Exothermic Reactions

Hey everyone!

I've been looking everywhere, but I cannot find the a detailed enough explanation for this question I have been thinking about:

Why is a reaction of HCl + NaOH exothermic?

I did an experiment at school, and I understand that in exothermic reactions bonds are made - energy is supplied. All I could find is that the excess energy is released and that's why it's an exothermic reaction.

Could someone please explain this in a bit more detail.

P.S: I learnt that it's something to do with enthalpy and the heat of neutralisation. Could someone explain this too.

THANKS!!

Okay, it's basically:
- when a reaction is exothermic - bonds are made and energy is released to the surroundings
- When a reaction is endothermic - Bonds are broken and energy is absorbed from the surroundings.

In your example of HCl + NaOH - this is a neutralisation reaction to form NaCl + H20.
Basically there is more bond making than bond breaking in this reaction so the Delta H is negative - it is more exothermic.

More energy is required to make the NaCl and H20 than having to break the HCl and NaOH.

Most Neutralisation reactions are exothermic

I hope I've clarified things. Remember, neutralisation is exothermic

(Just had my Chemistry AS Exam :O)

You're not quite there ...

Don't forget that ionic compounds (and strong acids) are completely dissociated in solution, so in the case of neutralisation of NaOH and HCl there are no (formal) bonds to be broken.*

However, the product H₂O is only very slightly dissociated and so you are forming 2 x O-H bonds. This is mainly responsible for the exothermic nature of the reaction.

* there are also interactions between the ions and the solvent to be broken and formed...

You're not quite there ...

Don't forget that ionic compounds (and strong acids) are completely dissociated in solution, so in the case of neutralisation of NaOH and HCl there are no (formal) bonds to be broken.*

However, the product H₂O is only very slightly dissociated and so you are forming 2 x O-H bonds. This is mainly responsible for the exothermic nature of the reaction.

* there are also interactions between the ions and the solvent to be broken and formed...

So the formation of H20 is what makes the reaction exothermic?

... in a nutshell, yes

Please could you explain why?

Because when bonds are formed energy is released ...

look at this interactive energy diagram

1. Exothermic reactions release energy to its surroundings. They release energy. Endothermic reactions do the opposite.
Basically, what you've got is a strong acid (HCl) reacting with a strong base (NaOH), this creates a neutralisation reaction, which forms water and a salt:
HCl + NaOH ---> H2O + NaCl

The reaction is exothermic because both HCl and NaOH are trying to balance each other out.
The reaction gives out energy. At first, this energy heats up the reacting mixture. Then heat is transferred from the mixture to its surroundings, and then the mixture cools to room temperature.

Okay, it's basically:
- when a reaction is exothermic - bonds are made and energy is released to the surroundings
- When a reaction is endothermic - Bonds are broken and energy is absorbed from the surroundings.

In your example of HCl + NaOH - this is a neutralisation reaction to form NaCl + H20.
Basically there is more bond making than bond breaking in this reaction so the Delta H is negative - it is more exothermic.

More energy is required to make the NaCl and H20 than having to break the HCl and NaOH.

Most Neutralisation reactions are exothermic

I hope I've clarified things. Remember, neutralisation is exothermic

(Just had my Chemistry AS Exam :O)

Thanks for the reply, but one thing I don't get is then why are the reactants at a higher potential energy level than the reactants. Shouldn't they be lower if "more energy is required to make the NaCl and H20 than having to break the HCl and NaOH"?

Thanks for the reply.

Btw: if we take a neutralisation reaction (i.e. HCl and NaOH) and they react to form NaCl and H20, does the release of energy (the delta H) represent the heat of neutralisation for water or for NaCl?

This really makes no sense ...

Sorry for the rubbish question. What I was I' trying to say is why in an energy diagram (the link you posted earlier) the reactants are at a higher energy than the products.

Thanks for taking your time out to answer to questions

Thanks for the reply, but one thing I don't get is then why are the reactants at a higher potential energy level than the reactants. Shouldn't they be lower if "more energy is required to make the NaCl and H20 than having to break the HCl and NaOH"?

I linked you the diagram to explain just that...

The total energy is made up of internal energy and kinetic (heat) energy.

When heat appears it MUST come from the transformation internal energy --> heat energy, hence the internal energy goes down.

Logically, this means that the internal energy of the reactants is greater than the energy of the products in an exothermic reaction.

I presume you mean that the reactants are at a higher potential energy than the products.

To be able to explain this, I think it's best to look at an energy level diagram e.g. http://www.gcsescience.com/Energy-Diagram-Exothermic.gif

Basically, what what this diagram shows is that the reactants have a high energy content (this is an exothermic reaction diagram btw). When the reaction occurs, energy is given out for example as heat and this means that the reactants lose energy as it is lost to the surroundings. The overall energy content of the reactants decrease so the product will have less energy overall.

In an endothermic reaction, we need to break bonds and to do that, we must put energy in to break the bonds - therefore the energy content of the product will be greater than reactants because energy has been put in.

Also, most reactions aren't purely exothermic or purely endothermic. The reaction will involve some bond breaking and some bond making thus, we say that the net energy absorbed or evolved is exothermic or endothermic.

You just need to remember that bond making releases energy and bond breaking absorbs energy.

I hope this helps :P Sorry it's a bit long