ThermochemistryHess’s Law of Additivity of

Reaction Enthalpies

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I will be able to use proper scientific terminology to describe Hess’s Law

I will be able to use Hess’s law to calculate enthalpies in multiple step reactions

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Learning Goals

describe Hess’s law use Hess’s law to calculate enthalpies in

multiple step reactions

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Success Criteria

Calorimetry is an excellent technique for determining enthalpy changes, BUT is no longer useful when reactions are too slow and temperature changes would be too small to measure

Chemists utilize methods to deal with this problem based on the principle that net (or overall) changes in some properties of a system are independent of the way the system changes from initial state (reactants) to the final state (products)

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Brick Laying Analogy

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The value of the ∆H for any reaction that can be written in steps equals the sum of the values of ∆H for each of the individual steps.

OR If two or more equations with known enthalpy

changes can be added together to form a new “target” equation, then their enthalpy changes may be similarly added together to yield the enthalpy change of the target equation

 

Hess’s Law

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 Written as an equation 

∆Htarget = ∆H1 + ∆H2 + ∆H3 + …

∆Htarget = ∑ ∆Hknown

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Example 1Carbon can react with oxygen to form carbon

monoxide. The carbon monoxide then reacts with oxygen to produce carbon dioxide. The two equations below represent this pathway.

C(s) + ½ O2(g) CO(g) ∆Hº = -110.5 kJ

CO(g) + ½ O2(g) CO2(g) ∆Hº = -283.0 kJ

 

Carbon can also react with oxygen to produce carbon dioxide directly.

 C(s) + O2(g) CO2(g) ∆Hº = -393.5 kJ

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Hess’s Law allows chemists to determine the enthalpy change of a reaction without direct calorimetry, using two familiar rules for chemical equations and enthalpy changes

1. If a chemical equation is reversed, then the sign of the ∆H changes.

2. If the coefficients of a chemical equation are altered by multiplying or dividing by a constant factor, then the ∆H is altered in the same way.

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Practice 1A typical automobile engine uses a lead-acid battery.

During discharge, the following chemical reaction takes place.

 2PbO(s) + 2H2SO4(l) 2PbSO4(aq) + 2H2O(l)

 Determine the enthalpy change of this reaction, given

the following equations. (1) 2PbO (s) + 2SO3(g) 2PbSO4(s) ∆Hº = -775 kJ

(2) SO3(g) + H20(l) H2SO4(l) ∆Hº = -133 kJ

  

Answer: -509 kJ11

Practice 2Mixing household cleansers can result in the

production of hydrogen chloride gas, HC1(g). Not only is this gas dangerous in its own right, but it also reacts with oxygen to form chlorine gas and water vapour. 

4HCl(g) + O2(g) 2Cl2(g) + 2H2O(g)

Determine the enthalpy change of this reaction, given the following equations.

 (1) H2(g) + Cl2(g) 2HC1(g) ∆Hº = -185 kJ

(2) H2(g) + ½ O2(g) H2O(l) ∆Hº = -285.8 kJ

(3) H2O(g) H2O(l) ∆Hº = -40.7 kJ

 Answer: -120.2 kJ 12

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Learning Checkpoint

Practice pp. 326-327 UC # 1, 2, 3

Multi-Step Enthalpy Practicep. 329 UC # 4, 5

Additional Practicep. 330 UC # 1, 2, 3