Why is the heat of formation for NaOH negative?

In summary, the conversation discusses the confusion surrounding the enthalpy change of dissociation for sodium hydroxide. While the heat of formation for NaOH is exothermic, the enthalpy change for dissociation becomes endothermic if the sign is flipped. This is due to the difference between formation and dissolution, where formation always involves elements and dissolution involves solvation by water.
  • #1
Nanomaly
1
0
Note: This isn't an actual homework question, and I tried asking others (including teachers), but no one could answer it.

Now, the heat of formation for sodium hydroxide is -425.6 kJ/mol (so it's exothermic).

When you dissolve NaOH in water, the reaction is exothermic. But this is dissociation, the opposite of formation. So logically, you would flip the sign around for the heat of formation to get the enthalpy change of the dissociation, right? But if you do, the enthalpy change becomes +425.6 kJ/mol, which means that it is endothermic. However, the dissolving of NaOH in water releases heat to the environment, so it's exothermic!

What's going on? This has been baffling me for a long time now.
 
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  • #2
Nanomaly said:
(snip) But this is dissociation, the opposite of formation. (snip)

Add NaOH to water and get elemental sodium, elemental oxygen, and elemental hydrogen? Guess again --- the words dissolution and dissociation both begin with "dis," but that does not make them equivalent. Formation is from the elements, ALWAYS. Things dissolve without dissociating. Things dissociate without being dissolved. Go back and learn the definitions.
 
  • #3
yeah, they are different cases. enthalpy of formation is different from enthalpy of dissolution. The former is associated with the overall process of formation from its most stable elements (which have the enthalpy of formation of zero), the latter is regards the process where water solvates the compound to its corresponding cation and anion.
 

Related to Why is the heat of formation for NaOH negative?

1. Why is the heat of formation for NaOH negative?

The heat of formation for NaOH is negative because the formation of NaOH releases heat energy. This means that the reaction is exothermic and the products have a lower energy than the reactants.

2. What is the significance of a negative heat of formation for NaOH?

A negative heat of formation for NaOH indicates that the reaction is energetically favorable and spontaneous. This means that the reactants will naturally form the products without the need for additional energy input.

3. How is the heat of formation for NaOH experimentally determined?

The heat of formation for NaOH can be experimentally determined using calorimetry. This involves measuring the change in temperature of a reaction mixture and using this data to calculate the heat released or absorbed during the reaction.

4. What factors can affect the heat of formation for NaOH?

The heat of formation for NaOH can be affected by the initial energy state of the reactants, the presence of catalysts or inhibitors, and the temperature and pressure of the reaction environment. Changes in these factors can alter the energy released or absorbed during the reaction.

5. Can the heat of formation for NaOH ever be positive?

No, the heat of formation for NaOH cannot be positive. This is because the formation of NaOH always releases heat energy, making it an exothermic reaction. A positive heat of formation would indicate an endothermic reaction, where the reactants have a lower energy than the products.

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