Answer to Question #151720 in General Chemistry for Alyssa Leblanc

Question

When asked to write the equation of something, how do you know what the subscripts and coefficients are? For example, we were told that solid calcium reacts with aqueous hydrogen chloride to yields aqueous calcium chloride and hydrogen gas. Then the answer shows 1Ca (s) + 2Hcl (aq) —-> 1 CaCl2 (aq) + 1H2
And as you can see I don’t know where they got the 2’s from. I’m also curious as to how you would figure out what state of matter to write if you weren’t given the background context, for example just predicting products. Also does knowing the charge of ions have anything to do with it? Thanks :)

Answer 1

Subscript:

The numbers appearing as subscripts in the chemical formula indicate the number of atoms of the element immediately before the subscript. If no subscript appears, one atom of that element is present.

To determine the correct subscripts in a chemical formula, you have to solve how many atoms you need to balance the charge.

For example if I had the compound Calcium Fluoride I would look at the periodic table and see that Calcium's ionic formula is Ca²⁺ . How do I know this? Well all elements want to have 8 valance electrons so they can be stable(happy). Seeing that Calcium has 2 valance electrons it is going to give away 2 electrons because that is easier than gaining 6 to be happy. Since Calcium has given away 2 electrons it has two more protons than electrons. We know that Protons have a Positive charge, Electrons have Negative charge, and the number of electrons is equal to the atomic number of an element in its pure non-ionic state. (Meaning it doesn't have a positive or negative charge; it is balanced.)

So if calcium gave away two electrons, it will have two more protons than an electron giving it a (2+) charge. The same process can be applied to Fluoride. Since fluoride is one to the left of the noble gases(group 18 or 8A) on the periodic table we know that it has 7 valance electrons because it is in group 7A or 17.

Knowing that we have 7 electrons the fluoride atom will gain an extra electron. Since the fluoride atom gained an extra electron it will have one more negative charge than a positive making it a F^- ion.

So you know that Calcium has a 2+ charge and that fluoride has a 1- charge, you then need these ions to balance out. So you need two fluorine atoms with a 1- ions to balance out the 2+ ion of calcium. Your final answer would be CaF₂ because you need two fluorine atoms to balance out the 2+ charge of the calcium.

Final Tip: Determine the charges then inverse the charges, remove the positive and negative superscipts, and write the charge numbers as a sub script. Ie. Calcium Fluoride Ca²⁺

 and F^- inversing and removing the charge signs would give you CaF₂

Coefficients are the numbers in front of each formula. If no number is shown, a one is understood. Three: The coefficients tell us how many molecules (moles) of each reactant used and how many molecules (moles) of each product made.

To know what the coefficients are you need to balance the reaction: the number of each atoms have to be the same. For example:

Ca + HCl —> CaCl₂ + H₂;

It is unbalanced, because there are 2 atoms of Cl on the right and only one on the left, the same for hydrogen. In means you need double the number of both H and Cl. To do that. you put the coefficient 2 befor HCl.

State

You can predict the state of a substance if you know its temperature, and its melting point and boiling point. If the temperature is: below the melting point, the substance is solid • between the melting point and boiling point, the substance is liquid • above the boiling point, the substance is gas.