A reaction described in a sentence is shortened to a word equation, then to formulae. An equation with correct formulae but unequal atom counts on the two sides is a skeletal equation.
“When magnesium ribbon is burnt in oxygen, it is converted to magnesium oxide.” The sentence is correct, but it takes 13 words to say which substances react and which substance forms.
A word equation keeps only that information: the names of the substances and the direction of the change.
The same reaction, shortened. Reactants on the left, products on the right, and an arrow showing the direction of change.
“Zinc reacts with sulphuric acid to give zinc sulphate and hydrogen” becomes: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen.
Write a word equation for: “Hydrogen gas combines with nitrogen gas to form ammonia.”
Hydrogen + Nitrogen → Ammonia
Hydrogen and nitrogen are the reactants, on the left, joined by a plus sign. Ammonia is the only product, on the right. The arrow stands for “combine to form”.
A word equation names the substances but does not show their atoms. Replacing each name with its chemical formula shows which atoms each substance contains, and how many.
Magnesium is Mg. Oxygen gas is made of molecules of two oxygen atoms bonded together, so it is written O₂. Magnesium oxide contains one magnesium atom for each oxygen atom, so it is MgO. The equation becomes:
Mg + O₂ → MgO
Formulae are the same in every language. They also make it possible to count the atoms on each side, which a word equation does not.
Count the oxygen atoms in Mg + O₂ → MgO. There are 2 on the left and 1 on the right.
An equation with correct formulae whose atom counts do not match on the two sides is called a skeletal equation. It names the right substances. It does not yet show the right amounts. As written, one oxygen atom is missing from the products, and in a real reaction no atom disappears.
Correct substances, unequal counts. The skeletal equation has 2 oxygen atoms on the left and 1 on the right. The next session shows how to make the counts equal.
In what sense is a skeletal equation correct, and in what sense is it incomplete?
It is correct about which substances react and form. The reactants, the products and their formulae are all right.
It is incomplete about how many of each. The number of atoms of each element is not the same on both sides. In a real reaction, atoms are rearranged but not created or destroyed, so the counts must match.
Balancing, in the next session, makes the counts equal by changing the number of each formula, without changing any formula.
A reaction can be written as a sentence, as a word equation, or as a skeletal equation in formulae. Each step is shorter and shows more detail about the atoms.
The skeletal equation still needs one more step. Making the atom counts equal on both sides depends on the law of conservation of mass, which is the subject of the next session.
1In a word equation, where are the reactants written?
2What does the arrow in an equation represent?
3An equation with correct formulae but unequal atom counts on the two sides is called a ____ equation.
4Which is the correct equation in formulae for “zinc reacts with sulphuric acid to give zinc sulphate and hydrogen”?
5What is wrong with the skeletal equation Mg + O₂ → MgO?
6Why are names replaced with chemical formulae?
Next: balancing equations, and the law that requires it.
Noescio