ENERGY CHANGES AND REVERSIBLE REACTIONS:

There are many types of energy among which heat energy is the most prominent in chemical reactions.

·       During a chemical reaction, there is always an energy change. Heat is either taken in or given out from/to the surroundings.

·       The reaction in which heat is taken in are known as endothermic reaction, in these the product has more energy than the reactants (due to absorption of heat energy). Endothermic reaction’s enthalpy change is represented by Positive(+) sign.

·      

The reaction in which heat is given out is known as exothermic reaction, in these the products has lesser energy than the reactants (due to emission(loss) of heat to surroundings). Exothermic reactions enthalpy change is represented by negative(-) sign

Ø Breaking Bonds takes in energy, B.B is endothermic.

Ø Bond forming is exothermic, energy is released.

·       In the chemical reaction the bonds are first broken and then the new bond form.

Ø If the energy needed to break bonds is greater than the energy released while bond formation, the reaction is endothermic. Whereas if the energy required to break bonds is lesser than the energy released while bond formation, the reaction is exothermic.

v The Examples of Exothermic Reactions are:

Ø Combustion: fuel burning (combustion) is a reaction with Oxygen in air.   Organic fuel compound (e.g. ethanol/Methane) react with oxygen to produce CO₂ and H₂O.   H_2­ reacts with O₂ to give H_2­O.

Ø Neutralization of acid by alkalis

Ø Respiration process(Breathing in and out)

v Examples of Endothermic Reactions are:

Ø Photosynthesis (reaction takes place in plants for making food)( 6CO₂   +   6H_2­O    →    C₆H₁₂O₆   +   6O₂ ) 

Ø Cooking

E NERGY PROFILE DIAGRAMS:

An energy profile diagram shows how the energy changes over time as a reaction proceeds

ENERGY CHANGES AND REVERSIBLE REACTIONS

There are many types of energy among which heat energy is the most prominent in chemical reactions.

·       During a chemical reaction, there is always an energy change. Heat is either taken in or given out from/to the surroundings.

·       The reaction in which heat is taken in are known as endothermic reaction, in these the product has more energy than the reactants (due to absorption of heat energy). Endothermic reaction’s enthalpy change is represented by Positive(+) sign.

·       The reaction in which heat is given out is known as exothermic reaction, in these the products has lesser energy than the reactants (due to emission(loss) of heat to surroundings). Exothermic reactions enthalpy change is represented by negative(-) charge

Ø Breaking Bonds takes in energy, B.B is endothermic.

Ø Bond forming is exothermic, energy is released.

·       In the chemical reaction the bonds are first broken and then the new bond form.

Ø If the energy needed to break bonds is greater than the energy released while bond formation, the reaction is endothermic. Whereas if the energy required to break bonds is lesser than the energy released while bond formation, the reaction is exothermic.

v The Examples of Exothermic Reactions are:

Ø Combustion: fuel burning (combustion) is a reaction with Oxygen in air. Organic fuel compound (e.g. ethanol/Methane) react with oxygen to produce CO₂ and H₂O.     H_2­ reacts with O₂ to give H_2­O.

Ø Neutralization of acid by alkalis

Ø Respiration process(Breathing in and out)

v Examples of Endothermic Reactions are:

Ø Photosynthesis (reaction takes place in plants for making food)( 6CO₂   +   6H_2­O    →    C₆H₁₂O₆   +   6O₂ ) 

Ø Cooking

ENERGY PROFILE DIAGRAMS:

·       An energy profile diagram shows how the energy changes over time as a reaction proceeds.

·       The minimum energy required to start the reaction is known as activation energy.

·       The activation energy for all chemical reactions is different, few reaction has smallest activation energy that the reaction is spontaneous(start on their own, without supply of energy)

 

 

 

ENERGY FROM FUELS:

Fuels are combusted (reacted with oxygen) to provide energy (as it’s exothermic reaction)

Qualities of Food Fuel:

ü  Cheap

ü  Available in large quantities

ü  Produce a large amount of heat when combusted(yield)

ü  Must not produce polluting gases(CO, NO)

ü  Easy and Safe to store and transport

GIVING OUT ENERGY AS ELECTRICITY (SIMPLE CELLS)

·      A cell is a device which converts chemical energy into electrical energy and is composed of two metals of different reactivity connected by an external circuit and an electrolyte.

Ø The process works due to the different reactivity of metals.

Ø Consists of a negative pole (the more reactive metal) and a positive pole (less reactive metal) and an electrolyte.

Ø The greater the difference in reactivity of the two

Metals, the greater the voltage will be.

Ø And hence the redox reaction(combining two half equations of the reactions) is giving out energy in the form of a current)

Ø The electrons flow because one metal is more reactive, so it has a stronger drive to give up its electrons.

Ø The atoms give up electrons and enter the solution as ions.

In the example:       Magnesium is at negative terminal as it is more reactive giving electrons and becoming positive ions while Copper is at positive terminal gaining electrons becoming negative ions.

At Negative Terminal:     Mg  →  Mg²⁺   +   2e^-    Or

                                            Mg   -   2e^-     →   Mg²⁺

At Positive Terminal:    Cu   +    2e^-   →   Cu^2-

 

HYDROGEN FUEL CELL:

Hydrogen fuel cell is being used in replacement of petrol engine, and as a alternative for electricity.

Obtaining Hydrogen for Fuel Cell:

 CH₄^(g)   +  2H₂O^(g)  →    CO₂^(g)    +   4H₂^(g)

 C₂H₆    →   C₂H₄    +    H₂

 2H₂O  →    2H₂    +   O₂

Working of Hydrogen Fuel Cell:

·      The electrolyte contains OH^- ions

·      Negative Pole is a carbon rod surrounded by Hydrogen that loses electrons to the OH^- ions. Hydrogen is oxidized.     2H₂^(g)    +  4OH^{- (aq)}  → 4H₂O^(l)  +  4e^-

·      The positive pole is carbon rod surrounded by Oxygen molecules that gains electrons.     

