1 Section 8.1 Describing Chemical Change l OBJECTIVES: –Write equations describing chemical reactions, using appropriate symbols

2 Section 8.1 Describing Chemical Change l OBJECTIVES: –Write balanced chemical equations, when given the names or formulas of the reactants and products in a chemical reaction.

3 All chemical reactions l have two parts: –Reactants - the substances you start with –Products- the substances you end up with l The reactants turn into the products. Reactants  Products

4 In a chemical reaction l The way atoms are joined is changed l Atoms aren’t created of destroyed. l Can be described several ways: 1. In a sentence Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation Copper + chlorine  copper (II) chloride

5 Symbols in equations-p.206 l the arrow separates the reactants from the products l Read “reacts to form” l The plus sign = “and” l (s) after the formula = solid l (g) after the formula = gas l (l) after the formula = liquid

6 Symbols used in equations l (aq) after the formula - dissolved in water, an aqueous solution.  used after a product indicates a gas (same as (g))  used after a product indicates a solid (same as (s))

7 Symbols used in equations l indicates a reversible reaction (more later) l shows that heat is supplied to the reaction l is used to indicate a catalyst is supplied, in this case, platinum.

8 What is a catalyst? l A substance that speeds up a reaction, without being changed or used up by the reaction. l Enzymes are biological or protein catalysts.

9 Skeleton Equation l Uses formulas and symbols to describe a reaction l doesn’t indicate how many. l All chemical equations are sentences that describe reactions.

10 Now, read these: Fe(s) + O 2 (g)  Fe 2 O 3 (s) Cu(s) + AgNO 3 (aq)  Ag(s) + Cu(NO 3 ) 2 (aq) l NO 2 (g) N 2 (g) + O 2 (g)

11 Balancing Chemical Equations

12 Balanced Equation l Atoms can’t be created or destroyed l All the atoms we start with we must end up with l A balanced equation has the same number of each element on both sides of the equation.

13 C + O 2  CO 2 l This equation is already balanced l What if it isn’t? C + O O  C O O

14 C + O 2  CO l We need one more oxygen in the products. l Can’t change the formula, because it describes what it is (carbon monoxide in this example) C + O  C O O

15 l Must be used to make another CO l But where did the other C come from? C + O  C O O O C

16 l Must have started with two C 2 C + O 2  2 CO C + O  C O O O C C

17 Rules for balancing:  Assemble, write the correct formulas for all the reactants and products  Count the number of atoms of each type appearing on both sides  Balance the elements one at a time by adding coefficients (the numbers in front) - save H and O until LAST!  Check to make sure it is balanced. Note: A balanced equation will also have the same masses on both sides as well.

18 l Never change a subscript to balance an equation. –If you change the formula you are describing a different reaction. –H 2 O is a different compound than H 2 O 2 l Never put a coefficient in the middle of a formula –2 NaCl is okay, Na2Cl is not.

19 Example H 2 +H2OH2OO2O2  Make a table to keep track of where you are at

20 Example H 2 +H2OH2OO2O2  Need twice as much O in the product RP H O

21 Example H 2 +H2OH2OO2O2  Changes the O RP H O

22 Example H 2 +H2OH2OO2O2  Also changes the H RP H O

23 Example H 2 +H2OH2OO2O2  Need twice as much H in the reactant RP H O

24 Example H 2 +H2OH2OO2O2  Recount RP H O

25 Example H 2 +H2OH2OO2O2  The equation is balanced, has the same number of each kind of atom on both sides RP H O

26 Example H 2 +H2OH2OO2O2  This is the answer RP H O Not this Note: the masses on both sides of equation add up to grams.

27 Balancing Examples _P + _O 2  _P 4 O 6 _ AgNO 3 + _Cu  _Cu(NO 3 ) 2 + _Ag _KClO 3  _O 2 + KCl _Mg + _N 2  _Mg 3 N 2 _C 2 H 4 + _O 2  _CO 2 + _H 2 O _FeCl 3 + _Ca(OH) 2  _Fe(OH) 3 +_CaCl 2

28 Answers to Balancing Examples l 4, 3  1 l 2, 1  1, 2 l 2  3, 2 l 3, 1  1 l 1, 3  2, 2 l 2, 3  2, 3

29 Section 8.2 Types of Chemical Reactions l OBJECTIVES: –Identify a reaction as combination, decomposition, single-replacement, double- replacement, or combustion

30 Section 8.2 Types of Chemical Reactions l OBJECTIVES: –Predict the products of combination, decomposition, single-replacement, double- replacement, and combustion reactions.

31 Types of Reactions l There are millions of reactions. l Can’t remember them all l Fall into several categories. l We will learn 5 major types. l Will be able to predict the products. l For some, we will be able to predict whether they will happen at all. l Will recognize them by the reactants

32 #1 - Synthesis Reactions

33 #1 - Synthesis Reactions l Combine - put together l 2 substances combine to make one compound. Ca +O 2  CaO SO 3 + H 2 O  H 2 SO 4

34 #2 - Decomposition Reactions

35 #2 - Decomposition Reactions l decompose = fall apart l one reactant falls apart into two or more elements or compounds. l NaCl Na + Cl 2 l CaCO 3 CaO + CO 2 l Note that energy is usually required to decompose

36 #3 - Single Displacement

37 #3 - Single Displacement l One element replaces another l Reactants must be an element and a compound. l Products will be a different element and a different compound. Na + KCl  K + NaCl F 2 + LiCl  LiF + Cl 2

38 #4 - Double Displacement

39 #4 - Double Displacement l Two things replace each other. l Reactants must be two ionic compounds or acids. l Usually in aqueous solution NaOH + FeCl 3  l The positive ions change place. NaOH + FeCl 3  Fe +3 OH - + Na +1 Cl -1 NaOH + FeCl 3  Fe(OH) 3 + NaCl

40 #4 - Double Displacement l Has certain “driving forces” –Will only happen if one of the products: –doesn’t dissolve in water and forms a solid (a “precipitate”), or –is a gas that bubbles out, or –is a covalent compound (usually water).

41 How to recognize which type l Look at the reactants: E + E =Combination C =Decomposition E + C =Single replacement C + C =Double replacement

42 #5 - Combustion l Means “add oxygen” l A compound composed of only C, H, and maybe O is reacted with oxygen l If the combustion is complete, the products will be CO 2 and H 2 O. l If the combustion is incomplete, the products will be CO (possibly just C) and H 2 O.

43 Examples C 4 H 10 + O 2  (assume complete) C 4 H 10 + O 2  (incomplete) C 6 H 12 O 6 + O 2  (complete) C 8 H 8 +O 2  (incomplete)

44 An equation... l Describes a reaction l Must be balanced in order to follow the Law of Conservation of Mass l Can only be balanced by changing the coefficients. l Has special symbols to indicate physical state, and if a catalyst or energy is required.

45 Reactions l Come in 5 major types. l Can tell what type they are by the reactants. l Single Replacement happens based on the activity series l Double Replacement happens if the product is a solid, water, or a gas.