Chapter 12 Chemical Reactions 12 1 Understanding Chemical

Chapter 12 Chemical Reactions

12. 1: Understanding Chemical Reactions O Forming New Substances O Chemical Reaction a process in which one or more substances change to make one or more new substances O The chemical and physical properties of the new substance differ from those of the original substances. O Examples of chemical changes: O Changing colors of leaves O baking muffins 2 Mg + O 2 2 Mg. O

12. 1: Understanding Chemical Reactions Signs of a Chemical Reaction O Gas bubbles energy change color change formation of a solid precipitate O O O precipitate Solid substance that is formed in a solution

12. 1: Understanding Chemical Reactions O A change of properties: O The signs can help you identify a chemical reaction, but they do not guarantee a reaction took place. O Ex. Boiling water gives off gas, but this is a physical change. O The most important sign that a chemical change took place is the formation of new substances that have different properties.

12. 1: Understanding Chemical Reactions O Chemical bond the attraction between atoms when electrons are transferred, shared, or pooled. O The original bonds must break O Atoms rearrange O new bonds must be formed.

12. 1: Understanding Chemical Reactions reaction? How do new substances form in a chemical O 1. Original bonds must break O Molecules are always moving O If molecules bump into each other with enough energy, the chemical bonds in the molecules break. O 2. Atoms rearrange O 3. New bonds must form

12. 1: Understanding Chemical Reactions O New Bonds, New Substances O What happens with Na + Cl 2? O Sodium—silver metal that reacts violently with water O Chlorine—green yellow gas that is poisonous O Na bonds and Cl Cl bonds break O Na+ Cl bonds form O Sodium chloride—harmless ionic compound that is white and dissolves in water 7

12. 1: Understanding Chemical Reactions O 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.

12. 1: Understanding Chemical Reactions Chemical Formulas and Equations O Subscript number written below and to the right of a chemical symbol in a formula. O *If no subscript is present, it means only 1 atom of that element is present. O Ex: H 2 O H = O = C 6 H 12 O 6 C = O= CO 2 C = H= O =

12. 1: Understanding Chemical Reactions O Chemical Equations- Uses chemical symbols and formulas as a shortcut to describe a chemical reaction O Reactants the starting materials in a chemical reaction O Products the substance formed from a reaction

12. 1: Understanding Chemical Reactions The Reason Equations Must be Balanced: O ·Atoms are never lost or gained in a chemical reaction O ·They are just rearranged O ·The # of atoms in the reactants must = # of atoms in the products O This is called balancing the reaction

12. 1: Understanding Chemical Reactions O Atoms are NEVER lost or gained in a chemical reaction O Lavoisier’s law of conservation of mass states that the total mass of the reactants before a chemical reaction is the same as the total mass of the products after the chemical reaction

12. 1: Understanding Chemical Reactions How to Balance an Equation: O You must use coefficients number placed in front of a chemical symbol or formula O Ex 1: 2 CO means 2 carbon monoxide molecules O Ex 2: 2 H 2 O means 2 dihydrogen monoxide molecules O Total there are 4 hydrogen atoms and 2 oxygen atoms O NEVER change the subscripts—this would change the IDENTITY of the element, molecule, or compound! ___H 2 + ___O 2 ___H 2 O |

Balancing Chemical Reactions ____ Na + ____Cl 2 → ____Na. Cl Na Cl ____ N 2 + _____ H 2 → ____ NH 3 N N H H

Balancing Chemical Reactions ____ Na 2 S+ ____Al. Cl 3 → ____Na. Cl + ____ Al 2 S 3 Na S S Al Cl

Balancing Chemical Reactions ____ Ca+ ____O 2 → ____Ca. O Ca O O ____ Ca. Cl 2+ ____Li 2 S → ____Ca. S + ____ Li. Cl Ca Cl Li S S

