CHEMICAL REACTIONS CH 5

5.1 NATURE OF CHEMICAL REACTIONS Chemical Reactions Change Substances Chemical reactions occur when substances undergo chemical changes to form new substances. You will be able to see changes: color change, light, bubbles In Bread making yeast + sugar carbon dioxide gas

Chemical reactions rearrange atoms --- new substances formed Gasoline (isooctane) reacts with oxygen to produce carbon dioxide, water, and energy C 8 H 18 + O 2 CO 2 + H 2 O Isooctane and oxygen are the reactants. (the substances or molecules that participate in a chemical reaction). Carbon dioxide, water and energy are the products (substances/molecules that form in a chemical reaction)

Reactants and Products contain the same type of atoms: carbon, hydrogen, oxygen New atoms not created. Old atoms are not destroyed Atoms are rearranged as bonds are broken and new bonds are formed. In ALL chemical reactions, mass is ALWAYS conserved

Energy and Reactions Most reactions require energy to get started. For isooctane-oxygen reaction, only a small spark is needed. All the bonds have to be broken. Energy used to break the bonds. Many forms of energy can be used to break bonds: heat, light, sound, electricity Energy can also be transferred in the reaction. When new bonds form, energy is released such as heat, light, sound

Energy is conserved in chemical reactions May not appear to be. Tiny spark sets off a large explosion (Fertilizer plant in Texas) Some of the energy comes from the bonds between the atoms in the reactants. This stored energy is called chemical energy. The total energy before the reaction EQUALS the total energy after the reaction

Exothermic Reactions Chemical reaction in which heat is released to the surroundings Temperature of the environment increases. The reaction will feel warm All combustion reactions are exothermic A common exothermic reaction is the setting of cement and plaster of paris

Endothermic reactions Chemical reactions that require heat. Temperature of the environment decreases. Reaction will feel cold. More energy is needed to break the bonds in the reaction than is given off by forming bonds in the products. EX: barium hydroxide + ammonium nitrate Some endothermic reactions cannot get enough heat from the surroundings, so additional heat must be added to cause the reaction to take place. Ex. Baking process (bread, cakes, cookies, etc…

5.3 BALANCING CHEMICAL EQUATIONS Describing Reactions Chemical equations summarize reactions Chemical equations are a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products. CH 4 + O 2 CO 2 + H 2 O Reactants are on the left of the arrow. Products are on the right of the arrow. Arrow means “yields” or “gives” To be useful, chemical equations must be “balanced”. The number of atoms of each element on the left side of the arrow must equal the number of atoms of each element on the right side. Obeys the law of conservation of mass.

How to Balance Chemical Equations If the equation is given to you, start at step 4 1.Identify the reactants & products 2.Write a word equation for the reaction (optional step) 3.Write the equation using the formula for the elements and compounds in your word equation. 4.Start with the most complex compound in the equation. 5.Balance the compound one element at a time. 5.Once that is done move on to the next substance in the equation. Balance it one element at a time. 6.You cannot change subscripts. That would change the nature of the substance. 7.You can only add/insert numbers in front of reactants or products. These numbers are called coefficients.

5.2 REACTION TYPES Classifying Reactions 5 Types of Reactions 1. Synthesis Reaction Called polymerization Many small molecules join together in chains to make larger structures called polymers. Ex: polyethene – black plastic trash bag material Ex: Water. Hydrogen reacts with oxygen to form water General Equation Form: A + B AB 2Na + Cl 2 2NaCl Product is always a more complex compound than the reactants.

2. Decomposition Reaction Reaction in which a single compound breaks down to form two or more simpler substances Ex, Digestion Ex. “Cracking” - process by which crude oil is broken down to form simpler substances such as gasoline and other fuels. General Equation Form: AB A + B 2H 2 O 2H 2 + O 2 Electrolysis – process in which an electric current is used to produce a chemical reaction.

