Notes CHEMICAL REACTIONS UNIT

 Think: When you hear the words “Chemical Reactions”, what comes to your mind?  Often times, people picture a scientist in a lab working with chemicals.  In real life, chemical reactions are taking place all around us when we breathe, eat, when fruit ripens, when we cook, and when plants go through photosynthesis. CHEMICAL REACTIONS

 A chemical reaction is a process that results in some chemical change to 1 or more substances.  If a substance changes in a way other than a physical change, a chemical reaction must have taken place. CHEMICAL REACTIONS

1.Release of a gas 2.Change in color/odor 3.Presence of a precipitate 4.Energy change (heat, light, etc.) 5.Change of identity of substances SIGNS THAT A CHEMICAL REACTION HAS TAKEN PLACE

 Reactants: substances that undergo a chemical change  Products: substances that form as a result of a chemical change  Reactants → Products  *Note: Products and reactants contain the same types of atoms. Remember the law of conservation of mass: Mass cannot be created or destroyed REACTANTS & PRODUCTS

 In order for chemical bonds to be broken, enough energy to overcome the attractive forces of the reactants must be added.  Chemical Energy: the energy stored within atoms and molecules that can be released when a substance reacts  The total energy before a reaction is equal to the total energy after a reaction. ENERGY AND CHEMICAL REACTIONS

1.Exothermic Reaction: a chemical reaction that gives off energy in the form of heat 2.Endothermic Reaction: a chemical reaction in which energy is absorbed from the surrounding area. 2 TYPES OF REACTIONS IN TERMS OF ENERGY

 exothermic+reactions exothermic+reactions  ENDOTHERMIC VS. EXOTHERMIC

 Chemical Equation: An equation that uses chemical formulas and symbols to show the reactants & products in a reaction.  In a chemical equation: the reactants, on the left side of the arrow, form products, on the right side of the arrow.  The symbol “ → ” means “gives” or “yields” CHEMICAL EQUATIONS

 An equation is balanced when the number of atoms of each element on the right side equals the number of atoms of each element on the left side.  A balanced chemical equation follows the Law of Conservation of Mass BALANCING CHEMICAL EQUATIONS

1.You CANNOT balance an equation by changing the chemical formulas.  You must leave the subscripts in the formulas alone!! 2.An equation can be balanced only by: putting numbers, called coefficients, in front of the chemical formulas.  The coefficients tell how many molecules of that compound are present. 3.The numbers of atoms for each element must be the same on each side of the equation. RULES FOR BALANCING CHEMICAL EQUATIONS

1.Determine number of atoms for each element. 2.Pick an element that is not equal on both sides of the equation. 3.Add a coefficient in front of the formula with that element and adjust your counts. 4.Continue adding coefficients to get the same number of atoms of each element on each side. STEPS USED TO BALANCE A CHEMICAL EQUATION

 Mg + O 2 → MgO Mg= O= Whenever you have a coefficient of 1, it is NOT written!! EXAMPLE #1

 CuSO 4 + Al → Al 2 (SO 4 ) 3 + Cu EXAMPLE #2

 The law of conservation of mass is followed in chemical equations: The total mass of the reactants is the same as the total mass of products. LAW OF CONSERVATION OF MASS

 The law of definite proportions: A compound ALWAYS contains the same elements in the same proportion, regardless of how the compound is made or how much of the compound is formed. LAW OF DEFINITE PROPORTIONS

There are 6 factors that affect the rate of chemical reactions: 1.Increase in temperature: Why? The particles are moving faster and have more chances to collide into each other to make a reaction. 2.Increase in Surface area: Why? More of the substance is exposed, so the particles have more opportunities to come into contact with each other.  Examples: crushing, chopping up RATES OF CHEMICAL REACTIONS

3.Increase in concentration: Why? Increasing the number of particles increases chance of collision resulting in a reaction. 4.Increase in pressure: Why? Particles are squeezed into a smaller volume, so there is less space and more collisions occur between particles. RATES OF CHEMICAL REACTIONS

5.Nature of the reactants: a. Size and Shape: Why? Heavier particles move slower than lighter particles at the same temperature & have less opportunity to collide with other molecules. b. Nature of reactant: Why? Some molecules must fit together in a certain way in order to react. RATES OF CHEMICAL REACTIONS

6.Adding a catalyst: A substance that alters the rate of a reaction, but is left unchanged in the reaction. a. Inhibitors: catalysts that slow down reactions. b. Enzymes: biological catalysts that speed up a biochemical reaction. RATES OF CHEMICAL REACTIONS