Thread: Exothermic and Endothermic reactions

I have just completed a lab at school about endothermic and exothermic reactions. I am wondering how it works.

We placed steel wool in oxygen and periodically recorded the temperatures. During the procedure the temperature in the sealed container increased causing the reaction to be endothermic. Is it the steel wool that absorbed heat from the oxygen but the thermometer measure the entire container the heat had to have been absorbed from somewhere.

We also did ammonium chloride and water and Sodium Hydroxide and water.

Please explain how this works and thank you in advance.

Originally Posted by Bigbear

I have just completed a lab at school about endothermic and exothermic reactions. I am wondering how it works.

We placed steel wool in oxygen and periodically recorded the temperatures. During the procedure the temperature in the sealed container increased causing the reaction to be endothermic. Is it the steel wool that absorbed heat from the oxygen but the thermometer measure the entire container the heat had to have been absorbed from somewhere.

We also did ammonium chloride and water and Sodium Hydroxide and water.

Please explain how this works and thank you in advance.

There is a form of potential energy called chemical energy. It is contained in any combination of chemicals that is not at its 'base' or equilibrium state. Iron (ferric or ferrous) oxide is closer to the base state for the combination of chemicals you had, and so it was possible for it to release energy as it moved to a less energetic state.

So it was the iron (steel wool) that released the energy (heat) shouldn't that make the temperature decrease? If it released heat it stayed because of the oxygen?

Originally Posted by Bigbear

So it was the iron (steel wool) that released the energy (heat) shouldn't that make the temperature decrease? If it released heat it stayed because of the oxygen?

The iron reacts with the oxygen, forming an oxide Fe₂O₃ or Fe₃O₄ depending upon the oxidation state.

It does this because being an oxide is a more stable state (given the atomic structure and the number of electrons in the shells) of the combination as a whole: the combination of iron and oxygen, that is.

As a result, when the two react, energy is given off, in the form of photons, which themselves interact with: the remaining gas in the container, the substance of the container itself, the iron oxide, the thermometer etc. The result of this interaction is that these molecules gain kinetic energy. This kinetic energy of the molecules is what shows up as a temperature increase.

Remember, to clarify your thinking on this matter, that heat is not the only form of energy. What is happening is that chemical energy (as I described above) is turning into kinetic energy (in molecules) which we measure as increased temperature (or, therefore, as an increase in heat energy).

No energy has been 'created' or 'destroyed' in this process. It has simply shifted from one form to another.

You can calculate the energy absorbed/released using the enthalpy of reaction. Add up the energy absorbed by bonds breaking, and the enregy released by bonds forming, and the difference is the energy released or absorbed, depending on whether it is positive or negative. You can find tables of the enrgy contained in certain common bonds, such as O=O.

I am confused. The heat had to have been absorbed not just when they mix they give of heat.

Originally Posted by Bigbear

I am confused. The heat had to have been absorbed not just when they mix they give of heat.

It might help us explain matters to you if you gave us a summary of your scientific/educational background so far. This will enable us to tailor our responses to you. Your age/school grade might help too.

Exothermic reactions remove heat from the reaction, giving temperature to other elements in the air. Endothermic is where heat is absorbed from the surroundings into the reaction, therefore in an endothermic reaction there is a net gain in temperature, which means the reaction has gained energy (as temperature is energy, or a form of it at least).

Heat is basically the amount atoms move I think, the more they move, the hotter they are. So the steel wool and the oxygen are reacting in such a way that atoms are being made to move faster.

So in an exothermic reaction, atoms get slower (the reacants slow down, getting cooler and giving the fast atoms out to the air), and in an endothermic reaction they get faster (the reactants are made faster and give the slow atoms into the air).

Thats why tea gets cold, air elements with slower motion collide and cools the tea down (slows the molecules in the tea down, (that is an exothermic reaction, its exothermic because heat is given out remember (the tea is losing heat))).

In reactions, electrons will move from atoms to other atoms, which alternates speeds. Some elements have a way of getting electrons easier than others, therefore they are bound to move more electrons in and out of that atom (oxygen does this well (because it is in group 6) but that is A-level stuff). The important thing is that.

Its also called thermodynamics. The second law of which states that two different temperature bodies when met will cool each other or heat up to meet a middle level of heat (that will then depending on the atoms speeds result in an exothermic or endothermic reaction).

Here is an exothermic reaction:



Here is an endothermic reaction:




On a final note, if the volume of the container changed, the temperature would change, and even the pressure. You could, in a certain special circumstance introduce a reaction that takes place common under natural conditions that would be exothermic, put it in a container and end up with an endothermic reaction :-D.

Hope I've been of some help .

Some people forget that when someone asks a question they have limited knowledge on that. The person that teaches then should keep it as simple as possible.

Was I right? No-one has replied since I posted here. Most of what I put down was me theorising from knowledge I already had.

During the procedure the temperature in the sealed container increased causing the reaction to be endothermic.

No, the reaction between iron and oxygen is exothermic, which just means heat is released by the reactants to the surroundings. The temperature increases because the heat produced goes into the oxygen in the container, the unreacted metal and the thermometer itself. As sunshinewarrior said, chemical energy has been converted to thermal energy.

If the reaction was endothermic you would see the temperature drop as heat was drawn in from the surroundings. You should have seen a temperature drop in the ammonium chloride experiment.

I don't know what's the difference between"endothermic" and "endogonic".


It;s said they've the same meaning in the dictionary.

Originally Posted by ArezList

I don't know what's the difference between"endothermic" and "endogonic".


It;s said they've the same meaning in the dictionary.

They are used interchangably in biology, an endogonic reaction just refers to an endothermic biochemical reaction.