3.Combustion Reaction The oxidation reaction of an organic compound in which heat is released Oxygen is reactant in every combustion reaction. One of the products will always contain oxygen. Water is a common product of combustion reactions. The amount of oxygen present determines the kind of product that may form. Ex: 2CH 4 + 4O 2 2CO 2 + 4H 2 O 2CH 4 + 3O 2 2CO +4 H 2 0 2CH 4 + 2O 2 2C + 4H 2 0

4. Single-Displacement Reaction A reaction in which one element or radical takes the place of another element or radical in a compound. General formula: AX + B BX + A Ex: 3CuCl 2 + 2Al2AlCl 3 + 3Cu Generally, a more reactive element takes the place of a less reactive one. All alkali metals undergo single-displacement reactions Alkali metals react with water to form ions Ex. K + H 2 O KOH + H 2 Na + H 2 O NaOH + H 2 (Very exothermic. H 2 may explode & burn instantly)

5. Double-Displacement Reaction A reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between compounds. General Formula: AX + BY AY + BX Basically an exchange of ions occurs in the reaction Ex: Pb(NO 3 ) 2 + K 2 CrO 4 Pb CrO 4 + 2KNO 3 In this reaction a yellow precipitate forms (lead chromate). Same yellow stuff that is painted on roads

Electrons & Chemical Reactions Another way to classify reactions. Involve the transfer of electrons. Oxidation-Reduction (Redox) Reactions – any chemical change in which one species is oxidized (loses electrons) and another species is reduced (gains electrons). Ex. Formation of rust. Fe + O 2 Fe 2 O 3 Substances that accept (gains) electrons are said to be “reduced” (Gain of electrons will reduce the positive charge on the ion. Will become more negative) Substances that give up (lose) electrons are said to be “oxidized”

In all redox reactions one or more reactants are reduce and one or more reactants are oxidized. Some redox reactions do not involve ions. In these reaction, oxidation is a gain of oxygen or a loss of hydrogen, and reduction is the loss of oxygen or a gain of hydrogen. Respiration and combustion are redox reactions because oxygen gas reacts with carbon compounds to form carbon dioxide. Carbon atoms in CO 2 are oxidized and oxygen (O 2 ) atoms are reduced.

5.4 RATES OF CHANGE Factors Affecting Reaction Rates 1. Most reactions go faster at higher temperatures. Kinetic theory states that particles move faster at higher temperatures. Faster particles cause more collisions between particles More collisions create more chances for particles to react. 2. A large surface area speeds up reactions Increases amount of exposure to other reactants. Increase exposure results in more reactions. 3. Concentrated solutions react faster Higher chance for particle collision because there are more particles to react with. 4. Reactions are faster at higher pressure Causes particles to have more collisions with other particles.

5. Massive, bulky molecules react slower Move more slowly than less massive molecules at the same temperature. Some large biological compounds must fit together in a particular way to react. If they do not collide in a certain way, they will not react with other compounds. 6. Catalysts change the rates of chemical reactions. Catalysts are substances that change the rate of a chemical reaction without being consumed or changed significantly. Catalysts are added to make a reaction go faster. Catalysts are not reactants or products. Catalysts that slow a reaction are called inhibitors. Many catalysts can be reused, cleaned or renewed. Sometimes the catalyst’s name is written over the arrow in the chemical equation. 7. Enzymes are biological catalysts They are proteins that speed up metabolic reactions in plants and animals without being permanently changed or destroyed. Very specific Controls one reaction or set of similar reactions Most fragile. Will not work or will decompose if temperature is too cold or too warm.

CH 4 + O 2 CO 2 + H 2 O Manganese dioxide, MnO 2, reacts with hydrogen peroxide, H 2 O 2, to form water, H 2 O, and oxygen gas, O 2. The manganese oxide remains unchanged in the reaction.

ASSIGNMENT COMPLETE 6.3 BALANCING CHEM. EQUATIONS