Chemical Reactions. Burlingame High School Chemistry 1

Transcription

1 Chemical Reactions 1

2 Chemical Equations Chemical equations are use to describe: the substances that react, the reactants the substances that are formed, the products the relative amounts of all substances involved in the reaction 2

3 Chemical Equations A chemical equation is a shorthand notation to describe a chemical reaction Just like a chemical formula, a chemical equation expresses quantitative relations 3

4 A Reaction between Mg and O 2 2Mg + O 2 magnesium and oxygen 4

5 A Reaction between Mg and O 2 2Mg + O 2 magnesium and oxygen react to form 5

6 A Reaction between Mg and O 2 2Mg + O 2 2MgO magnesium react to magnesium and oxygen form oxide 6

7 Quantitative Relationships in Equations 2Mg + O 2 2MgO 2 moles Mg + 1 mol O 2 2 moles MgO Moles are not conserved in a chemical reaction. 4 Cu(s) + O 2 (g) 2 Cu 2 O (s) The number and type of atoms are conserved. 7

8 Reactants and Products Reactants are the substances consumed Products are the substances formed Reactants Products 2Mg + O 2 2MgO 4Cu(s) + O 2 (g) 2 Cu 2 O (s) 8

9 What is a Coefficient? Coefficients are numbers before the formula of a substance in an equation 2 Mg + O 2 2 MgO 4 Cu(s) + O 2 (g) 2 Cu 2 O (s) A balanced equation has the same number of atoms of each element on both sides of the equation Subscripts in formulas must not be changed. 9

10 Law of Conservation of Matter In a chemical reaction, matter is neither created nor destroyed. All the atoms must be accounted for. A balanced equation must have the same number of each kind of atoms on both sides of an equation. 10

11 Balancing Equations All substances must be written with chemical formulas that describe them as they exist. The state of the substance is written in ( ). (s)= solid (l) = liquid (g) = gas 11

12 Balancing Equations The number and type of atoms must be the same on the reactant and the product side of the equation. 12

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14 Practice Balancing Equations Fe(NO 3 ) 3 + LiOH LiNO 3 + Fe(OH) 3 14

15 Set up Atom Inventory Fe(NO 3 ) 3 + LiOH LiNO 3 + Fe(OH) 3 Fe Reactants Fe Products NO 3 Li OH NO 3 Li OH 15

16 Practice Balancing Equations Fe(NO 3 ) 3 + LiOH LiNO 3 + Fe(OH) 3 Reactants Fe 1 NO 3 3 Li 1 OH 1 Products Fe 1 NO 3 1 Li 1 OH 3 Not Balanced 16

17 Balance the Equation Fe(NO 3 ) 3 + 3LiOH 3LiNO 3 + Fe(OH) 3 Reactants Fe 1 NO 3 3 Li 3 OH 3 Products Fe 1 NO 3 3 Li 3 OH 3 Now it is balanced! 17

18 Balance the Equation CH 3 OH + O 2 CO 2 + H 2 O 18

19 Set up an Atom Inventory CH 3 OH + O 2 CO 2 + H 2 O Reactants Products C H O C H O 19

20 Is it Balanced? CH 3 OH + O 2 CO 2 + H 2 O Reactants C 1 H 4 O 3 Products C 2 H 2 O 3 Not Balanced 20

21 Balance the Equation 2CH 3 OH + 3O 2 2CO 2 + 4H 2 O Reactants C 2 H 8 O 8 Products C 2 H 8 O 8 Now it is balanced! 21

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23 Balance these Equations Fe + HCl H 2 + FeCl 3 Sn(OH) 4 SnO 2 + H 2 O MgCO 3 MgO + CO 2 23

24 Check your answers 2Fe + 6HCl 3H 2 + 2FeCl 3 Sn(OH) 4 SnO 2 + 2H 2 O MgCO 3 MgO + CO 2 This was already balanced. 24

25 Writing Balanced Equations Write the correct formula for each substance H 2 + Cl 2 HCl Add coefficients so the number of atoms of each element are the same on both sides of the equation H 2 + Cl 2 2HCl H 2 atoms 2 atoms Cl 2 atoms 2 atoms 25

