How to Balance an Equation 2Mg + O2 → 2MgO

Step 1: Starting Out Mg + O2 → MgO Reactants Products Balancing equations is very simple with some basic tools. When starting to balance an equation you should make a list of all of the elements in the equation and identify the number of each element in that equation on both the reactant and product side. Mg + O2 → MgO Reactants Products Mg 1 1 O 2 1

Step 2 Reactants Products Not balanced. Once you have done step one you must decide if the equation is balanced or not. If the number of elements on each side of the equation are equal then the equation is balanced. If they are not then you will use coefficients to balance the equation. Coefficients are large numbers that indicate that we have multiples of that compound. The coefficients will be multiplied through each compound and change the number of elements in that compound. Mg + O2 → MgO Reactants Products Mg 1 1 O 2 1 Not balanced.

Step 2: continued Mg + O2 → MgO Reactants Products 2 This will affect the number you have written in your chart. To start, pick an element that is not equal in your chart. Once you’ve chosen the element decide whether the reactants or products should be changed. Remember, we can only multiply to get bigger, we cannot divide to get smaller. Once you add the coefficient it changes not just the element you were looking at but also all other element attached to it. Mg + O2 → MgO Reactants Products Mg 1 1 O 2 1 2 2 2

Step 3 Mg + O2 → 2MgO Reactants Products 2 Mg 1 1 O 2 1 2 2 2 Once you have changed one set of elements, you must change your chart to show the current number of elements now in the equation. This will often affect if your equation is balanced or not, if it is balanced then you are done and do not need to continue. If it is not balanced then you will need to add another coefficient to continue to balance the equation. In this example we added a 2 to the product which balanced the O but did not balance the Mg so we must go back to the reactants and place a coefficient in front of the Mg to balance that element. Mg + O2 → 2MgO Reactants Products Mg 1 1 O 2 1 2 2 2 2

Step 4: Final Step Once all of your elements in your balancing chart are equal across the reactant and product sides then your equation is balanced and you can stop. 2Mg + O2 → 2 MgO Reactants Products Mg 1 2 1 2 O 2 1 2

Special Circumstances 4 3 2 Number Neighbors What if my number of elements on the reactant and product side are number neighbors? When this occurs you must use the least common multiple (LCM) to balance your equation. Your number neighbors must then be multiplied to yield the LCM for that pair of numbers. Al + O2 → Al2O3 Reactants Products Al 1 2 O 2 3 Number Neighbors 4 4 6 6 Least common multiple: 6

Special Circumstances On some combustion reactions you will find that you cannot balance them because you end up with an odd number of element on one side of the equation and an even number of the same element on the other side. When this happens if you double the carbon compound you will find that the balancing will work out with no trouble. 2 C5H10 + O2 → CO2 + H2O Reactants Products C 5 1 H 10 2 O 2 3 15 10 5 10 5 10 5 10 10 20 20 15 30 30 11

Practice Problems Try balancing these equations on your own and then check your answers. Ag2O + HI → AgI + H2O H2 + O2 → H2O B + O2 → B2O3 2 2 2 2 4 3 2