Chapter 14 Chemical Reactions

1. Chapter 14 Chemical Reactions

2. Forming New Substances • Chemical Reaction- a process in which one or more substances change to make one or more new substances • The chemical and physical properties of the new substance differ from those of the original substances. • Examples of chemical changes: • Changing colors of leaves • baking muffins

3. Signs of a Chemical Reaction • Gas bubbles • energy change • color change • formation of a solid precipitate • precipitate- Solid substance that is formed in a solution

4. Signs of a Chemical Reaction • A change of properties: • The signs can help you identify a chemical reaction, but they do not guarantee a reaction took place. • Ex. Boiling water gives off gas, but this is a physical change. • The most important sign that a chemical change took place is the formation of new substances that have different properties.

5. Chemical Bond • Chemical bond-force that holds two atoms together in a molecule. The original bonds must break and new bonds must be formed. • Ex 1: H2 and Cl2 • Chlorine gas has a greenish-yellow color • Hydrogen gas is flammable • Hydrogen chloride is a nonflammable, colorless gas. • Ex 2: Na and Cl2 • Na is a metal that reacts violently with water • Cl is a greenish-yellow poisonous gas • Sodium chloride is a harmless substance that almost everyone uses.

6. Ch 14.2 Chemical Formulas and Equations • Chemical Formulas-a shorthand way to use chemical symbols and numbers to represent a substance. It shows how many atoms of each kind ore present in a molecule.

7. Chemical Formulas and Equations • Subscript- number written below and to the right of a chemical symbol in a formula. • *If no subscript is present, it means only 1 atom of that element is present. • Ex: H2O H = O = C6H12O6C = H= O= CO2C = O =

8. Writing formulas for Covalent Compounds • composed of 2 nonmetals • use prefixes • if there is only one atom of the first element drop the mono- prefix • change 2nd elements suffix to –ide • Ex: CO2 N2O

9. Writing formulas for Ionic Compounds • composed of a metal and a nonmetal • put the metal first, nonmetal secondoverall charge must be 0. • charges of the ions will cancel out • criss-cross rule • change suffix of nonmetal to –ide • Ex: NaCl MgCl2

10. Chemical Equations • Chemical Equations-Uses chemical symbols and formulas as a shortcut to describe a chemical reaction • Reactants-the starting materials in a chemical reaction • Products-the substance formed from a reaction • Ex: C + O2 CO2 reactant product

11. How do new substances form in a chemical reaction? • 1. Original bonds must break • Molecules are always moving • If molecules bump into each other with enough energy, the chemical bonds in the molecules break. • 2. Atoms rearrange • 3. New bonds must form

12. The Reason Equations Must be Balanced: • ·Atoms are never lost or gained in a chemical reaction • ·They are just rearranged • ·The # of atoms in the reactants must = # of atoms in the products • This is called balancing the reaction

13. Law of Conservation of Mass • Law of Conservation of Mass- Mass is neither created or destroyed in ordinary chemical and physical changes

14. How to Balance an Equation: • You must use coefficients- number placed in front of a chemical symbol or formula • Ex 1: 2CO means 2 carbon monoxide molecules • Ex 2: 2H2O means 2 dihydrogen monoxide molecules • Total there are 4 hydrogen atoms and 2 oxygen atoms • ·Only coefficients are changed when balancing equations – never subscripts!! • ___H2 + ___O2 ___H2O | | |

15. Balancing Chemical Reactions ____ Na + ____Cl2 → ____NaCl Na Na Cl Cl ____ N2 + ­­_____ H2 → ____ NH3 N N H H

16. Balancing Chemical Reactions ____ Na2S+ ____AlCl3 → ____NaCl + ____ Al2S3 Na Na S S Al Al Cl Cl

17. Balancing Chemical Reactions ____ Ca+ ____O2 → ____CaO Ca Ca O O ____ CaCl2+ ____Li2S → ____CaS + ____ LiCl Ca Ca Cl Cl Li Li S S

18. 14.3 Types of Reactions • By looking at the reactants, you can determine what kind of chemical reaction is taking place. • There are 4 kinds of reaction you will be able to identify • - synthesis • - decomposition • - single replacement • - double replacement

19. 14.3 Types of Reactions • 1. Synthesis Reaction- A reaction in which two or more substances combine to form one new compound. • A + B = AB • Ex: Sodium + Chlorine  Sodium Chloride • 2Na + Cl2 2NaCl • Clue: only one thing on right side

20. Synthesis Reaction

21. Types of Reactions • 2. Decomposition Reaction- A reaction in which a single compound breaks down to form two or more simpler substances. • AB = A + B • Ex: Carbonic Acid = Water and carbon Dioxide • H2CO3 H2O + CO2 • Clue: only one thing on the left side

22. Decomposition Reaction

23. Types of Reactions • 3. Single Replacement - A single element replaces another element that is part of a compound. • Products are a new compound and a different single element • A + BC  AC + B • Ex: Zn + 2HCl  ZnCl2 + H2 • *The more reactive element can displace a less reactive element. • Ex: Copper can replace Silver, but the opposite can’t occur. • Clue: element + compound

24. Single Replacement

25. Types of Reactions • 4. Double Replacement Reaction- A reaction in which ions from two compounds switch places. • AB + CD = AD + CB • Clue: Compound + Compound Sodium Chloride + Silver Fluoride  Sodium Fluoride + Silver Chloride NaCl + AgF  NaF + AgCl

26. D

27. 14.4 Energy and Rates of Chemical Reactions • Exothermic Reaction- Reaction in which energy is released. • EXO means “goes out” or “exits” • Ex: 2Na + Cl22NaCl + energy • Feels warm • Energy is released as a product

28. Endothermic Reaction- A reaction in which energy is taken in • ENDO means “goes in” • Ex: 2H2O + energy  2H2 + O2 • Feels cold • Energy is absorbed as a reactant

29. Law of Conservation of Energy • Law of Conservation of Energy- Energy cannot be created or destroyed • Energy can be transferred from one object to another • Rate of Reaction- the speed at which new particles form • Activation Energy-The initial boost of energy. The smallest amount of energy that molecules need to react.

30. Factors that Affect Reaction Rate • Temperature – the higher the temperature, the faster the reaction rate • Concentration- The higher the concentration of reactants, the faster the reaction rate • Surface Area- (amount of exposed surface on a substance) The larger the surface area, the faster the reaction rate • Inhibitor- A substance that slows down a reaction. Ex: Preservatives in food slow down bacteria growth. • Catalyst- A substance that speeds up a reaction without being permanently changed. NOT A REACTANT!! Ex: Enzymes speed up reactions in your body.