https://billvining.com/mmlib_sims/#gen_8_0
Use the simulation to visualise this.
Note 1. In the animation the bond strength is shown by the length and the thickness of the line.
Note 2. In the products two molecules are forming so the bond energy is for two bonds.
If the energy needed to break the bonds in reactants is less than the energy released when the bonds form in the products the reaction is
If the energy needed to break the bonds in reactants is more than the energy released when the bonds form in the products the reaction is
Reaction of nitrogen and oxygen in a car engine at high temperature and pressure to form the pollutant nitrogen oxide (this pollutant would be removed using a catalytic converter).
Bonds broken in reactants =
Bonds formed in products =
Energy to break bonds in reactants = 1 x 945 + 1 x 498 =
Energy to form bonds in products = 2 x 630 =
Energy change = energy to break bonds - energy to make bonds
Energy change = 1443 - 1260 = (sign )
The reaction is
Number of C-H bonds broken =
Number of Cl-Cl bonds broken =
Energy to break bonds = 4x413 + 243 =
Number of C-H bonds made =
Number of C-Cl bonds made =
Number of H-Cl bonds made =
Energy given out when bonds form = 3 x 413 + 327 + 432 =
Energy change = energy in to break bonds -energy out when bonds form
Energy change = 1895 - 1998 = (sign)
The reaction is
Identify the number and type of bonds broken in the reactants
Number of C-H bonds to be broken in reactants =
Number of C-C bonds to be broken in reactants =
Number of C-O bonds to be broken in reactants =
Number of O-H bonds to be broken in reactants =
Calculate the number and type of bonds formed in the products.
Number of C-H bonds to be formed in products =
Number of C=C bonds to be formed in products =
Number of O-H bonds to be formed in products =
Calculate the energy needed to break all of the bonds in the reactants.
Total energy to break bonds = sum of
5x C-H = 5 x 412 =
1 x C-C = 1 x 348 =
1 x C-O = 1 x 360 =
1 x O-H = 1 x 463 =
sum =
Calculate the energy needed to form all of the bonds in the products.
Total energy to form bonds = sum of
4x C-H = 5 x 412 =
1 x C=C = 1 x 612 =
2 x O-H = 2 x 463 =
sum =
Energy change = energy to break bonds in reactants- energy to form bonds in products
Energy change =
Energy change = sign
As the energy needed to break the bonds in the reactants is
Energy needed to break bonds in reactants =
Energy released to form bonds in products =
Energy change = energy to break bonds - energy to make bonds = sign
This is a combustion reaction so the temperature of the surrounding will
Note that we often talk of temperature change of the surrounding but energy in other forms can be absorbed or released.
The electrolysis of water is .
The energy of the products must be than the energy of the reactants.
The formulae of the products of electrolysis are O2 and H2.
Note in an exam I would expect the reactants and products to be balanced.
Calculate the energy change when one mole of water is electrolysed.
H-H = 436 kJ / mol
O-H = 463 kJ/mol
O=O = 496 kJ/mol
H₂O →H₂ + ½ O₂
Bonds broken =
Bonds formed =
Energy in break bonds = 2 x 463 =
Energy out in bond forming = 0.5 x 496 + 436 =
Energy change = bonds broken - bonds formed = 926-684 = (sign)
What would the energy change be if one mole of hydrogen was combusted?
(sign)
Energy to break bonds in reactant =
Energy to form bonds in products =
Energy change = energy to break bonds - energy to make bonds
Energy change = (sign)
This reaction is
Energy to break bonds =
Energy to make bonds =
Energy change in the reaction = energy needed to break bonds - energy needed to make bonds
Energy change in the reaction = (sign)
The energy change is
C-H bonds broken =
O=O bonds broken =
Energy needed to break bonds in reactants =
C-H bonds formed =
C-O bonds formed =
O-H bonds formed =
Energy given out when bonds form in the products =
Energy change for the reaction = (sign)
The reaction is
The temperature of the surroundings will
Reaction to show oxidation of methane to form ethanol
The reaction is
The temperature of the surroundings will
The starting temperature is
The final temperature is
The energy change is
The temperature of the surroundings has
Enthaly change of combustion of methane = (sign)
Energy profile diagram
Energy profile diagram
Energy profile diagram
Energy change and catalysts
Energy change and catalysts
Using ideas about bond breaking and bond formation, explain why the combustion of petrol in the petrol engine is exothermic.
Energy absorbed to break the chemical bonds in petrol and oxygen is than the energy released when chemical bonds are formed in carbon dioxide and water.
Overall, energy is into the surroundings and the temperature of the surroundings so the reaction is .
