Question

'chemical change involves change in energy i.e liberation or absorption of heat.' give two.examples to support this statement.

Answer 1

Answer:

You have probably seen a fire burning or burnt fuel for warmth or cooking or light. A fire burning is one of the most noticeable examples of a chemical reaction that produces a lot of energy.

All chemical reactions involve energy changes. In some reactions, we are able to observe these energy changes as either an increase or a decrease in the overall energy of the system. In some reactions we see this as a change in the temperature. In other reactions we can observe this change when a reaction starts to give off light or when a reaction will only work after light is shone on it.

The study of energy changes (particularly heat) in chemical reactions is known as chemical thermodynamics. This is also sometimes called thermochemistry.When a chemical reaction occurs, bonds in the reactants break, while new bonds form in the product. The following example explains this. Hydrogen reacts with oxygen to form water, according to the following equation:

2H2(g)+O2(g)→2H2O(g)

In this reaction, the bond between the two hydrogen atoms in the H2 molecule will break, as will the bond between the oxygen atoms in the O2 molecule. New bonds will form between the two hydrogen atoms and the single oxygen atom in the water molecule that is formed as the product.

For bonds to break, energy must be absorbed. When new bonds form, energy is released. The energy that is needed to break a bond is called the bond energy or bond dissociation energy. Bond energies are measured in units of kJ⋅mol−1.