26 Balancing Chemical Equations Assume one molecule of the most complicated substance C 5 H 12 + O 2 CO 2 + H 2 O Adjust the coefficient of CO 2 to balance C C 5 H 12 + O 2 5CO 2 + H 2 O Adjust the coefficient of H 2 O to balance H C 5 H 12 + O 2 5CO 2 + 6H 2 O Adjust the coefficient of O 2 to balance O C 5 H O 2 5CO 2 + 6H 2 O 26

27 Test your skill Balance the equation C 4 H 9 OH + O 2 CO 2 + H 2 O Answer: C 4 H 9 OH + 6O 2 4CO 2 + 5H 2 O 27

28 Balancing Equations C 5 H 10 + O 2 CO 2 + H 2 O C 5 H 10 + O 2 5CO 2 + H 2 O C 5 H 10 + O 2 5CO 2 + 5H 2 O C 5 H /2O 2 5CO 2 + 5H 2 O Sometimes fractional coefficients are obtained. 28

29 Balancing Equations C 5 H /2O 2 5CO 2 + 5H 2 O Multiply all coefficients by the denominator 2C 5 H O 2 10CO H 2 O 29

30 Combination (Synthesis) In this type of reaction, two elements or compounds form into a single compound. One type of synthesis reaction is when an element and oxygen combine to produce an oxide of the element. For example: 2Ba + O 2 2BaO When a metal reacts with a nonmetal, the result is often a salt. For example, 2Na + Cl 2 2NaCl When the oxides of metals react with water, a metal hydroxide is produced. An example of this is: CuO + H 2 O Cu(OH) 2 30

31 Examples of Synthesis Rxns The copper in the Statue of Liberty reacts with oxygen in the air to form the green copper oxide. 2Cu + O 2 2CuO Iron metal reacts with oxygen to form iron III oxide (rust). 4Fe + 3O 2 2Fe 2 O 3 31

32 Decompostion In a decomposition reaction, an element or compound is broken down into two simpler substances. The simplest kind of decomposition reaction is where a compound is broken down into its elements. An example of this is: 2HgO 2Hg + O 2 When some acids are decomposed, water and an nonmetallic oxide are formed. An example of this is sulfuric acid. The equation for this is: H 2 SO 4 SO 3 + H 2 O When a metal carbonate is heated, it breaks down into carbon dioxide and a metal oxide, as shown in this equation: CaCO 3 CaO + CO 2 32

33 An Example of a Decomposition Reaction Carbonic acid decomposes to form water and carbon dioxide. H 2 CO 3 H 2 O + CO 2 33

34 Combustion A substance combines with oxygen releasing a large amount of energy in the form of light and heat. An example of a combustion reaction is: C 3 H 8 + 5O 2 3CO 2 + 4H 2 O 34

35 Balance these combustion Rxns CH 4 + O 2 CO 2 + H 2 O C 2 H 6 + O 2 CO 2 + H 2 O C 5 H 12 + O 2 35

36 Balance these combustion Rxns CH 4 + O 2 CO 2 + H 2 O 2C 2 H 6 + 7O 2 4CO 2 + 6H 2 O C 5 H O 2 5CO 2 + 6H 2 O 36

37 Single Replacement In this type of reaction, one element replaces a similar element in a compound. One type of single-replacement reaction is when a metal in a compound is replaced by a more reactive metal. An example of this is: 2Al (s) + 3Fe(NO 3 ) 2 (aq) 3Fe (s) + 2Al(NO 3 ) 3 (aq) Another type of single-replacement reaction is the replacement of halogens. An example of this type of reaction is: Cl 2 (g)+ 2KBr 2KCl + Br 2 (g) 37

