Convention for Writing Chemical Equations – MCAT General Chemistry | MedSchoolCoach

Convention for Writing Chemical Equations

MCAT General Chemistry Chapter 1 - Section 2.1 - Chemical Equations

Sample MCAT Question - Convention for Writing Chemical Equations

Which of the following stoichiometric properties must be accounted for when balancing a reaction?


I. Number of atoms

II. Charge

III. Mass

a) I only

b) III only

c) I and II only

d) I and III only

C is correct. I and II only. When balancing a chemical reaction, we must add coefficients to each side of the reaction equation such that the number of atoms of each element in the reactants is equal to the number of atoms of each element in the products. Likewise, we must also account for charge, by ensuring that the total charge of the reactants is equal to the total charge of the products. This can sometimes be done simply by balancing the reactants and products, but in other, more complex, oxidation-reduction reactions it may require adding electrons and protons to either side of the equation. Mass, while a conserved value, does not need to be accounted for to balance the reaction equation.

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Convention for Writing Chemical Equations

Let’s go over the convention for writing chemical equations. Consider the following two chemical equations:


4 Fe(s) + 3 O2 (g) → 2 Fe2O3 (s)


3 H2(g) + 1 N2 (g) → 2 NH3 (g)


There are four basic components in any chemical equation. The first component is the directionality of the reactions, given by the arrow pointing from left to right. This indicates that the compounds on the left side of the reaction are our reactants, and the compounds on the right side are our products. While notation is traditionally shown this way, with the reactants on the left, note that an arrow can also point forward and backward together, indicating a reversible reaction.


The second component is the identity of the atoms themselves, in this case the reactants iron and oxygen on top and hydrogen and nitrogen on the bottom reaction, and the products ferric oxide and ammonia. 

Each substance is also denoted by the third component, the phase state, given in the parenthetical as a gas (g), solid (s), liquid (l), or aqueous solution (aq). 


The final component is the coefficient or the number before each compound. This represents the relative, or stoichiometric, ratio of each compound to the others in the reaction. In the case of our first reaction, for example, the coefficients tell us that for every four moles of iron and three moles of oxygen that react, two moles of ferric oxide will be produced. Note that most often a reactant or product with a coefficient of 1, such as nitrogen in our second reaction, will instead be left blank, with no coefficient shown.

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