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Chemical equations give information in two major areas.

Chemical equations give information in two major areas. First, they tell us what substances are reacting (those being used up) and what substances are products (those being made). Second, the coefficients of a balanced equation tell us in what ratio the substances react or are produced.

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Chemical equations give information in two major areas.

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  1. Chemical equations give information in two major areas. First, they tell us what substances are reacting (those being used up) and what substances are products (those being made). Second, the coefficients of a balanced equation tell us in what ratio the substances react or are produced. This last point has practical consequences whenever chemicals react. For example, the large middle tank of the space shuttle actually has two smaller tanks in it - one holding liquid oxygen and the other holding liquid hydrogen. The tank with the hydrogen holds twice as much as the oxygen-holding tank. Why? Answer - hydrogen and oxygen react in a 2:1 ratio. For any of oxygen used by the shuttle, twice as much hydrogen is needed. If the two tanks were equal sizes, the hydrogen tank would run dry when the oxygen tank was still half-filled.

  2. 2 H2 + O2 ---> 2 H2O Note the presence of a two in front of the hydrogen and also the water. These are called the coefficients. These numbers give two very important pieces of information about the equation. You must understand both in order to read and to use chemical equations successfully. First: the coefficients give the number of molecules (or atoms) involved in the reaction. In the example reaction, two molecules of hydrogen react with one molecule of oxygen and produce two molecules of water. Second: the coefficients give the number of moles of each substance involved in the reaction. In the example reaction, two moles of hydrogen react with one mole of oxygen and produce two moles of water. The point just made is CRITICAL. The coefficients of an equation tell us how many moles of each reactant are involved as well as how many moles of each product get produced.

  3. Quick Review There are three important things to remember about reading an equation. One: Reactants are on the left and products are on the right. Of what? The arrow. Two: Coefficients are the numbers in front of each formula. If no number is shown, a one is understood. Three: The coefficients tell us how many molecules (moles) of each reactant used and how many molecules (moles) of each product made.

  4. Identify the reactants, the products and the coefficients of these equations: 1) Zn + 2 HCl ---> ZnCl2 + H2 2) 2 KClO3 ---> 2 KCl + 3 O2 3) S8 + 24 F2 ---> 8 SF6 4) 4 Fe + 3 O2 ---> 2 Fe2O3 5) 2 C2H6 + 7 O2 ---> 4 CO2 + 6 H2O

  5. 3 Types of Chemical Reactions • Synthesis- put together • H2 + O H2O • 2) Decomposition- break apart • H2O H2 + O • 3) Replacement – • ZnCl + O ZnO + Cl

  6. Equation 1) Reactants Coefficient Products Coefficient Zn 1 ZnCl2 1 HCl 2 H2 1 Notice that a one is understood to be the coefficient of a formula with no number in front of it. Equation 2) Reactant Coefficient Products Coefficient KClO3 2 KCl 2 O2 3 Equation 3) Reactants Coefficient Product Coefficient S8 1 SF6 8 F2 24 Notice that a one is understood to be the coefficient of a formula with no number in front of it. Equation 4) Reactants Coefficient Product Coefficient Fe 4 Fe2O3 2 O2 3 Equation 5) Reactants Coefficient Products Coefficient C2H6 2 CO2 4 O2 7 H2O 6

  7. Chemical equations do not come already balanced. This must be done before the equation can be used in a chemically meaningful way. All chemical calculations to come must be done with a balanced equation. A balanced equation has equal numbers of each type of atom on each side of the equation. "Matter is neither created nor destroyed."

  8. So, now to balancing the example equation: H2 + O2 ---> H2O The hydrogen are balanced, but the oxygens are not. We have to get both balanced. We put a two in front of the water and this balances the oxygen. H2 + O2 ---> 2 H2O However, this causes the hydrogen to become unbalanced. To fix this, we place a two in front of the hydrogen on the left side. 2 H2 + O2 ---> 2 H2O This balances the equation.

  9. Two things you CANNOT do when balancing an equation. • You cannot change a subscript. • You cannot change the oxygen's subscript in water from one to two, as in: • H2 + O2 ---> H2O2 • True, this balances the equation, but you have changed the substances in it. H2O2 is a completely different substance from H2O. • 2) You cannot place a coefficient in the middle of a formula. • The coefficient goes at the beginning of a formula, not in the middle, as in: H2 + O2 ---> H22O • Water only comes as H2O and you can only use whole formula units of it.

  10. Balance this equation: H2 + Cl2 ---> HCl Remember that the rule is: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.

  11. The correctly balanced equation is: H2 + Cl2 ---> 2 HCl Placement of a two in front of the HCl balances the hydrogen and chlorine at the same time.

  12. How many oxygens are indicated: 3 Ca(NO3)2 Balance these equations: Zn + HCl ---> ZnCl2 + H2 KClO3 ---> KCl + O2 S8 + F2 ---> SF6 Fe + O2 ---> Fe2O3 C2H6 + O2 ---> CO2 + H2O

  13. Equation 1) Zn + HCl ---> ZnCl2 + H2 Upon examining this equation, you see that there is already one Zn on each side of the equation. We will attempt to leave it alone, if at all possible, since it is already balanced. One the right side, we see two chlorines and two hydrogens, with only one of each on the left. Putting a two in front of the HCl doubles the number of chlorine and hydrogen on the left side. This now leaves us with two chlorines and two hydrogens on each side of the arrow, making them both balanced. Since the zinc was already balanced, the entire equation is now balanced.

  14. Equation 2) KClO3 ---> KCl + O2 Start by noticing the the K and the Cl are ALREADY balanced in the skeleton equation. However, the oxygen is out of balance with three on the left and two on the right. It is important to emphasize that the oxygen on the left will increase only in steps of three, while the oxygen on the right will increase only in steps of two. The question to ask yourself is "What is the least common multiple between 2 and 3?" The answer of course is six. We need six oxygens on each side of the equation. We use a two on the left side since 2 x 3 = 6 and we use a three on the right side since 3 x 2 = 6. This causes the K and the Cl to become unbalanced, but putting a two in front of the KCl on the right side fixes that. This problem is interesting because you focused on the oxygens first. Normally, oxygen is the last (or next-to-last) element to be balanced

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