Chemical Reactions. Section 7.1: Nature of Reactions

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1 Chemical Reactions Section 7.1: Nature of Reactions When do chemical reactions take place? What is the role of energy in chemical reactions? 1

2 Chemical Reactions It is a change in matter that produces one or more NEW SUBSTANCES with different properties Chemical Reactions A chemical reaction is another name for a chemical change New properties are formed No new atoms are made, but atoms are rearranged New compounds can be made Old bonds are broken and new bonds are formed Always involves changes in energy 2

3 Chemical Reactions What forms of evidence show that a chemical reaction took place? Color Change. New phase is made Precipitation Gas or solid formation. Changes in Temperature. Changes in Properties. Energy change Energy Parts To A Chemical Reaction 1.Reactants: starting materials of a chemical reaction Chemical Reactions 2.Products: substances formed at the end Yields C O 2 CO 2 3

4 Chemical Reaction 2 2Na Cl 2 2NaCl What Do You Notice About All The Atoms? Energy and Reactions Chemical reaction occurs when chemical bonds break and new bonds form. Always takes a little energy Chemical reactions usually absorb heat or liberate (release) heat. Different forms of energy can be used:» Heat» Electricity» Light 4

5 Energy and Reactions Every Reaction Involves Energy 1. During a reaction bonds absorb energy and break 2. New bonds form energy released Energy and Reactions Forming bonds releases energy Energy is conserved Chemical Energyis energy stored in the bonds of the chemicals food fuel 5

6 Energy and Reactions Exothermic Reaction If breaking bonds takes less energy than making them- it releases energy (exothermic) Exo- outside therm- heat Exothermic reactions release energy Get hot/warm Give off light Or release electricity Energy and Reactions Exothermic Reactions 2 Na + Cl 2 2 NaCl + Energy Energy is in the products in an Exothermic Reaction 6

7 Heat Released Energy and Reactions Endothermic Reaction A chemical reaction in which energy is taken in. absorb heat Feels cool! Require heat or energy or they stop Endo-into Therm-heat 7

8 Energy and Reactions Endothermic Reactions Example: Photosynthesis 6 CO H 2 O + Energy C 6 H 12 O Energy is in the reactants in an Endothermic Reaction Heat is absorbed 8

9 Energy and Reactions Exothermic vs. Endothermic Exo and Endo Clip What is the law of the conservation of mass? During a chemical reaction, matter is neither created nor destroyed. Atoms in = atoms out 9

10 Section7.2 Balancing Chemical Equations What is a chemical equation? What can a balanced chemical equation tell you? Describing Reactions 10

11 Describing Reactions Chemical Equation A symbolic representation of a chemical reaction Putting chemical changes into words The plus mean and The arrow means yield or make 2Na + Cl 2 2 NaCl Describing Reactions Element Symbols All elements are represented by a 1 or 2 letter symbol C = Carbon Ne = Neon O = Oxygen The symbols are shown on the periodic table The burning of methane gas in oxygen is: CH O 2 CO H 2 O 11

12 Describing Reactions Chemical Formulas Shows the elements & number of atoms of each element in a molecule H 2 SO 4 Hydrogen: 2 atoms Sulfur: 1 atom Oxygen: 4 atoms 7 atoms total Subscript Describing Reactions Coefficients A formula may begin with a number. This number is called the coefficient. Represents the number of molecules of that compound or atom needed in the reaction. For example: 2H 2 SO 4 2 molecules of Sulfuric Acid Never put a coefficient in the middle of a formula (2 NaCl is okay, Na2Cl is not) If there is no number, then 1 is understood to be in front of the formula. 12

13 Coefficients 2H 2 SO 4 2 molecules of Sulfuric Acid A coefficient is distributed to ALL elements in a compound 2 H 2 (for a total of 4 H atoms) 2 S (for a total of 2 S atoms) 2 O 4 (for a total of 8 O atoms) Describing Reactions Reading Chemical Equations The two sides of the equation are separated by an arrow. REACTANTS: The combination of chemicals before the reaction are on the left side of the arrow PRODUCTS: The right side indicates the combination of chemicals after the reaction. CH 4 + 2O 2 CO 2 + 2H 2 O 13

