Chapter 15 Chemical Reactions

Section 1: Forming New Substances Chemical reaction – the process by which one or more substances undergo change to produce one or more new substances The new substances have different chemical and physical properties compared to the original substances New substances are formed in a chemical reaction because chemical bonds in the starting substances break, atoms rearrange, and new bonds form to make new substances

Clues to Chemical Reactions There are several clues that indicate when chemical reaction might be occurring Clues include: Gas Formation Solid Formation – the solid formed in a solution is called a precipitate Color Change Energy Change

Chemical Formulas Chemical Formula – a shorthand notation for a compound or a diatomic element using chemical symbols and numbers A chemical formula indicates how many of each kind of atom is present in a molecule Subscript – a number written below and to the right of a chemical symbol in a formula

Writing Formulas for covalent Compounds Covalent compounds are usually composed of two nonmetals The names of covalent compounds use prefixes to tell you how many atoms each element are in the formula

Writing Formulas for Ionic Compounds If the name of the compound contains the name of a metal and nonmetal, the compound is probably ionic To write the formula for an ionic compound, you must make sure the compounds overall charge is zero The formula must have subscripts that cause the charges of the ions to cancel each other out Remember the charge of many ions can be determined by looking at the periodic table

Chemical Equations Chemical Equation – a shorthand description of a chemical reaction using formulas and symbols i.e. 2 H2 + O2 → 2 H2O Reactants – the starting materials in a chemical reaction (to the left of the arrow) Products – the substances formed from a chemical reaction (to the right of the arrow) The symbol or formula for each substance in the reaction must be written correctly

Chapter 15 – Quiz 1 1) _________ is the process by which one or more substances undergo change to produce one or more new substances 2) Name two of the four clues that a chemical reaction has occurred 3) _________ is a shorthand notation for a compound or molecule using chemical symbols and numbers 4) _________ is the number written behind a symbol 5) __________ a shorthand description for a chemical reaction using chemical formulas and symbols

Section 2: Types of Chemical Reactions Most reactions can be grouped into one of four categories: synthesis, decomposition, single replacement, and double replacement

Synthesis Reactions Synthesis Reaction - a reaction in which two or more substances combine to form a single compound i.e. 2 H2 + O2 —> 2 H2O

Decomposition Reactions Decomposition Reaction – a reaction in which a single compound breaks down to form two or more simpler substances i.e. 2H2O → 2H2 + O2

Single Replacement Reactions Single-replacement reaction – a reaction in which an element takes the place of another element that is part of a compound i.e. Zn + 2HCl → ZnCl2 + H2

Double Replacement Reactions Double – replacement reaction – a reaction in which ions in two compounds switch places i.e. NaCl + AgNO3 → NaNO3 + AgCl

Section 3: Energy and Rates of Chemical Reactions All chemical reactions involve chemical energy, the energy stored in chemical bonds. Energy is released or absorbed in the overall reaction depending on how the chemical energy of the reactants compares with the chemical energy of the products

Exothermic vs. Endothermic Reactions If the chemical energy of the reactants is greater than the chemical energy of the products, energy is released during the reaction Exothermic – a chemical reaction were energy is released If the chemical energy of the reactants is less than the chemical energy of the products, the difference in energy is absorbed during the reaction Endothermic – a chemical reaction in which energy is absorbed

Energy is Conserved – It’s a Law! The law of conservation of energy – states that energy can neither be created or destroyed. i.e. the energy released in exothermic reactions was originally stored in the bonds of the reactants Also the energy absorbed in endothermic reactions is stored in the bonds of the products Activation energy – the minimum amount of energy needed for substances to react

Factors Affecting Rates of Reactions 1) Temperature An increase in temperature will increase the rate of reaction as the reactant particles move faster. A decrease will slow the reaction down 2) Concentration Increasing the concentration of the reactants increases the reaction rate as there is a greater chance for the molecules to collide. Decrease concentration and the reaction slows down

Factors Affecting Rates of Reactions (con’t) 3) Surface Area Increasing surface area increases the rate of reaction as more reactant particles are exposed to each other. Decrease surface area and the reaction slows down 4) Catalysts and Inhibitors Catalysts – a substance that speeds up a reaction without being permanently changed Inhibitor – a substance that slows down or stops a chemical reaction