1 Chapter 11 “Chemical Reactions” Mater Lakes Academy Chemistry Mrs.Nunez

2 Section 11.1 Describing Chemical Reactions l OBJECTIVES: –Describe how to write a word equation. –Describe how to write a skeleton equation. –Describe the steps for writing a balanced chemical equation.

3 All chemical reactions… l have two parts: 1.Reactants = the substances you start with 2.Products = the substances you end up with A+B C+D REACTANTS PRODUCTS

4 - Page 321 Reactants Products

5 Signs of a Chemical Reaction l Evolution of heat and light l Formation of a gas l Formation of a precipitate l Color change

6 Law of Conservation of Mass l Mass is neither created nor destroyed in a chemical reaction. l Total mass stays the same. l Atoms can only rearrange.

7 In a chemical reaction l A reaction can be described several ways: #1. In a sentence (every item is a word) Copper reacts with chlorine to form copper (II) chloride. #2. In a word equation (some symbols used) Copper + chlorine  copper (II) chloride #3.In a Skeleton Equation (Uses formulas and symbols) Fe (s) + O 2(g)  Fe 2 O 3(s

8 C. Johannesson Chemical Equations

9 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 in your body.

10 Writing Equations l Identify the substances involved. l Use symbols to show: 2H 2 ( g ) + O 2 ( g )  2H 2 O( g )  How many? - coefficient  Of what? - chemical formula  In what state? - physical state n Remember the diatomic elements.

11 Writing Equations Two atoms of aluminum react with three units of aqueous copper(II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride. How many? Of what? In what state? Al 2 (s)(s) + 3 CuCl 2 ( aq )  3 Cu (s)(s) + 2 AlCl 3 ( aq )

12 Describing Equations l Describing Coefficients: –individual atom = “atom” –covalent substance = “molecule” –ionic substance = “unit” 3 molecules of carbon dioxide 2 atoms of magnesium 4 units of magnesium oxide 3CO 2  2Mg  4MgO 

13 Describing Equations to produce How many? Of what? In what state? Zn( s ) + 2HCl( aq )  ZnCl 2 ( aq ) + H 2 ( g ) One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid one unit of aqueous zinc chloride and one molecule of hydrogen gas.

14 Write a skeleton equation for: Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. Read the following equation: Cu (s) + AgNO 3(aq)  Ag (s) + Cu(NO 3 ) 2(aq)

15 Balanced Chemical Equations l A balanced equation has the same number of each element on both sides of the equation. l They are balanced!!!

16 A. Balancing Steps 1.Write the unbalanced equation. 2.Count atoms on each side. 3.Add coefficients to make #s equal. Coefficient  subscript = # of atoms 4.Reduce coefficients to lowest possible ratio, if necessary. 5.Double check atom balance!!!

17 Helpful Tips l Balance one element at a time. l Update ALL atom counts after adding a coefficient. l If an element appears more than once per side, balance it last. (H and O) l Balance polyatomic ions as single units. “1 SO 4 ” instead of “1 S” and “4 O”

18 l Never change a subscript to balance an equation (You can only change coefficients) –If you change the subscript (formula) you are describing a different chemical. –H 2 O is a different compound than H 2 O 2 l Never put a coefficient in the middle of a formula; they must go only in the front 2 NaCl is okay, but Na 2 Cl is not.

19 C. Johannesson Al + CuCl 2  Cu + AlCl 3 Al Cu Cl  3  6   Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2  2  6

20 Section 11.2 Types of Chemical Reactions l OBJECTIVES: –Describe the five general types of reactions. –Predict the products of the five general types of reactions.

21 Synthesis (Combination) l the combination of 2 or more substances to form a compound l only one product A + B  AB

22 Combination Reactions l Combine = put together Ca + O 2  CaO SO 3 + H 2 O  H 2 SO 4 l We can predict the products, especially if the reactants are two elements. Mg + N 2  Mg 3 N 2 (symbols, charges, cross)

23 Combination Reactions Important Notes:  Some nonmetal oxides react with water to produce an acid: SO 2 + H 2 O H 2 SO 3  Some metallic oxides react with water to produce a base: CaO + H 2 O Ca(OH) 2

