Exothermic and Endothermic Reactions continued

Energy can be placed directly in a chemical reaction equation

Exothermic reactions have negative change in enthalpy (-delta H) and so energy appears on the right side of equation

Endothermic reactions have positive change in enthalpy (+delta H) and so energy appears on the left side of equation

Example:

 2C5H10(g)+15O2(g)--->10CO2 (g)+10H2O(l)+ 213kJ

This is an exothermic reaction because energy is released (i.e. appears on product side of equation)



MOLE IS BACK!

Delta H is the change in energy in the chemcal reaction and the units ar kilojoules (kJ) per 1 mole of the chemicals in the reaction.

For the above equation, the change in enthalpy is negative and so for C5H10, it would be -213kJ/ 1 mole C5H10

However, remember that for H2O, the change in enthalpy is -213kJ/ 10 mole H2O = -21.3 kJ/ 1 mole H2O

REMEMBER: value of delta H is not a fixed number, its value will depend on the chemical equation.

Now, lets start calculating with moles!

The unit conversion is kJ/mole and so if you want to convert from kJ to mole, the equation would be:
# of kJ x 1mole/# of kJ
If you want to convert moles to kJ, the conversion equation would be
# of moles x # of kJ/mole

Using the equation:
2C5H10(g)+15O2(g)--->10CO2 (g)+10H2O(l)+ 213kJ
How many kilojoules of energy is needed to produce 3.6 moles of O2.
3.6 moles x -213kJ/15 mole O2 = -51.12kJ = 51kJ (Remember, significant figures are also back)
the answer would be 51kJ of energy released

We can also use this mole conversion to convert to other units such as grams or molecules or atoms.
Example: How many grams of C5H10 is needed to produce 2200kJ of energy?
1100kJ x 2 mole C5H10/213kJ x 70.0g C5H10/1 mole C5H10 = 723.0 g = 720 g of C5H10
C5H10 molar mass = 70.0g/mol

When you are wondering which unit goes on top and which goes on the bottom of a fraction, remember to put the units you are looking for on top. This way, the others will cancel out after multiplying.

This worksheet has some problems about conversions between energy into moles and grams. Have fun and see how much you remembered~!