2. Chemical Equations & Chemical Reaction Types

3. 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 will react or are produced. Thirdly they also will represent the number of moles of each substance reacting or produced. The Meaning of a Chemical Equation

4. Equation Breakdown The reactants are on the left side of a chemical equation and the products are on the right side. However, you might ask, "On the left and right side of what?" Answer - the arrow. 2 H 2 + O 2 ---> 2 H 2 O On the left side are the reactants - hydrogen and oxygen. We will ignore the two in front of the hydrogen for a moment. On the right side is the product - water. We will also ignore the two in front of the water, but we will soon return to it.

5. Please be aware that there can be one, two, three, or more substances on either side of the arrow, as in this more complex equation: Ca(H 2 PO 4 ) 2 + CaSO 4 + HF ---> Ca 10 F 2 (PO 4 ) 6 + H 2 SO 4 Typically the arrow is replaced with "produces" or "yields" when the equation is said out loud. Coefficients are the numbers in front of the formulas. 2 H 2 + O 2 ---> 2 H 2 O 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.

6. First: the coefficients give the number of molecules (or atoms) involved in the reaction. Second: the coefficients give the number of moles of each substance involved in the reaction. 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. What is the relationship between molecules and moles? Yes!, Avogadro's Number is the factor between the two points above.

7. 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? 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 or moles of each reactant used and how many molecules or moles of each product made.

8. Practice Problems Identify the reactants, the products and the coefficients of these equations: 1) Zn + 2 HCl ---> ZnCl 2 + H 2 2) 2 KClO 3 ---> 2 KCl + 3 O 2 3) S 8 + 24 F 2 ---> 8 SF 6 4) 4 Fe + 3 O 2 ---> 2 Fe 2 O 3 5) 2 C 2 H 6 + 7 O 2 ---> 4 CO 2 + 6 H 2 O

9. Balancing Chemical Equations 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. Why? The Law of Conservation of Mass, Matter & Energy “Mass, Matter & Energy are neither created nor destroyed." Therefore, we must finish our chemical reaction with as many atoms of each element as when we started.

10. H 2 + O 2 ---> H 2 O In the example equation, there are two atoms of hydrogen on each side, BUT there are two atoms of oxygen on the left side and only one on the right side. Remember this: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow. An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. It is important to note that only the coefficients can be changed, NEVER a subscript. Here is the example equation for this lesson:

11. So, now to balancing the example equation: H 2 + O 2 ---> H 2 O 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. H 2 + O 2 ---> 2 H 2 O 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 H 2 + O 2 ---> 2 H 2 O This balances the equation.

12. Balance this equation: H 2 + Cl 2 ---> HCl Remember that the rule is: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow. H 2 + Cl 2 ---> 2 HCl Balance this equation: O 2 ---> O 3 3 O 2 ---> 2 O 3 The LCM tells you how many of each atom will be needed. Least common multiple.

13. Reaction Types Single Replacement Double Replacement or Ionic Decomposition or Analysis Composition or Synthesis Combustion

14. Single Replacement During single replacement, one element replaces another element in a compound. There are two different possibilities: 1.One cation replaces another. Written using generic symbols, it is: AX + Y ---> YX + A Element Y has replaced A (in the compound AX) to form a new compound YX and the free element A. Remember that A and Y are both cations ( positively-charged ions) in this example. Examples Rule Cu + AgNO 3 ---> Ag + Cu(NO 3 ) 2 More active metal Ca + H 2 O ---> Ca(OH) 2 + H 2 Metal replaces Hydrogen in Water Zn + HCl ---> ZnCl 2 + H 2 Metal replaces Hydrogen in an Acid

15. 2. One anion replaces another. Written using generic symbols, it is: A + XY ---> XA + Y Element A has replaced Y (in the compound XY) to form a new compound XA and the free element Y. Remember that A and Y are both anions (negatively-charged ions) in these examples. Examples Rule Cl 2 + NaBr ---> NaCl + Br 2 More Active Halogen Br 2 + KI ---> KBr + I 2 More Active Halogen ZnS + O 2 ---> ZnO + S 8 More Active Non-metal

16. Reaction Types: Double Replacement During double replacement, the cations or anions of two different compounds switch places. Written using generic symbols, it is: AB + XY ---> AY + XB A and X are the cations (postively-charged ions) in this example, with B and Y being the anions (negatively- charged ions). Examples Rule KOH + H 2 SO 4 ---> K 2 SO 4 + H 2 O Water is a Product AgNO 3 + NaCl ---> AgCl + NaNO 3 A precipitate is formed Use solubility table to see if a precipitate will form

17. Double Replacement In order for a double replacement reaction to occur, one of three things must show up as a product. 1.Water 2.Precipitate 3.Gas (Non-metallic oxides, diatomic substances)

18. Reaction Types: Decomposition During decomposition, one compound splits apart into two (or more pieces). These pieces can be elements or simpler compounds. Written using generic symbols, it is usually shown as: AB ---> A + B There are 6 general decomposition type reactions that can occur, the 1 st 5 given here require heat being applied to the reactant side. 1. Metallic oxide when heated: form a metal and oxygen Mercuric oxide  Mercury and Oxygen 2. Metallic carbonates when heated : form a metallic oxide and carbon dioxide (gas) Calcium carbonate  Calcium oxide and carbon dioxide gas.

19. Decomposition cont. 3. Many metallic hydroxides when heated form metallic oxides and water vapor. Calcium hydroxide  Calcium oxide and water (gas) 4. Metallic chlorates when heated form metallic chloride and oxygen (gas). Potassium chlorate  Potassium chloride and oxygen (g) 5. Some acids when heated decompose into nonmetallic oxides and water. Carbonic Acid  Carbon dioxide and water 6. Decomposition by electrolysis (electricity) Water  Hydrogen (g) and oxygen (g) or Sodium Chloride  Sodium (s) and chlorine (g)

20. Reaction Types: Synthesis or Composition Synthesis are, at this introductory level, almost always the reverse of a decomposition reaction. That means that two pieces join together to produce one, a more complex compounds. These pieces can be elements or simpler compounds. A + B ---> AB Reaction Types: Combustion Combustion, at its most general, can mean the reaction of oxygen gas (O 2 ) with anything. However, we will understand combustion to mean the reaction of oxygen with a compound containing carbon and hydrogen. A common synonym for combustion is burn. Written using generic symbols, it is usually shown as: CxHy + O 2 ---> CO 2 + H 2 O

21. Combustion C 6 H 12 O 6 + O 2 ---> CO 2 + H 2 O CH 4 + O 2 ---> CO 2 + H 2 O Notice that some compounds contain carbon, hydrogen AND oxygen. However, the products are all the same, in every reaction. We could vary it a bit by adding nitrogen (burns to form NO 2 ) to the compound formula or sulfur (burns to form SO 2 or SO 3 ). Like these: C 21 H 24 N 2 O 4 + O 2 ---> CO 2 + H 2 O + NO 2 C 2 H 5 SH + O 2 ---> CO 2 + H 2 O + SO 2

22. Phase Notation & Other forms of notation (g) gas (ppt) precipitate or (s) solid (l) liquid (s) solid (aq) aqueous – dissolved in water or solution of MnO 2 shows as a catalyst  heat is applied