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Understanding chemical formulas and what a mole is allows us to consider simple chemical equations. Chemical equations are used throughout chemistry and must be well understood in order to progress beyond the most fundamental level.
Chemical equations are a shorthand way of describing what happens when some initial reactants are converted to the final products.
During all chemical reactions the atoms that are initially combined to form the reactants are rearranged into new combinations that form the products. Since a mole of atoms is a specific number of atoms, chemical equations provide mole ratios between reactants (when there is more than one reactant), between reactants and products, and between products (when there is more than one product).The physical and chemical properties of the products are different from the physical and chemical properties of the reactants.
During this process no atoms are lost and it is important that all of the starting materials are accounted for either as products or left over reactants. A balanced chemical equation assumes no left over reactants and accounts for all of the reactants that are used.
This section will help you:
- Describe what a chemical equation is.
- Balance simple chemical equations.
- Understand atomic and mole ratios in chemical reactions.
An unbalanced chemical equation for the formation of gaseous water, H2O(g), from hydrogen gas, H2(g), and oxygen gas, O2(g), is:H2(g) + O2(g) → H2O(g)
This equation shows that the atoms from the reactants have been rearranged to form a new substance (water) that has different properties from the original materials, but what is wrong with this equation?There is one molecule (two atoms) of hydrogen and one molecule (two atoms) of oxygen on the reactant side of the equation while there is one molecule of water (two atoms of hydrogen and one atom of oxygen) on the product side. All of the original oxygens are not accounted for.
Now consider the following representation of the chemical reaction:
2H2(g) + O2(g) → 2H2O(g)
In this equation there are two molecules of hydrogen (four atoms) and one molecule of oxygen (two atoms) as reactants and two molecules of water (four atoms of hydrogen and two atoms of oxygen) as products. The number of atoms of hydrogen and the number of atoms of oxygen are the same on both sides.A chemical reaction is balanced when the number of atoms of each element in the reactants is equal to the number of atoms of each element in the products. The second chemical equation is a balanced chemical equation.
The balanced chemical equation can be interpreted as two molecules of hydrogen reacting with one molecule of oxygen to form two molecules of water. But a mole of molecules is just 6.02 x 1023 molecules. So the following statement is also true: two moles of hydrogen react with one mole of oxygen to form two moles of water.
The balanced chemical equation gives mole ratios. The mole ratio between the reactants hydrogen and oxygen is 2:1 (two moles of hydrogen to one mole of oxygen). The mole ratio between hydrogen and water is 2:2 (or 1:1, one mole of hydrogen to one mole of water) and the mole ratio between oxygen and water is 1:2 (one mole of oxygen to two moles of water).
Consider the reaction between solid phosphorous, P, and chlorine gas, Cl2, to form PCl3. The balanced chemical equation is:
2P + 3Cl2 → 2PCl3
The equation says that two moles of phosphorous will react with 3 moles of chlorine gas to make two moles of the new substance PCl3. The ratio of P to Cl2 (P:Cl2) is 2:3, the ratio of P to PCl3 is 2:2, and the ratio of Cl2 to PCl3 is 3:2.
These ratios can be used to convert between moles of reactants and moles of products as shown in the following lesson.
At this time we will balance equations by trial and error adjustment of the coefficients like we did in the water example. Don't change the subscripts, that will change the substance that is being used or produced!