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13. Energy flow.

posted May 4, 2016, 12:54 PM by Upali Salpadoru   [ updated May 10, 2016, 12:45 PM ]

Exothermic and Endothermic reactions.


Mehara.jpg

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.Nelly.jpg

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.

CO2 H.jpg

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.energy c.jpg















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.Ethanol.jpg



Requirements.

  • A light metal can

  • Thermometer.

  • A stand

  • Some cotton wool.

  • A few ml. of methyl alcohol.



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.



Formula of methanol =  CH3OH

Molar mass  ………..= 12 +3+17  =32

One tenth of a mole amounts to = 3.2 g.



Formula to calculate the heat gained = mass x specific heat x change in temperature.

Specific heat of water = 4.2 J/g for oC,

Specific heat of iron… = 0.45 J/g per oC


Here are some readings obtained by a student.


Readings


Mass of  methanol ………..3.2 g..

Mass of can……………...52 g.

Mass of water……………100 g.

Original temperature…….23 oC

Final temperature……….. 74 oC


Calculation


Heat gained by water = 100 x 4.2 x 51 .... = 21420

Heat gained by can = 100 x 0.45 x 51 .....= 2295

Total heat gained = 21420 + 2295 ........= 23715 J per 0.1mol.

Converting to kilo joules per mol. = 237.15 kJ/mol of methanol.

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

 


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