An endothermic reaction is one that gets colder as it proceeds; unlike an exothermic reaction which releases heat as it progresses. It all depends upon the bonds between the atoms in the reactants and the products. The first thing to say is that during a chemical reaction, exactly the same atoms appear in the products as in the reactants (see Conservation of Mass). If we take the combustion of methane as an example we can look at the bonds that need to break and those that need to form during the reaction.
In the graphic above, one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. Exactly the same type and number of atoms appear on the right as started on the left.
In order for the atoms to rearrange as shown in the diagram, the bonds between the reactant molecules must break so that the atoms are free to attach themselves in a different combination. For example four C-H bonds must break in the methane molecule if the four hydrogen atoms and one carbon atom are to bond with something else. Likewise, the O=O bonds must break so that the oxygen atoms are free to join up with the hydrogen and carbon atoms. To break a bond requires energy and fortunately we can look up in a table just how much energy is needed for each kind of bond to break.
So we can work out from this table how much energy is needed to break the bonds in the methane and the two oxygen molecules.
(4 × 413 kJ) + (2 × 498 kJ) = 2,648 kJ
We now have the atoms free and ready to recombine. When bonds form they release energy, so we can calculate how much energy will be released by using the table of bond energies above.
(2 × 805 kJ) + (4 × 464 kJ) = 3,466 kJ
So 818kJ more energy is released when the products formed than when the original bonds were broken. This is why the reaction is exothermic. A reaction in which more energy is needed to break the bonds than is released with the formation of new bonds is endothermic.
Question One…
Does ethane (C2H6) release more or less energy than methane when it reacts with oxygen? (Show your working.)
Question Two…
Ethanol is a compound of carbon, hydrogen and oxygen. It has the chemical formula C2H5OH. A model of an ethanol molecule looks like this…
A word equation for the combustion of ethanol would be…
What would a balanced symbol equation for this reaction look like?
How many oxygen molecules are needed to completely combust an ethanol molecule and how many carbon dioxide and water molecules are produced?
What additional piece of information would you need (that is missing from the table above) to calculate how much energy is released when a molecule of ethanol combusts completely in oxygen?
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