How do you write a balanced chemical reaction?

Answer

Before writing a chemical reaction, you must know a few things. You must know what the reactants are, and what the molecular formula of each one is. You must also know what the products of the reaction are, and how to write the molecular formulas for the products. Then you can go about writing the equation for the reaction and balancing the reaction.

In a chemical reaction, elements combine into compounds, compounds decompose back into elements, or transform existing compounds into new compounds. Because atoms are indestructible in chemical reactions, the same number of atoms of element must be present before and after any reaction. This is based on the principle of conservation of mass.

To balance a chemical equation, you must use stepwise reasoning. Sometimes there is some trial and error involved until you get enough practice to be good at it. It is something best learned with a lot of practice! The bottom line in any balanced reaction, and the way to test if you've done it correctly is this:

--HOWEVER MUCH OF ONE ELEMENT GOES IN, THE SAME AMOUNT MUST COME OUT!

Let's use this example: the decomposition of ammonium nitrate (NH₄NO₃) to dinitrogen oxide (N₂O) and water (H₂O). The unbalanced reaction is this:

NH₄NO₃ ---> N₂O + H₂O

On the LEFT side of the arrow are REACTANTS.

One the RIGHT side of the arrow are PRODUCTS.

Notice that the way the reaction is written not, 4 hydrogen atoms (H) go in, but only 2 come out (in the H₂O) and also 3 oxygen atoms (O) go in as reactants, only 2 come out as products. This is NOT balanced! The nitrogen (N), by chance, does happen to be balanced: 2 go in as reactant, and 2 come out as products. To balance the reaction, we will add coefficients, or numbers, in front of certain reactants and products to make sure what goes in comes out!

First, assign 1 as the coefficient of one species (or compound or element), and usually you want to chose the most complicated compound containing the most elements. Here we should pick NH₄NO₃.

Secondly, find the elements that appear in only one other place in the equation, and assign coefficients to balance the numbers of their atoms. In our example, N appears in only one other place (N₂O), and a coefficient of 1 for N₂O ensures that for the 2 N's that go in, 2 come out. Hydrogen appears in H₂O, and so its coefficient is 2 to balance the 4 H atoms on the left side. So now we have:

NH₄NO₃ ---> N₂O + 2 H₂O

Finally, check that the last element, O, is also balanced. In this case it is, as there are 3 on the left side and 3 on the right side. You don't need to put the "1" in front of the NH₄NO₃ and N₂O. The number one is implied if you don't write a number.

Let's do it again for a different example, the burning of butane, C₄H₁₀, in oxygen gas (O₂) to form carbon dioxide (CO₂) and water (H₂O). The unbalanced reaction is then

C₄H₁₀ + O₂ -----> CO₂ + H₂O

Here is a good tip: leave blanks in front of each compound or element in the reaction so you have room to add the coefficients. Like this:

___ C₄H₁₀ + ___ O₂ -----> ____ CO₂ + ____ H₂

Again, butane is the most complicated compound, so assign its coefficient to be one.

C₄H₁₀ + ___ O₂ -----> ____ CO₂ + ____ H₂

Since there are 4 C's in butane, and the only place the C can go is into CO₂, the coefficient in front of CO₂ must be 4. In the same way, there are 10 H's in butane, and they can only go into the water, and so the coefficient in front of H₂O must be 5 (10 H's divided by 2 H's per water).

C₄H₁₀ + ___ O₂ -----> 4 CO₂ + 5 H₂O

Finally, we have to balance the oxygens. On the right we have 4*2 = 8 in the CO₂, and 5*1 = 5 in the H₂O, so that gives a total of 8+5 = 13. So now we have to make sure that 13 oxygen atoms are also on the left. Since we have O₂ as the reactant, we can use a fraction as a coefficient to make the numbers come out right. Since we want 13 total, what number times 2 equals 13? That would be 13/2! So now we have:

C₄H₁₀ + ¹³/₂ O₂ -----> 4 CO₂ + 5 H₂O

Note that there is NOTHING WRONG WITH USING FRACTIONS in a balanced chemical equation. However, often people prefer to eliminate fractions because it makes it a bit easier to look at and work with. To do that, we simply multiply everything (on both side of the arrow) by 2:

2 C₄H₁₀ + 13 O₂ -----> 8 CO₂ + 10 H₂O

--TO SUMMARIZE:

Sometimes you can balance a reaction by just looking at it and figuring it out very easily (it's called doing it "by inspection"). Other times it can be trickier and these steps can help.

--Step 1: Assign the number 1 as the coefficient to one species. Choose the most complicated one, with the most different elements in it.

--Step 2: Identify, one by one, elements that appear in only one species whose coefficient is not yet determined. Choose that coefficient to balance the number of atoms of that element. Continue until all coefficients are identified. Check to make sure that the number of atoms of each element on the left is equal to the number on the right of the arrow!

--Step 3: If you want (this is optional!), you can multiply the whole equation by the smallest integer that will get rid of any fractions.

First answer by JEK. Last edit by JEK. Contributor trust: 1827 [recommend contributor]. Question popularity: 60 [recommend question]