mole of H
(g)
2
= H(compound) – H(atoms)
(2)
reactants
H
reactant
Critical thinking questions
1.
What is the relationship between Δ
H and Δ
H for a compound like H
?
atom
ac
2
2.
What is the value of ΔH for the overall process of separating one mole of H
(g) into its constituent
2
atoms and then reforming one mole of H
(g)?
2
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