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QUESTION

# When 2 moles of potassium chlorate crystals decompose to potassium chloride crystals and oxygen gas at constant temperature and pressure, 44.7 kJ are given off. What is the thermochemical equation?

2"KClO"_text(3(s]) -> 2"KCl"_text((s]) + 3"O"_text(2(g]). " "DeltaH_text(rxn) = -"44.7 kJ"

A thermochemical equation is simply a chemical equation that contains information about the change of reaction, DeltaH_"rxn".

The first thing to do when writing a thermochemical equation is to make sure that you get the balnced chemical equation right.

In your case, you know that 2 moles of potassium chlorate, "KClO"_3, undergo to form potassium chloride, "KCl", and oxygen gas, "O"_2.

So you know that you have

2"KClO"_text(3(s]) -> "KCl"_text((s]) + "O"_text(2(g])

Balance this equation by multiplying the products by 2 and 3, respectively

2"KClO"_text(3(s]) -> 2"KCl"_text((s]) + 3"O"_text(2(g])

Now focus on the enthalpy change of reaction. The problem tells you that the reaction given off "44.7 kJ", which means that the reaction is actually .

As you know, energy changes are being represent from the point of view of the system. This implies that for an exothermic reaction the enthalpy change will carry a negative sign, since the system is losing heat to the surroundings.

Therefore, the enthalpy change of reaction will be

DeltaH = -"44. 7 kJ"

As a result, the thermochemical equation for the decomposition of potassium chlorate looks like this

2"KClO"_text(3(s]) -> 2"KCl"_text((s]) + 3"O"_text(2(g]). " "DeltaH_text(rxn) = -"44.7 kJ"