chemistry chapter 7 – hess’s law teacher: h. michael hayes

Chemistry Chapter 7 – Hess’s Law

Teacher: H. Michael Hayes

‘The Reaction Mechanism’

• Reactions are complex and require several steps to convert Reactants to Products

• How do we determine the enthalpy change of complex reactions?

• Using Hess’s Law (and without a calorimeter) we can calculate the enthalpy change of complex reactions.

“A Reaction Mechanism is a series of simple reactions that convert reactants into products over the course of a complex reaction.”

‘The Reaction Mechanism ~ an example’

• x reactant(s) → y product(s)

• N2O4(g) → 2NO2(g) - a simple one step reaction.

• 2 NO(g) + O2 → 2NO2(g) – a complex two step reaction

• During the above reaction it is possible to detect the presence of another compound [N2O2] – This indicates that the reaction is complex.

“In fact, the reaction mechanism of dinitrogen dioxide formation involves two intermediate steps, each one constituting a simple reaction.”

1 – 2 NO(g) → N2O2(g)

2 – N2O2(g) + O2(g) → 2 NO2(g)

‘The Reaction Mechanism ~ an example’

• The overall balanced equation of the reaction corresponds to the sum of the equations of the two simple reactions.

“Reaction mechanisms cannot be directly observed. They are models that evolve to explain all observations over the course of a reaction.”

‘Graphical representation of a reaction mechanism’

• The progress of a complex reaction is visualized with an energy diagram.

“How many intermediate steps are involved in this hypothetical complex reaction where A → E ?”

Each simple reaction possesses its own activation energy [Ea1

, Ea2,

Ea3, Ea4

]. The overall activation

energy is [Ea]. Can you see why?

In a similar fashion the change in enthalpy [∆H] of the overall reaction is determined by combining [∆H1, ∆H2, ∆H3, ∆H4]

‘Summation of enthalpies’

• According to Hess’s Law, also know as the law of constant heat summation, if a reaction can be broken down into several simple reactions, its enthalpy change is equal to the algebraic sum of the enthalpy changes of each of its simple reactions.

‘It’s not always possible to use calorimetry to study enthalpy change. Some reactions occur very slowly and the resulting temperature changes are impossible to record’

‘We make use of standard molar enthalpies of formation tables to solve Hess’s Law Problems’ (pg 418 of Quantum)

‘Other reactions are fast and violent and for these reasons calorimetry is not a viable option for determining enthalpy changes’

‘Summation of enthalpies’

Hess’s Law – The enthalpy change of the overall reaction is equal to the sum of the enthalpies of the intermediate reactions.

The two intermediate reactions involved in the production of carbon dioxide.

The overall reaction

‘Summation of enthalpies’

ΔH= -393.5ΔH= -393.5

‘Summation of enthalpies’

ΔH= -393.5

• The RULES.

• Applying the RULES to the following problem.

‘Summation of enthalpies’

ΔH= -393.5

• Applying the RULES to the following problem.

• Break down the problem into several steps using data in tables.

‘Summation of enthalpies’

ΔH= -393.5

• Applying the RULES to the following problem.

• Flip the reactions and use the coefficients to change ΔH values.

‘Summation of enthalpies’

ΔH= -393.5

• Applying the RULES to the following problem.

• Sum up the reactions – cancel like terms – add up the ΔH values.

Hess’s Law – another example

ΔH= -393.5

Hess’s Law – another example

ΔH= -393.5

Hess’s Law – another example

ΔH= -393.5

Hess’s Law – Summary

ΔH= -393.5

Hess’s Law – Summary

ΔH= -393.5

Hess’s Law

ΔH= -393.5

• End of Chapter 7

• Now on to something different…

• Reaction Rates is Next