Question

An exothermic reaction releases 86.5 kJ. How many kilocalories of energy are released?

Answer 1

`1 " kJ"=0.239" kcal"`

`86.5 " kJ"=86.5*0.239=20.7" kcal"`

Explanation:

This problem is just a unit conversion, so you need to know the definition of a calorie. A calorie is a unit of energy equal to 4.184 J. This is the energy required to heat up one gram of water by one degree celsius at a pressure of one atmosphere. If 1 cal = 4.184 J, then 1000 cal = 4184 J.
Therefore:

`1 " kcal"=4.184" kJ"`

Divide both sides by 4.184 to get kJ in terms of kcal.

`1 " kJ"=0.239" kcal"`

Now you have your conversion factor and just need to multiply both sides by 86.5.

`1*86.5 " kJ"=0.239*86.5" kcal"`

`86.5 " kJ"=20.7" kcal"`