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IB CHEMISTRY

Reactivity 1.2.2

Hess's Law

"The enthalpy change for a reaction is independent of the route taken."

Reactants (A)

\Delta H_{reaction}

Products (B)

Route 1

Route 2

Intermediates (C)

Solving Cycle Problems

Equation Manipulation

You are given step-equations. Rearrange them to match the target.

Flip the equation? $\rightarrow$ Flip the sign of $\Delta H$ .
Multiply by 2? $\rightarrow$ Multiply $\Delta H$ by 2.
Add the equations? $\rightarrow$ Add the $\Delta H$ values.

Cycle Diagrams

Draw a triangle. Use the arrows like vectors.

Energy is conserved.
Clockwise Route = Anti-Clockwise Route.
$\Delta H_1 = \Delta H_2 + \Delta H_3$ .

Putting it into Practice

Solving a Hess's Law Cycle

Paper 2 Style

Given the following data:

$S(s) + O_2(g) \rightarrow SO_2(g) \quad \Delta H = -297 \: kJ$
$2SO_3(g) \rightarrow 2SO_2(g) + O_2(g) \quad \Delta H = +198 \: kJ$

Calculate $\Delta H$ for the reaction: $2S(s) + 3O_2(g) \rightarrow 2SO_3(g)$

Practice: Hess's Law Logic

[2 Marks]

State Hess's Law.

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