How to Balance Chemistry Equations

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In chemistry, many reactions produce substances that bear no resemblance to the original ones used in the experiment. For example, two gases, hydrogen and oxygen, combine to form water, a liquid. However, even though new chemicals are created, the number of elements remains the same both before and after a reaction takes place -- atoms trade partners but are never created or destroyed. Balancing chemical equations is an essential task by which chemists determine how much of each reactant a reaction needs, and the quantity of products it produces. You can work through the process in a few short steps.

  • Write down the original unbalanced equation, with reactants on the left side of the equation and the products on the right side of the equation. As an example, consider the reaction of magnesium nitride, a greenish yellow powder, with water. They react to form magnesium oxide, a white solid used as an antacid or dietary supplement, and ammonia, a pungent-smelling gas. Here is the reaction written as an unbalanced equation:

    Mg3N2 + H2O ---> MgO + NH3.

  • Choose one element, and see whether there are equal numbers of the element on both sides of the equation. For example, in the above equation, if you choose O (oxygen), you will see that there is one O on both sides of the equation, so this element is balanced. Other elements may not be balanced; for example, there are three Mg (magnesium) atoms in the reactant, and only one in the product.

  • Multiply the chemical which contains the lower amount of an element by the number of elements contained in the chemical on the other side of the equation. In the example used here, since there are three Mg atoms in the reactant and only one in the product, multiply the chemical that contains one Mg atom (in this case, MgO) by three. This gives

    Mg3N2 + H2O ---> 3MgO + NH3.

  • Count the number of elements in the new equation, and note any imbalances in the number of elements in the reactant and the product. In the equation balanced in the equation, there are now three O atoms in the product, and one in the reactant. This can be alleviated by adding a three in front of the chemical containing O in the reactant (H20). This new equation gives

    Mg3N2 + 3H2O ---> 3MgO + NH3.

  • Continue the process of counting the number of elements on either side of the equation, balancing the number of elements using the methods in the previous step. Concluding the example used here, there are two remaining unbalanced elements: N and H. In the reactant, there are two N atoms and six H atoms; in the product, there are three H atoms and one N atom. Since there are two times as many elements in the chemicals in the product, this equation can be balanced by placing a two in front of the chemical NH3 in the product. This gives

    Mg3N2 + 3H2O ---> 3MgO +2NH3.

    The equation is now balanced.

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  • Photo Credit Kamil Macniak/iStock/Getty Images
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