Which of the following reactions is not labeled correctly?
In reaction A, we have a dichromate ion reacting with a sulfite ion and a hydrogen ion, indicating we’re dealing with acidic conditions. The products are chromium ions with a charge of three plus, sulfate ions, and water. And this reaction has been labeled a redox reaction. In a redox reaction, reduction and oxidation processes are going on simultaneously.
During a reduction process, we expect the oxidation state of one or more components to reduce, while in an oxidation process, we expect the opposite. We expect to see the oxidation states of one or more components to increase. So let’s have a look at the oxidation states for the components in this reaction.
In the dichromate ion, the oxygen should have an oxidation state of minus two. And all the oxidation states should sum to the value of the charge. So seven times negative two plus two times the oxidation state of the chromium should sum to minus two. This means that chromium must have an oxidation state of plus six. Two times six minus seven times two is equal to negative two.
It’s a similar story for the sulfite ion, where the oxygen has an oxidation state of negative two. This means that sulfur has an oxidation state of positive four. And our last reactant, the hydrogen ion, has an oxidation state of positive one.
In order to be a redox reaction, we expect some of these numbers to increase and some of these numbers to decrease. The oxidation state of chromium three is positive three. The oxygen in the sulfate anion retains its oxidation state of negative two. But the oxidation state of the sulfur increases to positive six. Meanwhile, the hydrogen retains its oxidation state of positive one. And oxygen is again seen with an oxidation state of negative two.
During this reaction, we see the oxidation state of chromium decrease from positive six to positive three. That’s a reduction process. And we see the oxidation state of sulfur increase from positive four to positive six. That’s an oxidation process. We have oxidation and reduction occurring in the same reaction. It’s a redox reaction, which is correctly labeled. So it’s an incorrect answer to this question.
We’re looking for the reaction that’s not correctly labeled. So we can move on. In the second reaction, we have C₂H₄, which is the chemical formula for ethene, reacting with bromine, Br₂. Our product is 1,2-dibromoethane. Ethene has a double bond between the two carbon atoms. The bromines can insert into this double bond, forming a halo alkane. This reaction has been classified as an addition reaction, which is a type of reaction where two or more chemicals react to form only one. That’s exactly what we’re seeing here. The bromine is being added to a carbon–carbon double bond. And we don’t have any byproducts. So we have two components fusing together to form one. This reaction is correctly labeled, so we can move on.
In the next reaction, we see the salt sodium fluoride reacting with water, H₂O, producing hydrogen fluoride, otherwise known as hydrofluoric acid, and sodium hydroxide. This reaction has been classified as a hydrolysis, which is a chemical breakdown due to reaction with water. In this reaction, we’re seeing a direct reaction between sodium fluoride and water, producing the chemically distinct hydrogen fluoride and sodium hydroxide combination. This is not the same as dissolution, where, for instance, we would produce sodium ions and fluoride ions. This reaction is definitely an example of a hydrolysis reaction. The water is completely changing the chemical makeup of the sodium fluoride, producing an acid and a base, whereas before we had a salt.
If we did this reaction in reality, in solution under normal conditions, we produce very little hydrogen fluoride and sodium hydroxide, since they’re an acid and a strong base. However, this reaction can still be written as a valid reaction because it has a chance of happening and represents a very small part of a much bigger whole.
In the next reaction, we have methane, the smallest alkane, reacting with oxygen, producing carbon monoxide and water. The presence of carbon monoxide is an indicator we’re dealing with a combustion reaction which isn’t complete. In a complete combustion reaction, we’d expect carbon dioxide, not carbon monoxide. This reaction has been labeled decomposition, meaning a reaction where one chemical becomes two or more.
A good example of a decomposition reaction is the thermal decomposition of a carbonate like calcium carbonate, producing in this case calcium oxide and carbon dioxide. This is clearly not the case for the incomplete combustion of methane. Here we’re seeing multiple molecules reacting together, producing multiple other molecules. So we have our answer.
But just in case, let’s have a look at the last one. Here we have iron in its pure elemental form and sulfur, also in its pure elemental form. These come together to form iron sulfide. This has been labeled a synthesis reaction, which very much like addition means two or more simple chemicals combining to make one more complex chemical. This is indeed what we’ve got here.
We’re seeing two simple chemicals in their elemental form combining to form a binary inorganic solid. So before we round off, let’s just look at the difference between addition and synthesis.
An addition reaction is really talking about small molecules combining to make a big one, while synthesis reactions are specifically about the complexity of the starting materials versus the final materials. So an addition reaction can also be called a synthesis reaction. But generally, we talk about addition reactions in organic chemistry and synthesis reactions in inorganic chemistry. However, our incorrectly labeled reaction is 2CH₄ plus 3O₂ react to form 2CO plus 4H₂O, the incomplete combustion of methane.