Today we are going to discuss on Energy Changes. This is a relatively small chapter (in terms of content) but it can be slightly confusing! Questions on energy change usually appear only in MCQs, but, when it does come out in Paper 2, I always tell my students that its “FREE” Marks!


Exothermic Reactions

Exothermic Reactions are reactions that release heat to the surroundings.

Imagine an explosion.
Imagine what would happen if you were standing near it?
You would probably feel the heat from the fire, or get “blown” away by the amount of energy that’s being released.

Explosions release a lot of energy to the surroundings hence explosions are exothermic reactions!

Other examples include:-
1. Combustion,
2. Neutralisation (acid + alkali)
3. Respiration (production of energy in living things)
4. Condensation and Freezing (this can be confusing!)
5. Any bond FORMING reaction

Some students can be puzzled by why freezing is an exothermic process. To better understand this, it is good to recall the chapter on Kinetic Particle Theory.

Recall that in a liquid, particles are able to slide past one another, whereas in a solid, particles are only able to vibrate about fixed positions. Hence particles in a liquid have more energy than particles in solid. Since freezing involves a change in state from a liquid to solid, when a liquid freezes, it LOSES energy – hence freezing is an exothermic process! (viola!)

Endothermic Reactions

Endothermic reactions are reactions that absorb heat from the surroundings.

Imagine you are in Antartica, and everywhere around you is ice/snow.
Imagine you are very thirsty and have run out of water. There’s water(ice) around you but you cannot drink it! If only you had some heat/fire so that you could melt the ice!

To melt ice requires heat. (energy is required to break bonds between molecules… KPT!)
The energy needs to be absorbed by the ice/snow before it can melt. Hence melting is an ENDOTHERMIC process.

Other examples of endothermic reactions are:-
1. Thermal decomposition (using heat to breakdown a substance in simpler substances)
2. Photosynthesis
3. Dissolving of ammonium salts in water
4. Melting and boiling
5. Any bond BREAKING reaction

What Makes Reactions Exothermic/Endothermic?

Now that you know what exothermic and endothermic reactions are, and you also know some examples of exo & endo. reactions, what makes some reactions exothermic and others endothermic?

To put it simply, the reason why some reactions are exothermic and others are endothermic is because of the energy absorbed during bond breaking and the energy released during bond forming. (Its challenging to explain this online… but il do my best!)

Let me give you an analogy…

Lets assume you are walking along Orchard Road and happen to spot a $100 note on the floor… You look around, wait for awhile and… no one’s coming to claim it. So you pick it up and put it into your pocket.

Then several moments later, while taking out some items from your wallet, you drop a $10 note (unknowingly)… How much do you have now? $90! You “gained” $90.

If we use this theory and apply it to energy, if 100J of energy is used to break bonds in reactants, and 10J of energy is released during the formation of bonds in products, there will be an overall gain of 90J of energy. Since reactions that ABSORB/GAIN energy are endothermic reactions, the reaction will be ENDOTHERMIC!

Vice versa for exothermic reactions!

Once again…

Hope this helps!

If you’d like your child to gain a better understanding of O Level Chemistry (Pure or combined), I welcome your child to join us at our weekly coaching sessions! If you’d like to meet us in person, to find out more about the way we run our classes and the materials we use, feel free to schedule a Complimentary Consultation with us!