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美国高考SATII化学专题分析——Balancing Chemical Equations

来源:51SAT 2010-04-16   美国留学   免费评估   答疑
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SAT是Scholastic Aptitude Test的缩写,是申请几乎所有美国大学必须参加的考试。通常,希望继续接受高等教育的高中生需要参加SAT考试,并且SAT考试得分是获取奖学金的重要标准之一。大部分美国大学要求SAT考试作为录取的条件并根据SAT得分授予奖学金。

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

  1. Determine the correct formulas for all the reactants and products in the reaction.
  2. Begin balancing with the most complicated-looking group. A polyatomic ion that appears unchanged on both sides of the equation can be counted as a single unit.
  3. 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.
  4. If you get stuck, double the most complicated-looking group and try again.
  5. Finally, make sure that all coefficients are in the lowest-possible ratio.
  6. Know when to quit! None of the reactions you will encounter will 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 hands off the subscripts! Use only coefficients to balance chemical equations. Now let’s try an example. When you solve it yourself, make sure to follow the steps!
     Example
     Write the balanced equation for the reaction between chlorine and sodium bromide, which produces bromine and sodium chloride.
     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 a coefficient of 2 in front of NaBr. The balanced equation becomes
 
Cl2 + 2NaBrBr2 + 2NaCl
 
    Finally, count up everything to make sure you balanced the equation correctly. You have two chlorine atoms, two sodium 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 reaction between aluminum sulfate and calcium chloride, which produces aluminum chloride and calcium sulfate.
     Explanation
     Write the chemical formulas on their correct sides:
Al2(SO4)3 + CaCl2AlCl3 + CaSO4
    In this reaction, the aluminum sulfate looks the most complicated, so start there. Look at what happens with sulfate—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 the right 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 the right, 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.
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