Hess Law

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Heat of Reaction and Hess’s Law

Introduction: Hess’s Law is a very useful relationship that allows the calculation of the heat of reaction for reactions on paper (without carrying out an actual experiment of that particular reaction). You must first know the heats of reaction for related reactions that add algebraically to give the desired reaction. If it can be shown that reaction 1 + reaction 2 = reaction 3, then Hess’s Law states that H1 + H2 =H3. Thus, if you know the heats of reaction for the first two reactions, you simply need to add them to get the desired result. Hess’s law is easily extended when the relationship between the three equations is not so simple... multiplying equations by constants multiplies the heat of reaction by the same constant; reversing an equation just reverses the sign of H.

In this lab, you will perform three different experiments which are related to each other. From your data, you will determine the heats of reaction for all three reactions by use of a calorimeter. The calorimeter allows you to measure heat because all of the heat released by the reaction is assumed to be used to heat up the solution in which the reaction takes place. You will then use Hess’s law and the first two heats of reaction to calculate the value for the third and compare it to the experimental value. The three experiments are shown below as complete ionic equations. Use the given number code throughout this lab. All three reactions are exothermic.

1) Dissolving solid sodium hydroxide in water (this heat is actually a “heat of solution” since no true chemical reaction occurs):
NaOH (s) ---> Na+ (aq) OH¯ (aq)H1
2) Solid sodium hydroxide is dissolved in a hydrochloric acid solution:
H+ (aq) Cl¯ (aq)NaOH (s) ---> Na+ (aq) Cl¯ (aq)O (l)H2
3) A solution of sodium hydroxide is mixed with a hydrochloric acid solution:
H+ (aq) Cl¯ (aq)Na+ (aq) OH¯ (aq)---> Na+ (aq) Cl¯ (aq)O (l)H3

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