[化学] Balancing Chemical Equations

You may remember that the law of conservation of mass says that matteris neither created nor destroyed during a chemical reaction. This meansthat all chemical reactions must be balanced—the number of atoms,moles, and ultimately the total mass must be conserved during achemical process. Here are the rules to follow when balancingequations:

• Determine the correct formulas for all the reactants and products in the reaction.
• Begin balancing with the most complicated-looking group. Apolyatomic ion that appears unchanged on both sides of the equation canbe counted as a single unit.
• Save the elemental (single elements) reactant and products for last, especially if it is hydrogen or oxygen. Keep your eye out for diatomic molecules such as oxygen, hydrogen, and the halogens.
• If you get stuck, double the most complicated-looking group and try again.
• Finally, make sure that all coefficients are in the lowest-possible ratio.
• Know when to quit! None of the reactions you will encounterwill be that difficult. If the coefficients are getting wild,double-check what you’ve done since you may have a simple mistake.

    When balancing reactions, keep your handsoff the subscripts! Use only coefficients to balance chemicalequations. Now let’s try an example. When you solve it yourself, makesure to follow the steps!

    Example

    Write the balanced equation for the reactionbetween chlorine and sodium bromide, which produces bromine and sodiumchloride.

    Explanation

    First write the chemical formulas—be on the lookout for the diatomic elements (such as Cl2):

Cl2 + NaBrBr2 + NaCl

    Next, find the reagent with the scariest subscripts. In this case, start with Cl2.You need a coefficient of 2 in front of NaCl, which then requires acoefficient of 2 in front of NaBr. The balanced equation becomes

Cl2 + 2NaBrBr2 + 2NaCl

    Finally, count up everything to make sureyou balanced the equation correctly. You have two chlorine atoms, twosodium atoms, and two bromines on the reactants side and two bromines,two sodiums, and two chlorines on the products side. You’re done.

    Example

    Write the balanced equation for the reactionbetween aluminum sulfate and calcium chloride, which produces aluminumchloride and calcium sulfate.

    Explanation

    Write the chemical formulas on their correct sides:

Al2(SO4)3 + CaCl2AlCl3 + CaSO4

    In this reaction, the aluminum sulfate looksthe most complicated, so start there. Look at what happens withsulfate—since it remains sulfate on the right side of the reaction,treat it as a unit. You have three on the left side and only one on theright side, so place a coefficient of 3 in front of calcium sulfate.Now deal with the aluminum. You have three on the left and one on theright, so place a coefficient of 2 in front of aluminum chloride. Last,you must place a coefficient of 3 in front of calcium chloride.

Al2(SO4)3 + 3CaCl22AlCl3 + 3CaSO4

    Count the atoms on both sides of the reaction and you’ll see that you’re done.