12. 2: Types of Chemical Reactions O Synthesis Reaction A reaction in which two or more substances combine to form one new compound. O A + B AB O Ex: Sodium + Chlorine Sodium Chloride O 2 Na + Cl 2 2 Na. Cl

12. 2: Types of Chemical Reactions Synthesis Reaction

12. 2: Types of Chemical Reactions O Decomposition Reaction A reaction in which a single compound breaks down to form two or more simpler substances. O AB A + B O Ex: Carbonic Acid = Water and carbon Dioxide O H 2 CO 3 H 2 O + CO 2

12. 2: Types of Chemical Reactions Decomposition Reaction

O Single Replacement A single element replaces another element that is part of a compound. O Products are a new compound a different single element O A + BC AC + B O Ex: Zn + 2 HCl Zn. Cl 2 + H 2 O *The more reactive element can displace a less reactive element. O Ex: Copper can replace Silver, but the opposite can’t occur. 12. 2: Types of Chemical Reactions

12. 2: Types of Chemical Reactions Single Replacement

12. 2: Types of Chemical Reactions O Double Replacement Reaction A reaction in which ions from two compounds switch places. O AB + CD AD + CB

12. 2: Types of Chemical Reactions Double Replacement

Section 2: Types of Chemical Reactions O Combustion Reactions O A combustion reaction is a chemical reaction in which a substance combines with oxygen and releases energy O substance +O 2 substance(s) O Products are usually carbon dioxide and water

12. 3: Energy Changes and Chemical Reactions O The Law of Conservation of Energy O Similar to the law of conservation of mass O The law of conservation of energy states that ENERGY cannot be created or destroyed O Instead, energy changes forms O Stored in chemical bonds

O Exothermic Reaction in which energy is released. O EXO means “goes out” or “exits” O Ex: 2 Na + Cl 2 2 Na. Cl + energy O Energy listed in the products O Endothermic Reaction A reaction in which energy is taken in O ENDO means “goes in” O Ex: 2 H 2 O + energy 2 H 2 + O 2 O Energy listed in the reactants 12. 3 Energy and Rates of Chemical Reactions

12. 3: Energy Changes and Chemical Reactions O Activation Energy is the minimum amount of energy required to start a chemical reaction O Sources of Activation Energy O Friction O Electric spark O Light O Heat 28

12. 3: Energy Changes and Chemical Reactions O Factors Affecting Rates of Reactions O Surface Area O Temperature O Surface Area O Pressure O Concentration O Catalysts O Inhibitors

12. 3: Energy Changes and Chemical Reactions O Surface Area O Surface area is the amount of exposed, outer surface area of a substance O Increasing the surface area of solid reactants increases the rate of a reaction O More reactants exposed O More collisions

12. 3: Energy Changes and Chemical Reactions O Temperature O A higher temperature causes a faster rate of reaction O Particles move faster O More collisions O More energy O More have activation energy to react

12. 3: Energy Changes and Chemical Reactions O Concentration and pressure O A higher concentration of reactants causes a faster rate of reaction O More particles present O Smaller distance to travel O More collisions O A higher pressure pushes particles closer together and causes a faster rate of reaction O Smaller distance to travel O More collisions

12. 3: Energy Changes and Chemical Reactions O Catalysts O A catalyst is a substance that increases reaction rate by lowering the activation energy of a reaction O Is NOT changed or used up in the reaction O Allows a reaction to happen more quickly O You have thousands of them in your body O Enzymes are catalysts that speed up chemical reactions in living cells

12. 3: Energy Changes and Chemical Reactions O Inhibitors O An inhibitor is a substance that slows down, or even stops, a chemical reaction O A greater amount of inhibitor will slow down a chemical reaction

12. 3: Energy Changes and Chemical Reactions Factor Speeds up Reaction Surface Area Surface area Temperature Concentration Reactant concentration Pressure Catalysts Catalyst present Inhibitors No inhibitors present Slows down Reaction