Introduction to Chemical Reactions. Making new substances

Transcription

1 Introduction to Chemical Reactions Making new substances

2 Main Ideas Chemical Reactions are represented by Chemical Equations. Chemical Equations are balanced to show the same number of atoms of each element on each side. The Law of Conservation of Mass says that atoms won t be created or destroyed in a chemical reaction. That is why you have to balance chemical equations!

3 Chemical Reactions are Everywhere Cooking Respiration

4 Chemical Reactions are Everywhere Hair Dye Auto Fuel

5 How do you know when a chemical reaction takes place? Color Change Precipitate Formation

6 How do you know when a chemical reaction takes place? Gas Formation Odor

7 How do you know when a chemical reaction takes place? Temperature Change Change in Acidity

8 Representing Chemical Reactions Chemists observe chemical reactions and have come up with a way to represent or model what is happening. Making NaCl Solid Sodium combines with Chlorine gas to make solid Sodium Chloride: 2Na (s) + Cl2 (g) 2NaCl

9 Chemical Equations are different from Numerical Equations Numerical Equation: 3x + 2y = 47 Chemical Equation 2Na + Cl2 2NaCl ReactantA + Reactant B Product The reactants are used up in forming the product The arrow shows the direction of the reaction

10 Symbols used in Chemical Equations Symbol Purpose + Separates more than one reactant or product (s) Separates reactants from products. Indicates direction of reaction Identifies a solid state (aq) (l) Identifies that something is dissolved in water Identifies liquid state (g) Identifies gaseous state

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

12 An example When pellets of sodium hydroxide are dropped into an aqueous solution of iron(iii) chloride, iron(iii) hydroxide precipitates and sodium chloride stays dissolved. 3NaOH (s) + FeCl 3(aq) Fe(OH) 3 (ppt) + 3NaCl (aq) 12

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

14 Law of Conservation of Mass In a chemical reaction, matter is neither created nor destroyed. Atoms won t change their identity (e.g. a Carbon atom can t become an Iron atom) This means that you have to have the same number of each type of atom on each side of the chemical equation. Conservation of Mass Video

15 Balancing Equations After you write a chemical equation you have to balance it to make sure that the same number of atoms of each element are on each side.

16 C + O O C O O C + O 2 CO 2 This equation is already balanced What if it isn t already?

17 C + O C O O C + O 2 CO We need one more oxygen in the products. Can t change the formula, because it describes what is

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

19 C C + O O C C O O Must have started with two C 2 C + O 2 2 CO

20 Count the atoms, before & after 2H 2 + O H 2 O

21 Coefficients In a chemical reaction, the number of molecules is shown with a coefficient. 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Coefficients multiply subscripts through the whole molecule that follows, so 6CO 2 shows that there are 6 Carbon atoms and 12 Oxygen atoms. 21

22 How would you balance this equation? Li + H2O H2 + LiOH

23 Steps to Balancing a Chemical Equation 1. Write the Skeleton Equation Li(s) + H2O(l) H2 (g) + LiOH (aq) 2. Count the atoms of the elements in the reactants 1 atom Li, 2 atoms H, 1 atom O 3. Count the atoms of the elements in the products 1 atom Li, 3 atoms H, 1 atom O 4. Change to Coefficients to make the number of atoms of each element equal on both sides of arrow 2Li(s) + 2 H2O H2(g) + 2LiOH(aq) 5. Write the Coefficients in their lowest possible ratio 6. Check your work

24 Another Example CH4 (methane gas) + O2 CO2 + H2O Reactants Products # of Carbons = 1 # of Carbons = 1 # of Hydrogens = 4 # of Hydrogens = 2 # of Oxygens = 2 # of Oxygens = 3 Total atoms = 7 Total atoms = 6 7 6! Where did our atoms go?

25 Example Continued Change the Coefficients to make the number of atoms of each element equal Balance the Hydrogens: CH4 + O2 CO2 + 2 H2O Balance the Oxygens: CH4 + 2 O2 CO2 + 2 H2O

26 Example Continued CH4 + 2 O2 CO2 + 2 H2O Are your coefficients in their simplest ratio? Count your atoms again to check your work: Reactants Products # of Carbons = 1 # of Carbons = 1 # of Hydrogens = 4 # of Hydrogens = 4 # of Oxygens = 4 # of Oxygens = 4 Total atoms = 9 Total atoms = 9

27 Try These! C 2 H 6 + O 2 CO 2 + H 2 O Fe 2 O 3 + H 2 SO 4 Fe 2 (SO 4 ) 3 + H 2 O Hint : balance the polyatomic ion first! CaCl 2 + AgNO 3 AgCl + Ca(NO 3 ) 2 Think Pair - Share

28 Review Matter is not destroyed or created Atoms are rearranged in chemical reactions Chemical equations represent chemical reactions You have to have the same number of each type of atom on the left and right hand side of a chemical equation

29 WARNING! Don t mess with the insides of polyatomic ions put a square around them, or label them as X treat the WHOLE polyatomic ion as though it were an element! Don t ever play around with subscripts (those little numbers that tell you how many atoms are in a molecule) e.g. C 6 H 22 O 11

30 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 Never put a coefficient in the middle of a formula 2 NaCl is okay, Na2Cl is not.