14 Parts of a chemical Equation Coefficient the number of Molecules in an equation 2Na + Cl 2 Subscript the number of atoms in a formula 2 NaCl Reactants Everything to the left yields Products Everything to the right Balanced Chemical Equations A balanced chemical equation follows the law of conservation of mass. It can tell you the amount of reactants you will need, and the amount of products you will get from the reaction. For example, Methane + oxygen carbon dioxide + water CH 4 + O 2 CO 2 + H 2 O Does not tell you how much of each compound you will need. CH O 2 CO H 2 O This balanced equation does. 14

15 Balancing Chemical Equations Balance equations by changing coefficients 2Na + Cl 2 2 NaCl never by changing formula subscripts Remember the Law Conservation of Mass: Matter cannot be created or destroyed. That means we need to have the same amount of chemicals on each side of the. Example Balancing Chemical Equations 15

16 Rules for balancing 1 Write the correct formulas for all the reactants and products 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) 4 Check to make sure it is balanced. Balancing Chemical Equations Begin balancing chemical equations by putting numbers (coefficients) in front of them. Example H 2 O on one side could become 2 H 2 O Remember that each side needs to have same number of Hydrogen and Oxygen Note Don t change the subscript Example H 2 O becomes H 3 O 16

17 A way to start off the process is to create an inventory of your chemicals. Now look at the inventory and begin the process of balancing the equation. 17

18 One Sodium Add a 2 and check if it balance Did it balance? Are we done? NO 2 Time for a new chart.. Let s try again and add a 2 to the other side

19 Does it balance this time? This is a balanced equation. Photosynthesis Reaction Carbon dioxide + water Glucose (sugar) + oxygen 6CO H 2 O C 6 H 12 O 6 + 6O 2 Count the atoms on each side of the equation. C : 6 O : 18 H : 12 C : 6 O : 18 H : 12 19

20 Balancing Equations Practice 1. AgNO 3 + Cu Cu(NO 3 ) 2 + Ag 2. Mg + N 2 Mg 3 N 2 Elements Before After 3. P + O 2 P 4 O Na + H 2 O H 2 + NaOH 5. CH 4 + O 2 CO 2 + H 2 O More Balancing Equations Practice Homework Your Turn a) Pb(NO 3 ) 2 + K 2 CrO 4 PbCrO 4 + KNO 3 b) MnO 2 +HCl MnCl 2 + H 2 O+ Cl 2 c) C 3 H 6 + O 2 CO 2 +H 2 O d) Zn(OH) 2 + H 3 PO 4 Zn 3 (PO 4 ) 2 + H 2 0 e) CO + Fe 2 O 3 Fe + CO 2 f) CS 2 + Cl 2 CCl 4 +S 2 Cl 2 g) CH 4 + Br 2 CH 3 Br + HBr h) Ba(CN) 2 + H 2 SO 4 BaSO 4 + HCN 20

21 Diatomic Molecules There are 7 elements that exist in nature as diatomic molecules. What elements exist as diatomic molecules? H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 21

22 More Balancing Equations Practice A. Potassium iodide reacts with bromine forming potassium bromide plus iodine. B. Nickel (III) chloride reacts with beryllium oxide forming nickel (III) oxide plus beryllium chloride. 22

23 C. Chromium (II) sulfide decomposes producing chromium (II) plus sulfur. D. Titanium (III) oxide + copper (I) phosphate -> titanium (III) phosphate + copper (I) oxide 23

24 Balanced Equations and Mole Ratios A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction. 6CO H 2 O C 6 H 12 O 6 + 6O 2 In photosynthesis, for every 6 moles of carbon dioxide used, 6 moles oxygen is produced. This is a 1:1 molar ratio Mole Ratios Can Be Converted To Masses Mole Ratios can be converted to mass by taking the molar mass of the molecule times the number of moles required. O 2 has a molar mass of 32g/mol, mass of 32 g 3 O 2 has a mass of 3 mol x 32 g/mol = 96 g 24

