Exothermic and Endothermic reactions.

Mehara = What is the most important chemical reaction ?
Mehara asked. Can you help?
Ali = Is it the sun giving us energy?
No ! It is not a chemical reaction; it is a nuclear reaction similar to what happens in the atomic bomb
Nelly= Oh ! I know, It is breathing. We cannot live without breathing.
Breathing is not exactly a chemical reaction but you are very close.
Ali = It is respiration.
Yes. Respiration is an exothermic reaction. That means a reaction that releases energy.
What reaction has given energy to food?
Nelly = Photosynthesis, the endothermic reaction.
So it is nothing but correct to conclude that RESPIRATION and PHOTOSYNTHESIS are the two vital chemical reactions for us.
Exothermic Reactions
Respiration is a type of slow burning, where the glucose combines with oxygen to form carbon dioxide and water, releasing energy. Let us consider the oxidation of Carbon.
C + O2 = CO2 ………………….. (ΔH f =−393.5 kJ/mol )
Here is a diagram to explain the process.
Minus sign shows that energy has been given out to the environment.

Endothermic Reactions
Photosynthesis is a very complex reaction which may be summarised as follows:
6CO2 6H2O ⟶C6H12O6 + +6O2 ……… ΔH + 2802.5/mol.
Plant leaves having the green pigment, chlorophyll traps the light energy from sunlight. During this process carbon dioxide and water break down and new bonds are formed to give glucose.
Making and breaking of bonds.
Energy is required to break the bonds while energy is released when new bonds are made. Some bond energies are given below.
Activation Energy
If you mix petrol and air there may not be an observable change. But every chemist will know that an explosion is waiting to happen.
The reaction would be as follows.
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O
Hydrocarbon molecules in petrol combines with oxygen in the air to form carbon dioxide and water.
So why can't this happen when we are pumping petrol into a car?
The new bonds of the products to form atoms must be pulled out of their molecules. In other words the existing bonds must be broken. This requires energy. It is this energy we refer to as activation energy.
If you give a helping hand by a tiny help electric spark or by a cjgarette butt, a nasty explosion can result.
An Experiment to determine the ΔH of Methanol
Enthalpy of methanol (Methyl alcohol, sold in the shops as wine spirit), combustion may be determined by a carefully conducted experiment. .Set up the apparatus as shown.The idea is to burn the methanol and get all, at least as much as possible, into the water.
In this experiment we assume that all the heat generated is equal to the heat generated.
Requirements.
Method
Our aim is to determine the heat of combustion for one mole of methanol . Moisten some cotton wool, so that it may not burn. then add a weighed quantity of methanol into it. This could be done inside a fire proof bottle lid. If the mass is a fraction of the molar mass of methanol, which is 32, the calculation will become easier.
Record the temperature of water and start burning the methanol. Once all the alcohol has been burnt take the final temperature.
This is slightly less than the correct value.
Bond Energies
This is the energy absorbed to break a bond. unit- kilo Joules.
Bond Energies of some common compounds.
Carbon compounds.
Bond
|
C-H
|
C-C
|
C-N
|
C-O
|
C-F
|
C-Cl
|
C-Br
|
C-S
|
C-I
|
ΔH°
|
413
|
348
|
293
|
358
|
485
|
328
|
276
|
259
|
240 |
Nitrogen and Chlorine compounds
Bond
|
N-H
|
N-N
|
N-O
|
N-F
|
N-Cl
|
N-Br
|
Cl-Cl
|
|
|
ΔH°
|
391
|
163
|
201
|
272
|
200
|
243
|
243
|
|
|
Hydrogen and Oxygen compounds.
Bond
|
H-H
|
H-F
|
H-Cl
|
H-Br
|
H-I
|
O-H
|
O=O
|
O=Cl
|
O-I
|
ΔH°
|
436
|
467
|
431
|
366
|
299
|
463
|
146
|
203
|
234
|
Multiple
bonds
Bond
|
C=C
|
C=N
|
C=O
|
N=N
|
O=O
|
S=O
|
S=S
|
|
|
ΔH°
|
614
|
615
|
799
|
418
|
498
|
523
|
418
|
|
|
Q. 1.0
The equation shows the oxidation of hydrogen.
2H2 (g) + O2 (g) → 2H2O (g)
Simplified as H2 + 1/2 O2
---à H2O.
Find the heat of formation
of water.
Answer
Heat gained to break
bonds: = H-H = 436
kJ.
O=O = 498/2 =
249 kJ
Total =
685 kJ
Heat released forming
H-O-H = 463x2 =
926
Δ H˚ …………..= 685 – 926 = -241kJ