Energy changes in a chemical reaction
Almost all the changes are due to change in
energy. For example, breaking of existing bonds and making of new bonds
involve the energy change. During these changes, some amount of energy is
either absorbed or released. Based on the stability of products, these
changes are called by different names, like physical or
reversible change, chemical or irreversible change, etc.
Sometimes, a physical change is also called as temporary change and a
chemical change is called as a permanent change.
Best examples for
physical change are freezing, evaporation followed by condensation etc;
while respiration, photosynthesis, combustion of fuels represent a chemical
change. Let us study some more characteristics of these changes.
Touch your own forehead. It’s warm. All living organisms are warm to touch. When they die, their bodies are cold. Why do you think this is so?
The cells of living organisms are constantly make into a new chemicals and energy is released in these changes. This makes a living organism warm to touch.
Sometimes, chemical changes and physical changes can occur at the same time. For example, when sugar is heated, it first melts (physical change) and then
undergoes decomposition (chemical change).
Let us take another example. When sodium nitrate is heated, it melts (physical change), then it decomposes into a new chemical, sodium nitrite and oxygen (chemical change).
When wood is heated, gases such as carbon dioxide and water vapor are liberated. The ash that remains is mostly carbon and other non‐volatile
In simple words, when a physical change occurs, inter-molecular bonds are stretched or broken. But when a chemical change occurs, inter-molecular bonds
are first broken and then rearranged to form new bonds and new molecules.