25 Section 7.3 Chemical Reaction Types How does learning about reaction types help in understanding chemical reactions? In which kind of chemical reactions do the number of electrons in atoms change? All the reactants get turned into products No reactants left Some reactions go both directions They are called reversible reactions 2H 2 (g) + O 2 (g) 2H 2 O(g) + energy 2H 2 O(g) + energy 2H 2 (g) + O 2 (g) 2H 2 (g) + O 2 (g) 2H 2 O(g) + energy 25

26 Helps to identify the kinds of chemical reactions and to predict the products of the chemical reactions. Five general kinds of reactions: Synthesis Decomposition Single Displacement Double Displacement Combustion Reaction Synthesis A+ B AB Decomposition AB A + B Single displacement AC +B BC + A Double displacement AC + BD AD + BC Combustion Ex: CH 4 + 2O 2 CO 2 +2H 2 O 26

27 Synthesis Reaction Two or more substances (reactants) Combine to form only one substance (product) K + Cl 2 KCl Mg + O 2 MgO Fe + O 2 Fe 2 O 3 One substance (reactant) combine forms 2 or more substances (products) H 2 CO 3 H 2 O + CO 2 MClO 3 MCl + O 2 27

28 One element and one compound (in reactants) Produces one element and one compound (in Products) AC + B BC + A Zn + CuSO 4 --> Cu + ZnSO 4 F 2 + 2KI I 2 + 2KF Double displacement 2 compounds (in reactants) produce 2 compounds (in products) AgNO 3 + NaCl AgCl+ NaNO 3 Na 2 CO 3 + CaCl > CaCO 3 + 2NaCl 28

29 Combustion Reaction A reaction in which a compound and oxygen burn. Water is a common product A = Red B = Blue C = Green D = Yellow Type of Reaction Synthesis Definition Two or more elements or compounds combine to make a more complex substance Equation A + B AB Decomposition Compounds break down into simpler substances AB A + B Single Displacement Or Replacement Double Displacement or Replacement Occurs when one element replaces another one in a compound Occurs when different atoms in two different compounds trade places AB + C AC + B AB + CD AC + BD 29

30 Classifying Reaction Practice A. S 8 + 8O 2 8SO 2 + energy B. 6CO 2 + 6H 2 0 C 6 H 12 O 6 + 6O 2 Synthesis Synthesis C. 2NaHCO 3 Na 2 CO3 + H 2 O + CO 2 Decomposition D. Zn + 2HCl ZnCl 2 + H 2 Single-displacement Section 7.4 Reaction Rates & Equilibrium What kind of things speed up a reaction. What does a catalyst do? What happens when a reaction goes backward as well as forward? 30

31 Things that Affect Reaction Rate Anything that increases contact between particles will increase the rate of a reaction Particle size Surface area Temperature Concentration Pressure Things that Affect Reaction Rate Particle size Massive, bulky molecules react more slowly, because they move more slowly. 31

32 Things that Affect Reaction Rate Surface Area A large surface area speeds up reactions. Things that Affect Reaction Rate Temperature Most reactions go faster at higher temperatures. 32

33 Things that Affect Reaction Rate Concentration Higher concentrations of reactants react faster. Things that Affect Reaction Rate Pressure Reactions are faster at higher pressure. 33

34 Things that Affect Reaction Rate Catalysts - substances that speed up a reaction without being used up. Heat destroys catalysts Inhibitor a substance that blocks a catalyst, slowingthe reaction down Equilibrium Systems Chemical equilibrium-state of balance in which the rate forward equals the rate of the reverse reaction. Products and reactants remain unchanged. The reactions are still running. 34

35 Equilibrium Systems Equilibrium position-how much product and reactant there are at equilibrium. Shown with the double arrow. Reactants are favored Products are favored Catalysts speed up both the forward and reverse reactions so don t affect equilibrium position. Which is favored? Product Bill Nye Video 35