  O₂^(g)   +   2H₂O^(l)   +  4e^-    →   4OH^{- (aq)}

ü Adding the two half equations will form

2H₂   +   O₂    →   2H₂O

ü Hydrogen and Oxygen combine to form Water

Advantages of Hydrogen Fuel Cell:

ü No Pollutant as the only product formed is Water

ü Gives Plenty of energy as current(electricity)(high yield)

ü Reactants are renewable(will not runout/reproducible)

Disadvantages of HFC:

ü Hydrogen is highly flammable(therefore dangerous)

REVERSIBLE REACTIONS

The reactions in which the reactant form products, and then during the reaction products react together to give back the reactants. The reaction proceeds in both forward and Backward direction.

The symbol that is used to represent reversible reactions is ⇋

The reversible reaction is endothermic in one direction, and exothermic in the other. The same amount of energy is transferred each time.

In reversible reactions the amount of reactant and product reaches to dynamic equilibrium; in which there is continual change in both direction ;forward and backward(formation/breaking of reactant and products) at the same rate. There is no overall change.

 

Example of Reversible Reaction are (important):

·      Production of Ammonia by Nitrogen and Hydrogen gases is reversible(Haber process)

·       Formation of Sulphur trioxide(an intermediate step in production of sulfuric acid(contact process)

2SO₂^(g)   ₊   O₂^(g)     ⇋    2SO₃^(g)

·      Thermal Decomposition of CaCO₃

CaCO₃^(s)      ⇋      CaO^(s)     +    CO₂^(g)

 

INCREASING YEILD

LE-Chartier’s principle: “When a reversible reaction is in equilibrium and you make a change, the system acts to oppose the change and restore equilibrium. A new equilibrium mixture forms.”

A reversible reaction’s equilibrium position can be shifted towards the required product to increase the yield by changing physical conditions.

vThe conditions that can shift the position of equilibrium are:

·      Temperature

·      Pressure

·      Changing the concentration of reactant/product.

CHANGING THE TEMPERATURE:

·      When the reaction is endothermic in forward direction, the increase in temperature will shift the position of equilibrium towards right(product).

Ø An increase in temperature increases the energy of the surroundings,

Ø According to Le Chatellier’s principle, the reaction will go in the direction that opposes the increase in energy,

Ø So the reaction will go in the direction in which energy is absorbed, which is endothermic reaction(forward)

 

o  When the reaction is endothermic in forward direction, the decrease in temperature will shift the position of equilibrium towards left(reactant).

Ø A decrease in temperature decreases the energy of the surroundings,

Ø According to Le Chatellier’s principle, the reaction will go in the direction that opposes the decrease in energy,

Ø So the reaction will go in the direction in which energy is released, which in forward endothermic reaction will be backward(towards reactant)

 

Example of forward endothermic reaction(decomposition of Hydrogen Iodide)

_Ø  2HI     ⇋    H₂    +   I₂

 

·      When the reaction is exothermic in forward direction, the increase in temperature will shift the position of equilibrium towards left(reactant). Decrease the yeild

Ø An increase in temperature increases the energy of the surroundings,

Ø According to Le Chatellier’s principle, the reaction will go in the direction that opposes the increase in energy,

Ø So the reaction will go in the direction in which energy is releases, which is exothermic reaction(forward)(towards the product)

o  When the reaction is exothermic in forward direction, the decrease in temperature will shift the position of equilibrium towards right(product).

Ø An increase in temperature increases the energy of the surroundings,

Ø According to Le Chatellier’s principle, the reaction will go in the direction that opposes the decrease in energy,

Ø So the reaction will go in the direction in which energy is absorbed, which in forward exothermic reaction will be backward(towards reactant)

CHANGING THE PRESSURE:

Pressure is caused by the gas molecules when they collide with the walls of the container.

Pressure is the factor in equilibrium mixture when one or more reactants or products have gas molecules.

If the gas molecules(only to be considered for pressure) are greater at the reactant side, the increase in pressure will shift the equilibrium towards product side.

Ø When the pressure is increased the more gas  molecules will collide with each other(at higher frequency and will collide strongly) which will shift the equilibrium towards lesser gas molecules as more gas molecules will react with each other and shift the equilibrium towards other side.

Ø By increasing the pressure the molecules will be closer together.

Ø The position of equilibrium shifts to minimize the increase, therefore it will shift in the direction of fewer gas molecules(in the direction that opposes

the increase in pressure).

For Example:

The reactant have 4 gas molecule and the product have 2 gas molecules, therefore increasing the pressure will shift the equilibrium position towards product.

Question:

2A^(g)    +    B^(g)     +   3C^(s)      ⇋     5D^(g)

The increase in pressure will shift the equilibrium towards left as there 3 molecule on left(reactant side while 5 molecules on right(product side). The C molecules will not be considered when changing pressure is concerned as it’s not a gas.

 

CHANGING THE CONCENTRATION OF REACTANT/PRODUCT:

At equilibrium reaction, When we increase the concentration of the reactants(on the left) the reactant particle collide more frequently with each other forming products. When the concentration of reactants is increased their will be more particles of reactants, which will disturb the position of equilibrium, According to Le'Charlier's principle to offset this effect more reactant will convert into product.

  

Summary of Reversible reaction:

In endothermic Temp ↑ yield ↑

In Exothermic Temp ↑ yield ↓

When more reactant gas molecule: Pressure ↑ yield ↑

Concentration of product ↓ yield ↑