Unit 6 Chemical Reactions 1. Parts of a Chemical Reaction 2. Indicators of a Chemical Reaction 3. Enthalpy 4. Balancing Chemical Equations 5.

Transcription

1 Unit 6 Chemical Reactions 1. Parts of a Chemical Reaction 2. Indicators of a Chemical Reaction 3. Enthalpy 4. Balancing Chemical Equations 5. Word Equations 6. Classifying Chemical Reactions 7. Predicting Products 1

2 1. Parts of Chemical Reactions Can you think of any chemical reactions that occur naturally? Respiration Acid Rain Fermentation 2

3 How to read a chemical equation 2 H 2 (g) + O 2 (g) 2 H 2 O(g) reactants products Reactants are on the left and products are on the right Subscripts tell how many Coefficients show how many reaction taking place + is used to separate reactant (and product) substances from each other 3

4 Subscripts: H 2 O = H 2 O 1 A water molecule is made up of Coefficients: 3H 2 O There are 4

5 (g) (l) (s) (aq) Abbreviations of State 5

6 2. Indicators of Chemical Rxns Mg (s) + 2HCl (aq) MgCl 2(aq) + H 2(g) FeCl 3(aq) + 3KSCN (aq) Fe(SCN) 3(aq) + 3KCl (aq) Pb(NO 3 ) 2(aq) + 2KI (aq) PbI 2(s) + 2KNO 3(aq) FeS (s) + 2HCl (aq) H 2 S (g) + FeCl 2(aq) NH 4 SCN + Ba(OH) 2 + energy NH 3 + H 2 O + Ba(SCN) 2 14 KMnO4 + 4 C3H5(OH)3 > 7 K2CO3 + 7 Mn2O3 + 5 CO H2O + HEAT 6

7 3. Enthalpy 7

8 Energy Changes in Reactions When a chemical reaction occurs, energy (in the form of heat) must be absorbed in order for Once reactant bonds are broken, 2 outcomes: The amount absorbed is more than the amount released = heat is for reaction The amount released is more than the amount absorbed = heat is in the reaction 8

9 Energy ( ) for this reaction called an reaction Energy for this reaction called an reaction 9

10 Enthalpy (heat of reaction) H The change in Enthalpy (ΔH) is the energy difference between the reactants and the products, found by Product Energy Reactant Energy For an Endothermic reaction: product energy reactant energy = positive #, so ΔH is positive For an Exothermic reaction: product energy reactant energy = negative #, so ΔH is a negative 10

11 Endothermic Reaction Endothermic reaction a reaction in which heat is absorbed The products are higher in energy than the reactants 2NH 4 SCN (s) + Ba(OH) 2(s) Ba(SCN) 2(aq) + 2H 2 O + 2NH 3(l) Is heat a product or a reactant (Was the reaction warmer before or after the reaction?) heat + 2NH 4 SCN (s) + Ba(OH) 2(s) Ba(SCN) 2(aq) + 2H 2 O + 2NH 3(l) 11

12 Exothermic Reaction Exothermic reaction a reaction in which heat is released The products are lower in energy than the reactants 2K (s) + 2H 2 O (l) 2KOH (aq) + H 2(g) Is heat a product or a reactant (Was the reaction warmer before or after the reaction?) 2K (s) + 2H 2 O (l) 2KOH (aq) + H 2(g) + heat 12

13 Summary There are two ways to show an endothermic reaction in a chemical equation: 2N 2 + O kJ 2N 2 O 2N 2 + O 2 2N 2 O ΔH = +58kJ 13

14 There are two ways to show an exothermic reaction in a chemical equation: 1. CH 4 + 2O 2 CO 2 + 2H 2 O + 102kJ 2. CH 4 + 2O 2 CO 2 + 2H 2 O ΔH = 102kJ 14

15 HOMEWORK: Hebden p #

16 4. Balancing Chemical Equations 16

17 Parts of an Equation: Reactant Product 2Ag + H 2 S Ag 2 S + H 2 Reaction symbol 17

18 Reactant The chemical(s) you start with before the reaction. Product The new chemical(s) formed by the reaction. 18

19 Subscript In a chemical equation, the number that shows how many atoms of an element are in a specific molecule. H 2 O There is one atom of oxygen in this water molecule There are two atoms of hydrogen in this water molecule 19

20 Coefficient In a chemical equation, the number that shows how many molecules there are of this chemical. 3 H 2 O There are three molecules of water 20

21 A balanced chemical equation has the same number of atoms on both sides of the equation. 4Al + 3O 2 2Al 2 O 3 4 Aluminum 6 Oxygen 4 Aluminum 6 Oxygen 21

22 Balancing Rules Matter can never be created nor destroyed. A subscript can never be added, removed or changed. You may change only the coefficients in the equation. You must end up with the same number and type of atoms on the reactant and product sides of the equation. Generally, balance main elements first, then try hydrogens, then try oxygens 22

