Introduction to Quantitative Chemistry 1

During a chemical reaction, the atoms are rearranged from the reactants  (starting substances) to the products (end substances). Chemical bonds between atoms are broken, and new bonds are formed, but the number of atoms in the reactants is equal to the number of atoms in the products: the total mass of the reactants at the beginning is equal to the total mass of products after the reaction. 

No atoms are created or destroyed, therefore no matter, is lost or gained. (BBC Bitesize, 2014).

The volume of gas produced can be used to follow the course of a reaction. Gas volumes can be measured using a gas syringe or an upturned measuring cylinder or burette. 

The mass of gas produced can be measured using a sensitive balance. 

Using a balanced chemical equation to calculate amounts of reactants and products is called stoichiometry. It is a super technical-sounding word that simply means the amalgamation of Chemistry and Mathematics (Khan Academy, 2016).

Stoichiometric calculations are based on the law of conservation of mass. 

The total mass of all products is equal to the total mass of all reactants in a chemical reaction.

Balancing chemical equations help ensure we have followed the law of conservation of mass.

2H2(g)  +  O2(g)  -->  2H2O(l) 

Reactants              Products

A chemical equation should be balanced on both sides. A chemical equation shows us the substances involved in a chemical reaction: the substances that react (reactants) and the substances that are produced (products). In general, a chemical equation looks like this:

Reactants     -->  Products

According to the law of conservation of mass, when a chemical reaction occurs, the mass of the products should be equal to the mass of the reactants. Therefore, the amount of the atoms in each element does not change in the chemical reaction, the chemical equation that shows the chemical reaction needs to be balanced. A balanced chemical equation occurs when the number of the atoms involved in the reactants side is equal to the number of atoms in the products side (Study, 2016).

In general, to balance an equation, here are the things we need to do:

• • Count the atoms of each element in the reactants and the products.

  • Use coefficients; place them in front of the compounds as needed.

  • Where possible, leave an element until last as any coefficient placed in from of an element only affects that element (eg 2Li) whereas any coefficient placed in front of a compound affects all atoms in the compound (eg 2NaCl)

The steps are simple, but it is sometimes a process of trial and error. 

This is where we bring stoichiometry and the law of conservation of mass together. When a chemical reaction is written as an equation, and is balanced, we can get an idea of the relative amounts of each substance present as either reactant(s) or product(s). As we learn more about stoichiometry, we will be able to make direct calculations to demonstrate the conservation of mass in any chemical reaction (Chan, et al., 2018).

Chemical reactions may involve one or more products being in a different state at room temperature to the reactant(s). Chemists need to take into account the different states which may be produced when applying the law of conservation of mass to the reaction. In addition to the balancing of chemical equations, we will also use subscripts to show these states (Chan, et al., 2018).