Chemistry I Notes Unit 6: Chemical Reactions A chemical reaction

Chemistry I Notes Unit 6: Chemical Reactions A chemical reaction—process by which substances are changed into different substances. • Reactants—substances present at the beginning of a chemical reaction • Products—substances present after a chemical reaction. Law of conservation of mass—total mass of reactants must equal the total mass of products for a chemical reaction. A chemical equation represents, with symbols and formulas, the identities and amounts reactants and products in a chemical reaction. 2 Types of Chemical Reactions 1. Endothermic—chemical reaction in which energy is added (also called an endergonic reaction). 2. Exothermic—chemical reaction in which energy is released (also called an exergonic reaction). Indications of a Chemical Reaction Certain easily observed changes usually indicate that a chemical reaction has occurred. 1. Evolution of energy as heat and light 2. Production of a gas 3. Formation of a precipitate. • A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate. 4. Color change 7 Diatomic Elements N2 H2 O2 I2 Br2 Cl2 F2 A word equation is an equation in which the reactants and products in a chemical reaction are represented by words. example: methane + oxygen à carbon dioxide + water A formula equation represents the reactants and products of a chemical reaction by their symbols or formulas. example: The formula equation for the reaction of methane and oxygen is CH4(g) + O2(g) à CO2(g) + H2O(g) (not balanced) Symbols found in a chemical equation: Image Source: http://image.slidesharecdn.com/unit1ale-‐140911145133-‐phpapp02/95/unit-‐1-‐chemical-‐reactions-‐in-‐our-‐surroundings-‐8-‐
638.jpg?cb=1410447261 Balancing Chemical Equations 1. Write a word equation. water à hydrogen + oxygen 2. Write a formula equation from the word equation. H2O(l) à H2(g) + O2(g) (not balanced) 3. Balance the formula equation. A. Balance the different types of atoms one at a time. B. First balance the atoms of elements that are combined and that appear only once on each side of the equation. C. Balance H atoms and O atoms last D. Count atoms to be sure that the equation is balanced. 5 Major Types of Chemical Reactions: a. Synthesis (also known as a metathesis or combination reaction) b. Decomposition c. Single-‐displacement (single-‐replacement) d. Double-‐displacement (double-‐replacement) e. Combustion Other Types of Reactions f. Oxidation/reduction (redox) g. Acid/base *An equation may be classified more than one way. Synthesis (Composition/Metathesis) Reactions • In a synthesis reaction, also called a composition reaction, two or more substances combine to form a new compound. • In a synthesis reaction, simple compounds make more complex compounds. General equation: A + X à AX Two or more reactants à one product Predicting the Products of a Chemical Reaction Synthesis (Composition) Reactions • element + oxygen à oxide • element + sulfur à sulfide • element + halogen à ionic compound • metal oxides + water à metal hydroxides Decomposition Reactions • In a decomposition reaction, a single compound produces two or more simpler substances. • Opposite of synthesis reaction • More complex substances make less complex ones. General equation: AB à A + B One reactant à two or more products The decomposition of a substance by an electric current is called electrolysis. Decomposition Reaction • Binary compound à individual element • metal carbonates à metal oxide + carbon dioxide • metal chlorate à metal chloride + oxygen. • Acid à nonmetal oxides + water Single-‐Displacement Reactions • In a single-‐displacement reaction, also known as a single-‐replacement reaction, one element replaces a similar element in a compound. General Equation: A + BX à AX + B Element + compound à new compound + new element Single Replacement (Single Displacement) • Replacement of a metal in a compound by another metal (the two metals switch places). • Replacement of hydrogen in water by a metal produces a metal hydroxide and hydrogen. • Replacement of Hydrogen in an acid by a metal produces a metal compound and hydrogen • Replacement of halogens by another halogen (the two halogens change places). Double-‐Displacement Reactions • In double-‐displacement reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds. • Two atoms trade places General Equation: AX + BY àAY + BX compound + compound à 2 new compounds Double Replacement (Double Displacement) • Formation of a precipitate (ions of compounds change places) • Formation of a gas (ions change places, an insoluble gas is formed that bubbles out) • Formation of water (also called an acid-‐base reaction) Combustion Reactions • In a combustion reaction, a substance combines with oxygen, releasing a large amount of energy in the form of light and heat. example: combustion of propane C3H8(g) + 5O2(g) à 3CO2(g) + 4H2O(g) Substance + oxygen à CO2 + H2O Oxidation-‐Reduction (Redox) Reactions Redox occurs in many single displacement reactions. The redox component of these types of reactions is the change of charge on certain atoms. Reduction—gain of electrons Oxidation—loss of electrons Oxidation and reduction always occur in pairs. Example: Fe + CuSO4 → FeSO4 + Cu • In this reaction, Fe goes from not having a charge to having a +2 charge (as part of a compound) • Cu goes from having a +2 charge to not having a charge The loss of 1 or more electrons from a reactant is called oxidation. When a reactant is oxidized, it loses electrons and, consequently, energy. • The gain of 1 or more electrons is reduction. When a reactant is reduced, it gains electrons and, therefore energy. • The reactions of cellular respiration are oxidation-‐reduction (redox) reactions because electrons are transferred from one reactant to another during the electron transport chain. REMEMBER—“OILRIG”—Oxidation is Loss, Reduction is Gain Acid/Base Reactions An acid–base reaction is a chemical reaction that occurs between an acid and a base. Acid-‐Base reactions are often double displacement reactions. Some acid-‐base reactions are neutralization reactions. Most acid-‐base reactions have the general form: Acid + Base à Ionic Compound + H2O The goal of any chemical reaction is to create a more stable arrangement of matter. Through chemical reactions elements and compounds become more stable than they were before a chemical reaction. Summary of Major Reaction Types
5. Combustion
Image Source: https://lh6.googleusercontent.com/LASySxSAAUVRxeaAuh2vQXp0da1Is88RHnWTV5gX3mCcX7MNYMvnnXJeet_GPmtNPfbRlSt0nlTih-‐
_SF9oyNZMQJ5lhEgyS3krLsIknUe4zBod54C_DBS8fTXiLyejryA Image Source: http://www.iun.edu/~cpanhd/C101webnotes/chemical%20reactions/images/organicb.jpg Catalysts Many reactions occur with the use of a catalyst. Catalyst—substance that lowers the amount of activation energy required for a reaction to occur. • A catalyst speeds up the chemical reaction, but is not a part of the chemical reaction. Enzymes are biological catalysts. Chemical Kinetics Chemical Kinetics—branch of chemistry that deals with the rates of chemical reactions and how the reactions occur. 4 Factors that Affect Reaction Rates 1. The number of collisions between the substances that are reacting. Materials with more surface area will have a higher number of collisions (powders vs. big chunks). 2. Higher temperature is accompanied by an increase in the reaction rate. 3. Medium in which the reaction occurs. (Does the reaction occur in water, are the materials polar or nonpolar, are the materials solids, liquids or gases). 4. Presence of a catalyst. Equilibrium Reactions Reversible Reaction—a chemical reaction in which the products can react to re-‐form the reactants. Chemical Equilibrium—occurs when the rate of a forward reaction equals the rate of its reverse reaction and the concentrations of products and reactants remain unchanged. LeChatelier’s Principle If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established. 4 Factors that affect Equilibrium 1. Pressure 2. Volume 3. Temperature 4. Concentration An activity series—list of elements organized according to the ease with which the elements undergo chemical reactions. The ability of an element to react is referred to as the element’s activity. • The most-‐active element is placed at the top. • Used to help predict whether certain chemical reactions will occur. See Activity Series chart on next page. Image Source: http://crescentok.com/staff/jaskew/isr/tigerchem/reaction/series.gif

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