23 ** Balancing Demo H 2 + O 2 H 2 O Reactants Products Hydrogen Oxygen 23

24 You try! use a tally if needed! C + O 2 CO 2 Fe + O 2 Fe N 2 O N 2 + O 2 Fe + O 2 Fe 2 O 3 24

25 Balance this C 4 H 10 + O 2 CO 2 + H 2 O 25

26 Polyatomic Ions If you have the same polyatomic ion on both sides of the equation, balance the whole polyatomic rather than each element in the polyatomic.see below Na 2 SO 4 + Al(NO 3 ) 3 NaNO 3 + Al 2 (SO 4 ) 3 26

27 PRACTICE PROBLEMS BALANCING CHEMICAL EQUATIONS Balance the following chemical equations. The answers are found on the next slide. 1. Na 2 SO 4 (aq) + BaCl 2 (aq) BaSO 4 (s) + NaCl(aq) 2. Al(s) + H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + H 2 (g) 3. NO 2 (g) + H 2 O(l) + O 2 (g) HNO 3 (aq) 4. Al(s) + HCl(aq) AlCl 3 (aq) + H 2 (g) 5. C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O(g) 27

28 HOMEWORK: Balancing Chemical Equations I Worksheet 28

29 5. Word Equations we must be able to build balanced formula equations from word equations hints: keep in mind that seven elements naturally occur as diatomic molecules: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 acids can be found on the back of periodic table ammonia is the common name for NH 3 yields is another word for produces 29

30 Example hydrogen and nitrogen yields ammonia Now balance: 30

31 Another aluminum sulphate + phosphoric acid yields aluminum phosphate + sulfuric acid 31

32 HOMEWORK: Balancing Chemical Equations II Worksheet 32

33 6. Classifying Chemical Reactions 33

34 Classifying Types of Chemical Reactions 34

35 Six types of chemical reactions. Classified according to patterns of reactants & products Study pattern to predict products Synthesis combining 2 (or more) substances into 1 can start with elements or compounds follow rules for writing formulas for product E.g. 2Na + Cl 2 2NaCl 2. Decomposition 1 substance breaking down into 2 (or more) products are elements or simpler compounds E.g. 2H 2 O 2H 2 + O 2 35

36 Single Replacement element reacts with compound single element replaces one element in compound metal replaces metal, non metal replaces non metal e.g. Fe + CuSO 4 FeSO 4 + Cu Cl 2 + 2KBr 2KCl + Br 2 36

37 Double Replacement 2 compounds switch partners 2 elements replace each other new compounds will be ionic (+ ion/ ion) follow formula writing rules e.g. CaCl 2 + Na 2 CO 3 CaCO 3 + 2NaCl 37

38 Acid Base Neutralization (special kind of Double Replacement) H + from acid and OH from base neutralize to form H 2 O Acid + Base Salt + Water e.g. HBr + LiOH LiBr + HOH (H 2 O) Combustion burning of hydrocarbons always requires oxygen gas products are always CO 2 and H 2 O e.g. CH 4 + 2O 2 CO 2 + 2H 2 O 38

39 HOMEWORK: Classifying Reactions Worksheet 39

40 7. Predicting Products of Chemical Reactions We have many tools in our tool belt. We can: make formulas from names balance equations give reaction types for different reactions Now we ll use these tools to build reactions and predict the products! 40

41 Predict the products: Ca(OH) 2 + Al 2 (SO 4 ) 3? Step 1: Determine the reaction type using only the reactants. i.e. Is this synthesis, decomposition, single replacement, double replacement, neutralization, combustion? The reactants are both compounds, but there is no acid present, therefore the type is.. 41

42 Step 2: Build the products using ion charges Ca(OH) 2 + Al 2 (SO 4 ) 3 Step 3: Balance the equation 42

43 Try these (use the 3 steps) 2H 2 + O 2 2NaCl 2C 2 H 2 + 5O 2 H 2 SO 4 + 2NaOH 43

44 Predicting Single Replacement If you have reactants composed of a single element reacting with a compound, you most likely have a single replacement reaction There are two outcomes to a single replacement reaction: The reaction proceeds as you would predict The reaction will not occur 44

45 Some elements are more reactive than others If a more reactive element is replacing a less reactive element in a compound, the reaction WILL proceed If a less reactive element is attempting to replace a more reactive element in a compound, the reaction WILL NOT occur We have a reactivity list called an ACTIVITY SERIES 45

46 Metals Activity Series Lithium most active Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Tin Lead Hydrogen Copper Silver Mercury Platinum Gold least active Nonmetals Fluorine Chlorine Bromine Iodine An element higher on the list will replace an element lower on the list in a single replacement reaction. NOTE: separate lists for metals & non metals 46

47 Al + NiCl 2 Pb + LiBr F 2 + CaCl 2 Br 2 + AICI 3 47

48 HOMEWORK: 1. Classifying & Predicting Worksheet 2. Activity Series Worksheet 48