THE RETURN OF THE MOLE.. (dun dun dun)

Hello all! Today we will be talking about ENERGY and how to calculate it! This means that we will be bringing the mole concept back...

Yes I know I know... we've all missed the beloved mole... Who didn't?!


Without further ado, let's get back to business. When we talk about energy in a chemical equation, we are talking about the change in energy, expressed in kilojoules (kJ) per mole, that is either absorbed or released in the chemical reaction. 


As you know already from the last post:
energy being released - exothermic
energy being absorbed - endothermic


In exothermic reactions, since energy is being released, the change in enthalpy will be negative and the energy term will be in the products' side.
Endothermic reactions will have a positive change in enthalpy because energy is being absorbed (increased) rather than having a decrease amount of it. The energy term will be on the reactants' side.
eg. The freezing of water shows that heat/energy is being released.
       H₂O (s) → H₂O (l) + 6.01 kJ          
In this case, the reactant has a higher energy than the compound in the product.
**Note: Even though ΔH is negative, it is written just as what is included in the result of the product so the sign is neglected.  **


When we calculate the energy change of a reaction, the ΔH is expressed in kJ per mole of one of the chemicals in the equation. The coefficients of the compounds in the balanced equation is used for the ratio of kJ/mol.


If the exothermic equation was: 2H₂O (s) → 2H₂O (l) + 12.02 kJ
The ΔH of one mole for H2O would be: -12.02 kJ/ 2 mol H2O     OR     -6.01 kJ/1 mol H2O


3 things to keep in mind:

1. ΔH changes with different reactions (the energy term is different for each)
2. ΔH is different for each compound, like molar mass, ΔH depends on the chemical being referred as the coefficients may be different for each chemical reaction.
3. ΔH is NOT a constant like Avogadro`s number.

When given the amount of moles of a compound produced, refer to the mole chart to calculate the energy! However, energy is now used so be sure to add that into your original chart!

Here are some questions you can try! The answers are posted below. :)

Remember to use sig figs!!

1. How many moles of CH4 are needed to produce 1500kJ of energy?

CH4 + 2O2 → CO2 + 2H2O + 812kJ

2. Calculate the amount of energy required to produce 23.0g of NO2.

N2 + 2 O2 + 67.6 kJ → 2 NO2

3. How many atoms of Oxygen are needed to produce 1000. kJ of energy?

4NH3 + 5 O2 → 6 H2O + 4 NO + 905 kJ

Answers:

1. 1.8 mol CH4

2. 16.9 kJ

3. 6.654 x 10²⁴ atoms of O

GOODLUCK!