General question for writing chemical equations

Question

Basically need a rundown when writing a chemical equation based on word descriptions (eg write the chemical equation of hydrochloric acid reacting with calcium chloride).

How do I know what element has which/how many subscript? I understand that the co-efficient is for balancing the equations afterwards but writing the equation itself is difficult. I've been taught that gasses often bonds to itself like O₂ but metals and metalloids are more of a challenge (Ie how can we tell a metal's subscript as sometimes on one side it is bonded to itself but on the other it is singular).

Also what is the method for arranging the reactants, using brackets correctly and finding it's charge if it has one?

Answer 1

Wow, that's a lot of stuff to cover in a written response, but I'll try to get you going. To tell what the subscripts are, you need to understand a little about valence and valence electrons, etc. You look at the periodic table to get some guidance. Examples might help:

For sodium chloride we have Na and Cl. Na is in group one so is +1. Chlorine is group 7 and is -1, so the bond in a 1:1 ratio = NaCl

If you have magnesium chloride, Mg is group 2 so is +2 and you need 2 Cl to balance it = MgCl₂

For aluminum chloride, Al is group 3 so it is +3 and you need 3 Cl to balance it = AlCl₃

No parentheses are needed because you don' t have a polyvalent cation or anion. For that, an example might be magnesium hydroxide. Mg is +2 and hydroxide is OH (polyatomic) and is -1, so you need two of them.

Magnesium hydroxide = Mg(OH)₂ and you need parentheses to show the subscript 2 applies to both the O and the H and not just the H.

The question you posed is hydrochloric acid reacting with calcium chloride.

First you must be able to write each compound.

Hydrochloric acid = HCl

Calcium chloride = CaCl₂ (Ca group 2 has +2 charge and Cl group 7 has -1 charge)

In fact, there really is no reaction between HCl and CaCl₂, but that's another lecture.

Hope this was of some help. If you don't get it, you should probably have a 1 hour tutoring session online. Find yourself a good tutor, and go for it.