I got a rather frustrating homework assignment in science class. Although I have a good grade in that class, I'm really struggling with this new chapter based around ions, chemical compounds, and chemical equations.
How do you write chemical compounds? I tried googe'ling it but nothing good came up because it just kept giving me stuff for chemical equations, which are different.
An example part of the homework is that I somehow need to combind Ca and C(2)H(3)O(2) (the numbers in ( ) are NOT oxidation numbers, they are the amount of atoms for that element).
How would I do that? We learned about this in class but I struggled to understand it. For the example above, I tried CaC(2)H(3)O(3) to try and balance them out but I highly doubt that answer is correct.
Answers & Comments
Verified answer
In introductory chemistry, to write a chemical formula you combine two elements, usually a metal and a nonmetal or a metal and a polyatomic ion. This is the case for your calcium and acetate, C2H3O2. (You can drop the ( ), that just confuses the issue. Everyone knows that the numbers are subscripts.)
The key to the correct formula is oxidation number. The sum of the oxidation numbers of the element or ion must be zero. The ionic charge is the same as the oxidation number. For instance, calcium has an oxidation number of +2, while acetate is -1. Therefore, we need two acetates to give a total of -2 so that the sum of the oxidation numbers is zero, and we get Ca(C2H3O2)2 for the formula. Note that C2H3O2 is placed inside parentheses since there is more than one.
Another example is a compound that contains aluminum and oxygen. Aluminum has an oxidation number of +3 and oxygen has an oxidation number of -2. Therefore, for the sum of the oxidation numbers to equal zero we need two Al atoms and three O atoms, for a formula of Al2O3.
Another "trick" is to crisscross the oxidation numbers.
+3 -2 ..... write the oxidation numbers above the elements or ions
Al...O .... start by writing the element with the positive oxidation number first
+3 -2 .... crisscross the numbers, and ignore the signs
Al2O3 ... The 2 over O goes with Al. The 3 over Al goes with O.
The sum of the oxidation numbers will equal zero.
............ +3+3 -2 -2 -2 = 0
Al2O3 = Al Al O O O
Can you crisscross to answer your question? Sure you can.
+2.......-1 .............. The entire C2H3O2 ion has a -1 oxidation number
Ca C2H2O2
+2.......-1 .............. Don't crisscross a 1. 1's are invisible.
Ca(C2H2O2)2 ...... Don't forget the parentheses around a polyatomic ion
There is more at my website: http://c7chemistry.wikispaces.com/Formula+Writing
It would also be handy to have an oxidation number reference. Here is a periodic table of common oxidation numbers: https://c7chemistry.wikispaces.com/file/view/oxida...
Chemical compounds are combinations of positive and negative ions, which may or may not be composed of more than one element (referring to ions).To write chemical compounds, you need to know the charges of the ions involved. For the example you've given,
Ca - calcium ion - Ca{2+} and C2H3O2 - acetate ion - C2H3O2{1-}
The numbers in braces are the charges of the ions. The acetate ion is an example of a polyatomic ion, meaning an ion with many different atoms but having one common charge. To write the formula of the resulting compound (calcium acetate), you just place the absolute value of the charge of one ion and place it in the subscript position of the other. Applying this, we get:
Ca1(C2H3O2)2 (calcium acetate)
Notice that the absolute value for the charge of the acetate ion (1) is now the subscript of the calcium ion, and the absolute value for the charge of the calcium ion (2) is now the subscript of the acetate ion. We enclose the polyatomic ion in parentheses to show that they are one unit, and that we need two units of acetate to combine with one calcium. In chemistry however we no longer write the subscript 1 so the compound youre looking for is
Ca(C2H3O2)2
Another example would be bismuth sulfide, a combination of Bi and S:
Bi{3+} + S{2-} -> Bi2S3
If the charges are the same, you simply assume that we only need one of each, as in the example of calcium oxide, a combination of Ca and O:
Ca{2+} + O{2-} -> CaO