Chapter 7. Chemical Reactions

Transcription

1 Chapter 7 Chemical Reactions 1

2 All chemical reactions have two parts Reactants - the substances you start with Products- the substances you end up with The reactants turn into the products. Reactants Products

3 In a chemical reaction The way atoms are joined is changed Atoms aren t created or destroyed What goes into the kettle must come out of the kettle. 3

4 Chemical Reactions can be described several ways In a sentence Copper reacts with chlorine to form copper (II) chloride. In a word equation Copper + chlorine copper (II) chloride 4

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

6 Symbols used in equations (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)) 6

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

8 What is a catalyst? A substance that speeds up a reaction without being changed by the reaction. Enzymes are biological or protein catalysts. Amylase in your saliva catalyzes starch hydrolysis, and H O decomposition. 8

9 Skeleton Equation Uses formulas and symbols to describe a reaction Doesn t indicate how many. All chemical equations are chemical sentences that describe reactions. 9

10 10 Convert these to equations Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (II) chloride and hydrogen sulfide gas. Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

11 How did you do? Fe S 3(s) + HCl (g) FeCl + H S(g) HNO 3(aq) +Na CO 3 (s) H O (l) +CO + NaNO 3 11

12 Try the other way Fe(g) + O (g) Fe O 3 (s) Cu(s) + AgNO 3 (aq) Ag(s) + Cu(NO 3 ) (aq) Pt NO N (g) + O (g) 1

13 13 Balancing Chemical Equations

14 Balanced Equation Atoms can t be created or destroyed All the atoms we start with we must end up with A balanced equation has the same number of each element on both sides of the equation. 14

15 C + O O C O O C + O CO This equation is already balanced What if it isn t already? 15

16 C + O C O O C + O CO We need one more oxygen in the products. Can t change the formula, because it describes what is 16

17 C + O O C C O O Must be used to make another CO But where did the other C come from? 17

18 C C + O O C C O O Must have started with two C C + O CO 18

19 Rules for balancing 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) Check to make sure it is balanced. 19

20 0 Never Change a subscript to balance an equation. If you change the formula you are describing a different reaction. H O is a different compound than H O Never put a coefficient in the middle of a formula NaCl is okay, NaCl is not.

21 Example H + O H O Make a table to keep track of where you are at 1

22 Example H + O H O R H O P 1 Need twice as much O in the product

23 Example H + O H O R H O P 1 Changes the O 3

24 Example H + O H O R P H O 1 Also changes the H 4

25 Example H + O H O R P H 4 O 1 Need twice as much H in the reactant 5

26 Example H + O H O R P H 4 O 1 Recount 6

27 Example H + O H O R P 4 H 4 O 1 The equation is balanced, has the same number of each kind of atom on both sides 7

28 Example H + O H O R P 4 H 4 O 1 Not this This is the answer 8

29 Examples AgNO 3 + Cu Cu(NO 3 ) + Ag AgNO 3 + Cu Cu(NO 3 ) + Ag Mg + N Mg 3 N 3Mg + N Mg 3 N 9

30 P + O P 4 O 10 More Examples 4P + 5O P 4 O 10 Na + H O H + NaOH Na + HOH H + NaOH CH 4 + O CO + H O CH 4 + O CO + H O 30

31 31 Types of Reactions

32 3 Types of Reactions There are millions of reactions. Can t remember them all Fall into several categories. We will learn 5 types. Will be able to predict the products. For some we will be able to predict whether they will happen at all. Will recognize them by the reactants

33 #1 Combination Reactions Combine - put together elements, or compounds combine to make one compound. Ca +O CaO 33 SO 3 + H O H SO 4 We can predict the products if they are two elements. Mg + N

34 Ca + Cl Write and balance Fe + O iron (II) oxide Al + O Remember that the first step is to write the formula Then balance 34

35 # Decomposition Reactions decompose = fall apart one reactant falls apart into two or more elements or compounds. electricity NaCl Na + Cl CaCO 3 CaO + CO 35

