Chemical reactions usually involve change of heat due to energy absorbed or evolved during reaction.
The study of heat changes of chemical reactions is known as THERMOCHEMISTRY
Energy can come in different forms;
like heat, light, electrical, nuclear, etc…
Chemical energy is stored in all chemical substances.
With the law of conservation of energy, energy is never destroyed or created, only to be transferred.
Bond making / breaking
Energy is absorbed or released because there are bonds being broken and there are bonds being formed when a chemical reaction occurs
When a bond is being broken, heat energy is absorbed from the surrounding to break the bond;
Bond breaking is endothermic .
The amount of energy absorbed depends on the strength of the bond being broken
The stronger bond, more energy will be required to break the bond.
When a bond is being formed, heat energy is evolved, given out to the surroundings
Bond formation is exothermic .
The amount of heat energy evolved depends on the strength of the bond being formed
The stronger the bond that is formed, more energy will be released.
Net change in energy
When the amount of energy evolved in a reaction > the amount of energy absorbed, there will be a net release of energy to the surroundings.
The temperature of the surroundings will increase and the reaction mixture will feel warm.
This is known as an exothermic reaction.
Net change in energy
If the amount of energy absorbed in a reaction > the amount of energy evolved, there will be a net absorption of energy from the surroundings.
The temperature of the surroundings will drop and the reaction mixture feels cold.
This is known as an endothermic reaction.
summary Reaction mixture feels cold Reaction mixture feels warm Temperature of surroundings drops Temperature of surroundings increases Net absorption of heat energy from surroundings Net release of heat energy to surroundings More energy absorbed than energy evolved More energy evolved than energy absorbed Strength of bonds broken stronger than strength of bonds formed Strength of bonds formed greater than strength of bonds broken Endothermic Reaction Exothermic Reaction
Examples of exothermic and endothermic reactions: Thermal decomposition Dissolving some ionic salts in water, like ammonium chloride, potassium nitrate and copper(II) sulphate etc… Photosynthesis Action of light on silver bromide Combustion Respiration Neutralisation Dissolving acids Dissolving alkalis Rusting Oxidation of metals Nuclear Endothermic reactions Exothermic reactions
The overall energy change during a reaction is known as the enthalpy change ( H) of the reaction.
The unit for enthalpy change is kJ/mol.
The unit for energy is kJ.
An exothermic reaction has a negative enthalpy change,
indicating more energy is lost than gained in the reaction.
An endothermic reaction has a positive enthalpy change,
indicating more energy is gained than lost in the reaction.
In a reaction, the reactant molecules must be able to overcome an energy barrier before they can turn into products.
This energy barrier is known as the activation energy, Ea.
Hence, activation energy is the minimum amount of energy required to initiate a chemical reaction.
Reactions with high activation energy will not occur spontaneously and may require heating or addition of catalyst to initiate the chemical reaction.
Exothermic reaction Energy Time Reactants Products Activation energy H : -ve
Endothermic reaction Energy Time Reactants Products Activation energy H : +ve
Energy Profile Diagram
An energy profile diagram is a plot of energy against time to represent the relative amount of energy of the reactants and the products.
An exothermic reaction has an overall energy loss during the reaction.
Hence the energy content of the reactants is higher than the energy content of the products.
An endothermic reaction has an overall energy gain during the reaction
Hence the energy content of the products is higher than the energy content of the reactants.
Exothermic reaction Energy Time Reactants Products Enthalpy change ( H) : negative
Endothermic reaction Energy Time Reactants Products Enthalpy change ( H) : positive
Bond energy is the amount of energy required to break a given bond.
Likewise, when the same amount of energy is evolved when that bond is formed.
Bond energy values can be used to calculate the enthalpy change of a reaction.
When a bond is formed, the bond energy is assigned a negative value, indicating energy is lost.
When a bond is broken, the bond energy is assigned a positive value, indicating energy gained.
Enthalpy change of a reaction
= Sum of [energy absorbed due to bond breaking in reactants (assigned positive value)]
+[energy evolved due to bond formation in products (assigned negative value)]
An example of enthalpy change calculation from bond energies:
Hydrogen reacts with oxygen to form water.
2 H 2 (g) + O 2 (g) 2 H 2 O (g)
Bond energies 463 O – H 496 O = O 436 H – H Bond energies(kJ/mol) Bond
In the reaction,
2 hydrogen-hydrogen bonds (1 bond in each hydrogen molecule) and 1 oxygen-oxygen bond are broken,
while 4 oxygen-hydrogen bonds (2 bonds in each water molecule) are formed.
Hence enthalpy change for the above reaction
= 2 (+436) + (+496) + 4 (–463) = –484 kJ/mol.
Hence, the thermochemical equation of the reaction between hydrogen and oxygen to form water can be written as
2 H 2 (g) + O 2 (g) 2 H 2 O (g) H = -484 kJ/mol