24 C. Johannesson Decomposition l a compound breaks down into 2 or more simpler substances l only one reactant AB  A + B

25 Decomposition Reactions l decompose = fall apart l NaCl Na + Cl 2 l CaCO 3 CaO + CO 2 l Note that energy (heat, sunlight, electricity, etc.) is usually required

26 Decomposition Reactions l We can predict the products if it is a binary compound –It breaks apart into the elements: lH2OlH2O l HgO

27 Decomposition Reactions l If the compound has more than two elements you must be given one of the products –The other product will be from the missing pieces l NiCO 3 CO 2 + ___ H 2 CO 3 (aq)  CO 2 + ___ heat

28 C. Johannesson Single Replacement l one element replaces another in a compound –metal replaces metal (+) –nonmetal replaces nonmetal (-) A + BC  B + AC

29 C. Johannesson Fe(s)+ CuSO 4 (aq)  Cu(s)+ FeSO 4 (aq) C. Single Replacement l Products: –metal  metal (+), or H –nonmetal  nonmetal (-) –free element must be more active (check activity series) Br 2 (l)+ NaCl(aq)  N.R.

30 Single Replacement Reactions l We can even tell whether or not a single replacement reaction will happen: –Because some chemicals are more “active” than others –More active replaces less active l There is a list on page called the Activity Series of Metals l Higher on the list replaces those lower.

31 The “Activity Series” of Metals Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead Hydrogen Bismuth Copper Mercury Silver Platinum Gold 1)Metals can replace other metals, provided they are above the metal they are trying to replace (for example, zinc will replace lead) 2)Metals above hydrogen can replace hydrogen in acids. 3)Metals from sodium upward can replace hydrogen in water. Higher activity Lower activity

32 The “Activity Series” of Halogens Fluorine Chlorine Bromine Iodine Halogens can replace other halogens in compounds, provided they are above the halogen they are trying to replace. 2NaCl (s) + F 2(g)  2NaF (s) + Cl 2(g) MgCl 2(s) + Br 2(g)  ??? No Reaction! ??? Higher Activity Lower Activity

33 C. Johannesson AB + CD  AD + CB Double Replacement l ions in two compounds “change partners” l Must be ionic compounds in aqueous solution l cation of one compound combines with anion of the other

34 Pb(NO 3 ) 2 (aq)+ KI(aq)  PbI 2 (s)+ KNO 3 (aq) E. Double Replacement l Products: –switch negative ions –one product must be insoluble (check solubility table) NaNO 3 (aq)+ KI(aq)  N.R. 2 2

35 Double Replacement Reactions l Have certain “driving forces”, or reasons –Will only happen if one of the products: a) doesn’t dissolve in water and forms a solid (a “precipitate”), or b) is a gas that bubbles out, or c) is a molecular compound (which will usually be water).

36 C. Johannesson Combustion CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) l Combustion means “add oxygen” l the burning of any substance in O 2 to produce heat A + O 2  CO 2 + H 2 O

37 Combustion Reactions 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 (or possibly just C) and H 2 O.

38 Section 11.3 Reactions in Aqueous Solution l OBJECTIVES: –Describe the information found in a net ionic equation. –Predict the formation of a precipitate in a double replacement reaction

39 Net Ionic Equations l Many reactions occur in water- that is, in aqueous solution l When dissolved in water, many ionic compounds “dissociate”, or separate, into cations and anions l Now we are ready to write an ionic equation

40 Net Ionic Equations l Example (needs to be a double replacement reaction) AgNO 3 + NaCl  AgCl + NaNO 3 1. this is the full balanced equation 2. next, write it as an ionic equation by splitting the compounds into their ions: Ag 1+ + NO Na 1+ + Cl 1-  AgCl + Na 1+ + NO 3 1- Note that the AgCl did not ionize, because it is a “precipitate”

41 Net Ionic Equations 3. simplify by crossing out ions not directly involved (called spectator ions) Ag 1+ + Cl 1-  AgCl This is called the net ionic equation Let’s talk about precipitates before we do some other examples

42 Predicting the Precipitate l Insoluble salt = a precipitate [note Figure 11.11, p.342 (AgCl)] l General solubility rules are found: a)Table 11.3, p. 372 in textbook

43 Let’s do some examples together of net ionic equations, starting with these reactants: BaCl 2 + AgNO 3 → NaCl + Ba(NO 3 ) 2 →