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Understanding Chemical Reactions: Formation and Equations

Learn about chemical reactions and how new substances are formed, explore chemical formulas and equations, understand balancing reactions for mass conservation, and identify different types of chemical reactions.

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Understanding Chemical Reactions: Formation and Equations

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  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.

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