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Exo and Endothermic Reactions // Bond Energies (We use energy level…
Exo and Endothermic Reactions // Bond Energies
Exo
thermic - chemical reactions that release heat energy to the surroundings.
Endo
thermic - chemical reactions that absorb energy from the surroundings
Common exothermic reactions
Magnesium and Hydrochloric Acid in a test tube
Neutralisation reaction
Common endothermic reactions
Photosynthesis
Ammonium nitrate in a conical flask of water on a wet wooden block (flask freezes to wood)
In order to make products from the atoms in the reactants, we must:
1) Break the bonds between the atoms in the reactants.
2) Rearrange the atoms.
3) Make new bonds with the newly arranged atoms.
The
breaking
of bonds is an
endothermic
process because it
takes in
energy from the
surroundings
in order to break the forces of attraction.
The
formation
of bonds is an
exothermic
process because it
gives off
energy as the bonds are snapped together.
We use
energy level diagrams
to show whether the reaction is exothermic or endothermic. Energy level diagrams also help us to interpret the overall heat exchange for a reaction.
Here, we can see that the products on the right of the hump are lower than the reactants on the left of the hump. This means that the energy of the products is lower than that of the reactants.
Therefore, the overall heat change is negative. This means that energy is given out to the surroundings because bonds have been made - we can interpret that this is an exothermic reaction.
The energy released by making new bonds is greater than the energy needed to break the old original bonds as per
step 1
Here, we can see that the products on the right of the hump are higher than the reactants on the left of the hump. This means that the energy of the products is higher than that of the reactants.
Therefore, the overall heat change is positive. This means that energy is taken in from the surroundings because bonds have been broken - we can interpret this as an endothermic reaction.
The energy taken in by breaking new bonds is greater than the total energy given out by the formation of the original bonds as per step 1
Activation energy is defined as the minimum energy needed to break the bonds in the reactants to start the reaction.
In both exothermic and endothermic reactions, the activation energy is higher than that of the reactants. This is because the initial step of all reactions is bond breaking which is an endothermic process. Therefore, energy must be supplied from the surroundings to break the bonds and start the reaction.
In the case of combustion of methane in a Bunsen burner, the activation energy is provided by the spark or flame required to light the gas.
Bond Energies
Every chemical bond has a particular amount of energy associated with it., no matter what compound the bond is found in. This value is the amount of heat energy taken in or given out to the surroundings.
H2 + Cl2 --> 2HCl
H-H + Cl-Cl --> H-Cl + H-Cl
Bond Energies
H-H = 436kJ mol^-1
Cl-Cl = 242 kJ mol^-1
H-Cl = 431 kJ mol^-1
We want to know an overall energy change in a reaction. To do this, we need to find the difference between the energy taken in and the energy released.
In order to work this out, we need to identify the bonds broken and the bonds formed. The bonds broken here are the H-H and Cl-Cl bonds, and the bonds formed are 2x H-Cl
Energy taken in as bonds are broken
436kJ mol^-1 + 242kJ mol^-1 = 678kJ mol^-1
Energy given out as bonds are formed
2 (431kJ mol^-1) = 862kJ mol^-1
Overall Energy Change
Here, more energy has been given off to the surroundings than has been taken in from the surroundings. Therefore we know the reaction is exothermic.
To calculate the overall, energy change, we would find the difference between the two. 678 - 862 = -184kJ mol^-1