31 Techniques If an atom appears more than once on a side, balance it last. If you fix everything except one element, and it is even on one side and odd on the other, double the first number, then move on from there. C 4 H 10 + O 2 CO 2 + H 2 O

32 Types of Reactions Predicting the Products

33 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

34 1. Synthesis (Combination) Reactions combination of two or more simple substances combining to form a more complex substance. A + B AB +

35 1. Synthesis (Combination) Reactions Combine - put together 2 elements, or compounds combine to make one compound. Ca +O 2 CaO SO 3 + H 2 O H 2 SO 4 We can predict the products if they are two elements. Mg + N 2

36 Synthesis of Magnesium Oxide 2 Mg (s) + O 2(g) MgO 2

37 Other Examples Synthesis (Combination) Reactions: C (s) + O 2(g) CO 2(g) S (s) + O 2(g) SO 2(g)

38 Write and balance Ca + Cl 2 Fe + O 2 iron (II) oxide Al + O 2 Remember that the first step is to write the formula Then balance

39 2. Decomposition Reactions Breaking down large, complex molecules or ionic compounds into smaller, simpler entities. AB A + B +

40 #2 Decomposition decompose = fall apart Reactions one reactant falls apart into two or more elements or compounds. electricity NaCl Na + Cl 2 CaCO 3 CaO + CO 2

41 Decomposition of Hydrogen Peroxide 2 H 2 O 2(l) O 2(g) + 2 H 2 O (l)

42 #2 Decomposition Reactions Can predict the products if it is a binary compound Made up of only two elements Falls apart into its elements H 2 O HgO electricity

43 #2 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 2 CO 3 (aq)

44 #3 Single Displacement (Replacement) reaction of an element with a compound that produces a new element and new compound. A + BC AC + B

45 Single Displacement of Zinc and Chlorine 2 HCl (aq) + Zn (s) H 2(g) + ZnCl 2(aq)

46 #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 2 + LiCl LiF + Cl 2

47 #3 Single Replacement Exceptions we ve missed along the way Zinc, Zn, always forms a +2 ion doesn t need parenthesis ZnCl 2 is zinc chloride Silver, Ag, always forms a +1 ion AgCl is silver chloride

48 #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

49 #3 Single Replacement We can tell whether a reaction will happen Some are more active than other More active replaces less active Refer to the Activity Series in your Reference Table Higher on the list replaces lower. If the element by itself is higher, it happens, in lower it doesn t

50 #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

51 #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 2, Cl 2, Br 2, I 2 The order of activity is that on the table. Higher replaces lower. F 2 + HCl Br 2 + KCl

52 4. Double Displacement reaction of two compounds, where the positive and negative ions in their respective compounds switch places to form two new compounds. This can only happen with ionic compounds AB + CD AD + CB

53 Double Displacement A double displacement reaction only occurs if one of the following three results are seen: a precipitate is formed a gas is produced a change of ph occurs (a neutralization reaction) ** If the products are both soluble then the reaction is NR (no reaction)

54 Double Displacement of Lead and Potassium Pb(NO 3 ) 2(aq) + 2 KI (aq) PbI 2(s) + 2KNO 3(aq) Precipitate: A solid that is formed as a result of the chemical reaction of two aqueous solutions.

55 #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

56 #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.

57 Complete and balance assume all of the reactions take place. CaCl 2 + NaOH CuCl 2 + K 2 S KOH + Fe(NO 3 ) 3 (NH 4 ) 2 SO 4 + BaF 2

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

59 Examples H 2 + O 2 H 2 O Zn + H 2 SO 4 HgO KBr +Cl 2 AgNO 3 + NaCl Mg(OH) 2 + H 2 SO 3

60 5. Combustion Reactions More commonly known as burning Easily identified (release of heat) For combustion 3 things need to be present: Fuel Oxygen Heat

61 Last Type Combustion A compound composed of only C H and maybe O is reacted with oxygen If the combustion is complete, the products will be CO 2 and H 2 O. If the combustion is incomplete, the products will be CO and H 2 O.

62 Examples C 4 H 10 + O 2 (complete) C 4 H 10 + O 2 (incomplete) C 6 H 12 O 6 + O 2 (complete) C 8 H 8 +O 2 (incomplete)

63 23. Distinguish between complete an incomplete combustion,. 24. Write a balanced equation for the complete combustion of each of these compounds. a) acetic acid, HC 2 H 3 O 2 c) glycerol, C 3 H 8 O 3 b) decane, C 10 H 22 d) sucrose, C 12 H 22 O Write a balanced equation for the incomplete combustion of each of these compounds. a) glycerol, C 3 H 8 O 3 c) acetic acid, HC 2 H 3 O 2 b) glucose, C 6 H 12 O 6 d) acetylene, C 2 H 2

64 Unit 8 Summary

65 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.

66 Reactions Come in 5 types. 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.

67 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.