Enthalpy Of Reaction Worksheet Page 2
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1
2
3
4Enthalpy Of Reaction (∆
H)
rxn
Model 3: Enthalpy of Reaction using ∆
H and ∆
H
atom
ac
To determine the overall value of ΔH for a reaction, we can imagine the reaction taking place by:
(i)
breaking apart all of the reactant molecules into their constituent atoms: Δ
H (reactants)
atom
(ii)
reassembling or combining these atoms into the product molecules: Δ
H (products)
ac
The overall enthalpy of the reaction is then the sum of these parts:
Δ
H = Δ
H (reactants) + Δ
H (products)
(3)
rxn
atom
ac
Critical thinking questions
1.
Why is the ΔH associated with the upward arrow in Model 3 a positive number?
2.
Why is the ΔH associated with the downward arrow in Model 4 a negative number?
3.
What is the value of ΔH for the overall reaction in Model 3?
4.
Using your answer to Q2, rewrite the equation below so that it involves only Δ
H (reactants) and
ac
Δ
H (products).
ac
Δ
H = Δ
H (reactants) + Δ
H (products) =
rxn
atom
ac
5.
Using your answer to Q2, rewrite the equation below so that it involves only Δ
H (reactants) and
atom
Δ
H (products).
atom
Δ
H = Δ
H (reactants) + Δ
H (products) =
rxn
atom
ac
6.
If Δ
H for the reactants is more negative than Δ
H for the products in a chemical reaction, will Δ
H
ac
ac
rxn
be positive or negative? Explain your reasoning.
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