A chemical equation is the graphical representation, in the form of symbols indicating the chemical elements, of a reaction. The reactants used in the reaction are listed inside the left side of the equation, while the products resulting from the reaction are listed on the right side of the same equation. The law of conservation of mass (also known as Lavoisier's law) states that, in the course of any chemical reaction, no atom can be created or destroyed. We can therefore deduce that the number of atoms of the reactants must balance the number of atoms that make up the products obtained from a chemical reaction. Read this article to learn how to balance chemical equations in two different ways.
Steps
Method 1 of 2: Traditional Balancing
Step 1. Make a note of the equation to balance
In our example, we will use the following:
- C.3H.8 + O2 H.2O + CO2
- This chemical reaction occurs when propane gas (C.3H.8) is burned in the presence of oxygen producing water and carbon dioxide.
Step 2. Take note of the number of atoms that make up each element within the two sides of the equation
Look at the subscript number of each element of the equation to calculate the total number of atoms involved.
- Left limb: 3 carbon atoms, 8 hydrogen and 2 oxygen atoms.
- Right member: 1 atom of carbon, 2 of hydrogen and 3 of oxygen.
Step 3. Always leave hydrogen and oxygen at the end of the balancing process
Start by analyzing the other elements in the equation.
Step 4. If there is more than one element to balance on the left side of the equation, choose the one that appears as a single molecule as both reactant and product
In our example, this means that we will have to start by balancing the carbon atoms.
Step 5. Add a coefficient to the single carbon atom on the right side of the equation to balance the 3 carbon atoms present as reactants (listed on the left side)
- C.3H.8 + O2 H.2O + 3CO2
- The coefficient 3, which precedes the carbon symbol on the right hand side of the equation, indicates three carbon atoms exactly like the number 3 in the subscript of the carbon symbol on the left side of the reaction.
- When working with chemical equations, it is possible to modify the coefficients of the elements (which represent the number of molecules of reagent or product to which they refer), but it is never possible to modify the values placed in subscript (which indicate the number of atoms).
Step 6. Let's proceed by balancing the hydrogen atoms
In the left side of the reaction under consideration, we have 8 hydrogen atoms. This means that, even on the right side of the equation, we will need to have 8 hydrogen atoms.
- C.3H.8 + O2 4H2O + 3CO2
- Within the right side of the equation, we added the number 4 as the coefficient of the compound in which hydrogen appears, since the latter is already present with 2 atoms.
- Multiplying the coefficient (4) by the subscript value (2) of the hydrogen produced by the reaction, we will obtain exactly the desired result: that is 8.
- The reaction naturally produces another 6 oxygen atoms, in the form of 3CO carbon dioxide2, which added to those added give as a result (3 x 2 = 6 oxygen atoms + 4 added by us = 10).
Step 7. Let's proceed by balancing the oxygen atoms
- Since we added a coefficient to the molecules on the right side of the equation, the number of oxygen atoms has changed. We now have 4 oxygen atoms in the form of water molecules and 6 atoms in the form of carbon dioxide molecules. So, in total, the reaction produces 10 oxygen atoms.
- Add the number 5 as the coefficient of the oxygen molecule on the left side of the equation. Each member now has 10 oxygen atoms.
- C.3H.8 + 5O2 4H2O + 3CO2
Balance Chemical Equations Step 7Bullet3 - The equation is perfectly balanced because it has the same number of carbon, hydrogen and oxygen atoms in each member, so the job is done.
Method 2 of 2: Algebraic Balancing
Step 1. Make a note of the equation including the chemical elements and variables, in the form of coefficients needed to perform the balance
Let's take as an example the equation shown in the image accompanying the passage, so let's assume that the variable "a" is equal to 1 and write the formula to carry out the balancing.
Step 2. Substitute the correct values for the respective variables
Step 3. Check the number of elements obtained as reactants, on the left side of the equation, and those obtained as products, on the right side
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For example: aPCl5 + bH2O = cH3PO4 + dHCl. We assume that a = 1 and that the values of the variables b, c and d are unknown. At this point separate the single elements present in the reaction, which are P, Cl, H, O, and balance the number of atoms obtaining: a = 1, b = 4, c = 1 and d = 5.
Balance Chemical Equations Step 10Bullet1
Advice
- Always remember to simplify the final equation.
- If you get stuck, you can balance the equation you are studying by using one of the countless websites that offer this type of service. However, remember that you will not have access to these types of tools during the exam or verification in the classroom, so do not abuse them and run the risk of becoming dependent on them.
- It is best to balance chemical equations using the algebraic method.
Warnings
- The coefficients present within an equation describing a chemical reaction cannot be fractions. This is because it is not possible to split a molecule or atom in half during a chemical reaction.
- During the various steps that make up the process of balancing a chemical equation, it is possible to help yourself by using fractional coefficients, but once the balancing is completed all the coefficients must be represented by whole numbers, otherwise the reaction will never be balanced.
- To remove fractional coefficients from a chemical equation, multiply both sides (both the reactant and the product members) by the common denominator of all the fractions present.
