1 Chapter 5 “Chemical Reactions” CP Chemistry

2 Describing Chemical Reactions l OBJECTIVES: –Describe how to write a word equation.

3 Describing Chemical Reactions l OBJECTIVES: –Describe how to write a skeleton equation.

4 Section 11.1 Describing Chemical Reactions l OBJECTIVES: –Describe the steps for writing a balanced chemical equation.

5 All chemical reactions… l have two parts: 1.Reactants = the substances you start with 2.Products = the substances you end up with l The reactants will turn into the products. Reactants  Products

6 - Page 321 Reactants Products

7 In a chemical reaction l Atoms aren’t created or destroyed (according to the Law of Conservation of Mass) 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

8 Symbols in equations l the arrow (→) separates the reactants from the products (arrow points to products) –Read as: “reacts to form” or yields l The plus sign = “and” l (s) after the formula = solid: Fe (s) l (g) after the formula = gas: CO 2(g) l (l) after the formula = liquid: H 2 O (l)

9 Symbols used in equations l (aq) after the formula = dissolved in water, an aqueous solution: NaCl (aq) is a salt water solution  used after a product indicates a gas has been produced: H 2 ↑  used after a product indicates a solid has been produced: PbI 2 ↓

10 Symbols used in equations ■ double arrow indicates a reversible reaction (more later) ■ shows that heat is supplied to the reaction ■ is used to indicate a catalyst is supplied (in this case, platinum is the catalyst)

11 Summary of Symbols

12 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.

13 #3. The Skeleton Equation l Uses formulas and symbols to describe a reaction –but doesn’t indicate how many; this means they are NOT balanced l All chemical equations are a description of the reaction.

14 Write a skeleton equation for: 1. Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. 2. Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

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

16 #4. Balanced Chemical Equations l Atoms can’t be created or destroyed in an ordinary reaction: –All the atoms we start with we must end up with (meaning: balanced!) l A balanced equation has the same number of each element on both sides of the equation.

17 Rules for balancing: 1)Assemble the correct formulas for all the reactants and products, using “+” and “→” 2)Count the number of atoms of each type appearing on both sides 3)Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last) 4)Double-Check to make sure it is balanced.

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 Practice Balancing Examples _AgNO 3 + _Cu  _Cu(NO 3 ) 2 + _Ag _Mg + _N 2  _Mg 3 N 2 _P + _O 2  _P 4 O 10 _Na + _H 2 O  _H 2 + _NaOH _CH 4 + _O 2  _CO 2 + _H 2 O

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21 Types of Chemical Reactions l OBJECTIVES: –Identify the five general types of reactions.

22 Indicators of chemical reactions Formation of a gas Emission of light or heat Formation of a precipitate Color change Emission of odor

23 Types of Reactions l There are probably millions of reactions. l We can’t remember them all, but luckily they will fall into several categories. l We will learn: a) the 5 major types

24 #1 – Synthesis (Combination) Reactions l 2 substances combine to make one compound Ca + O 2  CaO SO 3 + H 2 O  H 2 SO 4

25 #2 - Decomposition Reactions l decompose = fall apart l one reactant breaks apart into two or more elements or compounds. l NaCl Na + Cl 2 l CaCO 3 CaO + CO 2 l Note that energy (heat, sunlight, electricity, etc.) is usually required

26 #2 - 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

27 #3 - Single Replacement Reactions 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 (Cations switched) (Anions switched)

28 #4 - Double Replacement Reactions l Two things replace each other. –Reactants must be two ionic compounds, in aqueous solution NaOH + FeCl 3  –The positive ions change place. NaOH + FeCl 3  Fe +3 OH - + Na +1 Cl -1 = NaOH + FeCl 3  Fe(OH) 3 + NaCl

29 #4 - 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).

30 Complete and balance: l assume all of the following reactions actually take place: CaCl 2 + NaOH  CuCl 2 + K 2 S  KOH + Fe(NO 3 ) 3  (NH 4 ) 2 SO 4 + BaF 2 

31 #5 – Combustion Reactions l Combustion means “add oxygen” l Hydrocarbon + O 2  CO 2 + H 2 O l If the combustion is complete, the products will be CO 2 and H 2 O.

32 Combustion Reaction Examples: C 4 H 10 + O 2  C 6 H 12 O 6 + O 2  C 8 H 8 + O 2 

33 SUMMARY: 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 the physical state, if a catalyst or energy is required, etc.

34 Reactions l Come in 5 major types. l Synthesis (Combination) l Decomposition l Single Replacement l Double Replacement l Combustion