Topic: Chemical Equation Naming

Back again with another "Chemistry" question. I'm not cheating on my HW, I swear, I just need help with naming chemical equations. How can I tell if an equation is combustion, decomposition, single displacement or double displacement. If you can help me there, I can do the rest. Thanks again.

Combustion reactions are reactions involving an oxidant. Take the combustion of methane for example; CH4 + 2 O2 -> CO2 + 2 H2O, methane is the fuel in this example; the oxidant is oxygen. You'll find that for most equations, oxygen is used as the oxidant, ozone (O3) is sometimes used as an example as well. It will most likely not get much more complicated than that, as there are many more oxidants.

Decomposition occurs when a compound is separated in smaller compounds. The electrolysis of water is a decomposition reaction; 2 H2O -> 2 H2 + O2. The basic structure of a decomposition reaction is AB -> A + B.

Single and double displacement reactions are both a type of redox reaction (reduction-oxidation reaction). In a single displacement reaction; one ion is displaced with another, in a double displacement reaction, two ions are displaced.

Single:

2 HCl + Mg -> MgCl2 + H2
(AB + C -> CB + A)

Double:

2 NaBr + BaCl2 -> 2 NaCl + BaBr2
(AB + CD -> AD + CB)

Gotcha.

Combustion is a reaction that breaks down a compound into two compounds in the presence of oxygen. Usually, the products are CO2 and H2O, which is your tell tale sign. Sometimes if you have a non carbon compound you are combusting it will be slightly different, though the products are similar in size and composition (for example Sulfur may combust into SO3 and H2O)

Decomposition is pretty simple to identify. It's when you basically have multiple products forming from one reactant. The idea is that the reactant is broken down into components, like H2O being electrolytes into H2 and O2

Single replacement is exactly that. You are switching the ionic bond of an ion. The tell tale sign for this is that one of your reactants isn't bonded to anything. It's just a lone element. And in the product, that element becomes an ion.

Double replacement is when you have two ionic compounds trade ions. So if you have NaNO3 react with AgCl, you'd get NaCl and AgNO3. So the Cl and NO3 both switched places.

I may push my promise here, but will this be a Single displacement:

2 Na + Cl2 = 2 NaCl
Also, does the coefficients matter?

EDIT: 2Na + Cl2 = 2NaCL is not a single displacement because it doesn't have a lone element.

No, since there is no ion displaced. It is a redox reaction though, as sodium loses an electron to chlorine.

Coefficients don't matter in this case.

How do you tell if something is gaining/losing an electron here?

Generally their position on the periodic table indicate that. Elements on the left usually/always lose electrons and elements on the right gain electrons

Oh so their oxidation state? I know what you mean. One of the question my Chem. teacher gave me is an impossible reaction. I'm skipping it.

EDIT: Thanks for the help. I got it from here.

No, see, there are lone elements (as in elemental Sodium and Chlorine). But that's a synthesis (aka composition) reaction.

A single replacement means you have an ION changing places.

So a single replacement would be like

Na + KCl -> K + NaCl

So let me get this straight:

SR: Lone element (in which case K)
DR: Two elements swap
Combustion: When Oxygen's a reactant
Decomposition: When hydrogen * oxygen's a reactant
Synthesis: ??

Obligatory "I hate chemistry" statement here.

Close.

DR is when two ions swap, so like in NaCl, there is Na+ and Cl-, which is different from elemental Na.

Combustion is a specific reaction that uses Oxygen as a reactant, though you can have non-combustion reactions with Oxygen (So all combustion reactions have Oxygen as a reactant, but not all reactions with Oxygen as a reactant are combustion reactions). It's hard to fully describe exactly what combustion is, but the rule of thumb you should use is if Oxygen is a reactant, and CO2 and H2O are in the products, it's likely a combustion reaction.

Decomposition doesn't have to by hydrogen and oxygen. It's just when one compound breaks down to multiple compounds. H2O into H2 + O2 is an example of that. Another common one is H2CO3 (Carbonic acid) becoming H2O and CO2. The key thing to recognize here is that you are getting new neutral molecules. NOT ions. So NaCl -> Na+ and Cl- isn't a decomposition reaction. The rule of thumb here is that you have one reactant (as in one compound, if there is a number in front to balance the equation, that's fine) and multiple products.

