Bond Energy Calculations
- Each type of chemecal bond has a different bond energy. This bond energy can very depending on what type of compound the bond is in.
- We can use the bond energy to calculate the enthalpy change for a reaction.
- You add up the energy of the bonds that are broken and subtract the energy of the bonds that are made.
enthalpy change = total energy used to break bonds - total energy released on making bonds
The Formation of Hydrochloric Acid:
- H-H = +436 kJ/mol
- Cl-Cl = +242 kJ/mol
- H-Cl = +431 kJ/mol
- Breaking one mole of H-H and Cl-Cl requires -> 436 + 242 = 678 kJ
- Forming two moles of H-Cl releases -> 2 x 431 = 862 kJ
- /\H = 678 - 862 = -184 kJ
- This tell us that the reaction is exothermic as the /\H is negative.
A reversible reaction is a reaction where the products can react together and convert back to the original reactants. The reaction for ammonia is reversible.
Dynamic Equilibrium -> the relative quantities of reactants and productswill reach a certain balance and stay there.
There are different things that affect the way in which the dynamic equilibrium works.
Change in Temperature:
- All reactions are endothermic in one direction and exo in the other.
- If you raise the temperature, the endothermic reaction will occur and if you lower the temperature, the exothermic reaction will occur to replenish the heat.
Change in Pressure:
- If you increase the pressure, the reaction will occur where the products have less molecules than the reactants and vice versa.