36 # Decomposition Reactions Can predict the products if it is a binary compound Made up of only two elements Falls apart into its elements electricity H O HgO 36

37 # Decomposition Reactions 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 NiCO 3 H CO 3 (aq) 37

38 #3 Single Replacement One element replaces another Reactants must be an element and a compound. Products will be a different element and a different compound. Na + KCl K + NaCl F + LiCl LiF + Cl 38

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40 Metals Reactivity Series Lithium Li 1+ Most Reactive Potassium K + Calcium Ca + Sodium Na + Magnesium Mg + Aluminum Al 3+ Manganese Mn + Zinc Zn + Chromium Cr +, Cr 3+ Iron Fe +, Fe 3+ Lead Pb + Copper Cu + Mercury Hg + Silver Ag + Platinum Pt + Gold Au +, Au 3+ Least Reactive

41 #3 Single Replacement Exceptions we ve missed along the way Zinc, Zn, always forms a + ion doesn t need parenthesis ZnCl is zinc chloride Silver, Ag, always forms a +1 ion AgCl is silver chloride 41

42 #3 Single Replacement Metals replace metals (and hydrogen) K + AlN Zn + HCl Think of water as HOH Metals replace one of the H, combine with hydroxide. Na + HOH 4

43 #3 Single Replacement We can tell whether a reaction will happen Some are more active than others More active replaces less active There is a list on page 191 Higher on the list replaces lower. If the element by itself is higher, it happens, in lower it doesn t 43

44 #3 Single Replacement Note the * H can be replaced in acids by everything higher Only the first 5 (Li - Na) react with water. Fe + CuSO 4 Pb + KCl Al + HCl 44

45 45 #3 Single Replacement What does it mean that Au And Ag are on the bottom of the list? Nonmetals can replace other nonmetals Limited to F, Cl, Br, I The order of activity is that on the table. Higher replaces lower. F + HCl Br + KCl

46 46 #4 Double Replacement Two things replace each other. Reactants must be two ionic compounds or acids. Usually 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

47 #4 Double Replacement Will only happen if one of the products doesn t dissolve in water and forms a solid or is a gas that bubbles out. or is a covalent compound usually water. 47

48 Complete and balance assume all of the reactions take place. AlCl 3 + NaOH CuCl + K S KOH + Fe(NO 3 ) 3 (NH 4 ) SO 4 + BaF 48

49 How to recognize which type Look at the reactants E + E Combination C E + C C + C Decomposition Single replacement Double replacement 49

50 Examples H + O H O Zn + H SO 4 HgO KBr +Cl AgNO 3 + NaCl Mg(OH) + H SO 3 50

51 Combustion Last Type A compound composed of only C H and maybe O is reacted with oxygen If the combustion is complete, the products will be CO and H O. If the combustion is incomplete, the products will be CO and H O. 51

52 Examples 1. C 4 H 10 + O (complete). C 4 H 10 + O (incomplete) 3. C 6 H 1 O 6 + O (complete) 4. C 8 H 8 +O (incomplete) 5

53 Balance Alphabetically 1. C 4 H O 8CO + 10H O. C 4 H O 8CO + 10H O 3. C 6 H 1 O 6 + 9O 6CO + 6H O 4. C 8 H 8 +6O 8CO + 4H O 53

54 54 Chapter 7 Summary

55 An equation Describes a reaction Must be balanced because to follow Law of Conservation of Energy Can only be balanced by changing the coefficients. Has special symbols to indicate state, and if catalyst or energy is required. 55

56 Come in 5 types. Reactions Can tell what type they are by the reactants. Single Replacement happens based on the activity series using activity series. Double Replacement happens if the product is a solid, water, or a gas. 56

57 The Process Determine the type by looking at the reactants. Put the pieces next to each other Use charges to write the formulas Use coefficients to balance the equation. 57