Synthesis is just the opposite of decomposition. Instead of breaking a compound into multiple compounds. You are combining multiple reactants into one product. The rule of thumb here is that you have one product (again if there is a number in front to balance the equation that's fine) and multiple reactants.

A synthesis (composition reaction) occurs when two or more elements/molecules form a single compound (A + B -> AB). Decomposition is the exact opposite; one compound is separated into smaller products (AB -> A + B).

Combustion are reactions involving an oxidant (for example; O2).

Examples of single and double displacement reactions already have been posted;

Single:

2 HCl + Mg -> MgCl2 + H2
Hydrogen is swapped out for magnesium in the compound HCl -> MgCl2 (AB + C -> CB + A).

Double:

2 NaBr + BaCl2 -> 2 NaCl + BaBr2
In this example sodium ''trades'' bromine for chloride with barium (AB + CD -> AD + CB).

Displacement reactions involve the direct removal of an atom or molecule with another atom or molecule. Heard of the Electrochemical Series? This is a table like description of the reactivity of an element in the periodic table. Flourine is the most electro negative element while francium, or rather, caesium, is the most electro positive element. We use the series to tell us what can displace what. Let's take this equation. for example:
2NaF(s) + Cl2(g) > 2NaCl(aq) + F2(g) CAN NEVER HAPPEN because chlorine can't displace fluorine. If the sodium fluoride is, however, a sodium bromide, we can count on chlorine displacing bromine.
Combustion reactions involve oxygen gas as part of the reactants.
Single Decomposition is the breakdown of a compound into two or more simpler substances. If heat is needed, then we say the reaction is thermal dissociation.
Double Decomposition involves the reactants decomposing to form new substances by an exchange of radicals. For example:
AgNO3(AQ) + NaCl(aq) > AgCl2(s) + NaNO3(aq)

I thought combustion could happen with elemental fluorine instead of oxygen.

(I am interested in Chemistry but haven't done any for 12 years).

This equation is a combination reaction i.e. the combination of two elements to form a compound. In some cases, that compound can dissociate into its reactant elements. That is called a single decomposition reaction.

It is possible. Any reaction with an oxidant is technically a combustion. Actually, all halogens are able to take the role of an oxidant during a combustion. However most combustions occur in the presence of oxygen.

Hmm. Didn't know that. So how can you distinguish combustion from any redox reaction?

Since electrons are transferred during a combustion reaction; they are considered redox reactions as well. Take the example of a combustion reaction I gave earlier:

CH4 + 2 O2 -> CO2 + 2 H2O

Even though it seems difficult to keep track of the electrons during a reaction like this; they are in fact moving around. Use oxidation numbers to keep track of the electrons:

CH4 + 2 O2 -> CO2 + 2 H2O
(-4)(+4) + 2 (0) -> (+4)(-4) + 2 (+2)(-2)

With this knowledge, we can easily identify the half-reactions:

Oxidation reaction: CH4 -> CO2 + 8 e-
Reduction reaction: 2 O2 + 8 e- -> CO2 + 2 H2O

The rule of thumb that is usally given when explaining combustion reactions is that CO2 and H2O are reaction products. Although this is the case in the combustion reaction of methane in oxygen, this doesn't apply to the combustion of methane in, say, fluorine. Fluorine has a high electronegativity, causing it to react instantly with most other elements. Since a combustion is nothing more than a rapid redox reaction, the instant and violent reaction caused by fluorine in the presence of a reducing agent (fuel) will ignite the fuel, just like it would in the presence of oxygen.

Now, fluorine doesn't naturally occur in high concentrations, and since storing it is quite difficult, a combustion reaction with fluorine as the oxidant isn't very common. Therefore, the rule of thumb still applies in the majority of combustion reactions.

Now for combustion, does it have to be both CO2 and H2O or one or the other?