38 Single Replacement Rxn To determine if a single replacement reaction will take place, you must use a reactivity series. Metals can be arranged in order of how reactive they are. Metals high in the reactivity series can displace metals lower down. 38

39 A Single Replacement Reaction Copper wire reacts with silver nitrate solution to form silver and copper nitrate solution Cu + 2AgNO3 Cu(NO3)2 + 2Ag 39

40 Single Replacement Reactions Use the reactivity of halogens to determine if the following reactions would occur. F 2 Cl 2 Br 2 I 2 I 2 (g) + 2NaBr F 2 (g) + 2NaI No Reaction 2NaF + I 2 40

41 Double Replacement In a double-replacement reaction, the two compounds exchange places in a solution to form two new compounds. For a double replacement reaction to occur, one of the products must be: 1. A solid precipitate 2. A gas 3. water 41

42 Double Replacement Reactions One of the types of double-replacement reactions is when a precipitate is formed. An example of this type of reaction is: Pb(NO 3 ) 2 (aq) + 2KI (aq) PbI 2 (s)+ 2KNO 3 (aq) When one of the products is a gas, doublereplacement reactions also occur. For example: FeS (aq)+ 2HCl (aq) H 2 S (g)+ FeCl 2 (aq) An example of a double-replacement reaction where water is one of the products is: HCl (aq) + NaOH (aq) NaCl (aq)+ H 2 O(l) 42

43 When two ionic compounds combine, the ions may recombine to form a precipitate. In this example, Pb2+ and I- combine to form insoluble PbI2Ña precipitate. 43

44 Double Replacement Reactions When two ionic compounds combine, the ions may recombine to form a precipitate. In this example, Pb 2+ and I - combine to form insoluble PbI 2 precipitate. 44

45 Double Replacement Reactions Will a precipitate form when aqueous solutions of Ca(NO 3 ) 2 and NaCl are mixed? Ca(NO 3 ) 2 + NaCl CaCl 2 + NaNO 3 CaCl 2 is soluble NaNO 3 is soluble No precipitate forms 45

46 Double Replacement Reactions Will a precipitate form when aqueous solutions of CaCl 2 and K 3 PO 4 are mixed? CaCl 2 + K 3 PO 4 Ca 3 (PO 4 ) 2 + KCl KCl is soluble Ca 3 (PO 4 ) 2 is insoluble 3Ca PO 4 3- Ca 3 (PO 4 ) 2 Precipitate forms 46

47 Predicting Products To predict the product you must recognize the type of reaction. Combination(synthesis) 2 or more elements combine Decomposition Single reactant Single Replacement An element replaces another element from a compound in an aqueous solution. Double Replacement Exchanging cations Usually occurs in aqueous solution to produce a ppt, gaseous product or water. Combustion reacting with O 2 to produce CO 2 and H 2 O. 47

48 Types of Reactions Type of reaction Double Replacement Single Replacement Decomposition Combination Combustion General Equation AB + XY AY + BX A + XY AY + X A + XY XA + Y XY X + Y X + Y XY C x H Y + O 2 CO 2 + H 2 O 48

49 Types of Reactions Combination 2 or more elements combine Decomposition single reactant Single Replacement An element replaces another element from a compound in an aqueous solution. Double Replacement Usually occur in aqueous solution to produce a ppt, a gas or water. Combustion Usually O 2 reacting with a hydrocarbon to produce CO 2 and H2 O 49

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53 Ionic Equations Molecular equation (Normal balanced Equation) 2AgNO 3 (aq) + Cu(s) 2Ag(s) + Cu(NO 3 ) 2 (aq) Total ionic equation (ionic compounds as individual ions) 2Ag + (aq) + 2NO 3 - (aq) + Cu(s) 2Ag(s) + Cu 2+ (aq) + 2NO 3- (aq) Net ionic equation (Remove all spectator ions) 2Ag + (aq) + Cu(s) 2Ag(s) + Cu 2+ (aq) 53