Chemical Reactions Little book chapter 2 (page 26-54)

Section 1 Objectives Describe how chemical reactions produce new substances that have different chemical and physical properties. Identify four signs that indicate that a chemical reaction might be taking place. Explain what happens to chemical bonds during a chemical reaction.

Chemical Reactions A chemical reaction is a process in which one or more substances change to make one or more new substances. The chemical and physical properties of the substance will change.

Chemical Reactions Signs of a chemical reaction: Precipitate – solid substance that is formed in a solution. Light Heat change Electricity Color change Gas formation

Signs of chemical reactions The dark red precipitate is called silver chromate. It is formed when potassium chromate is added to a silver nitrate solution. Nitrogen dioxide gas is formed when copper is placed into nitric acid. Bleach reacts with the dye on the fabric and causes the color of the material to change. Energy is released during some chemical reactions.

Section 2 Objectives Interpret and write simple chemical formulas. Write and balance simple chemical equations. Explain how a balanced equation shows the law of conservation of mass.

Chemical Formulas The names of many covalent compounds use prefixes to express the number of atoms of each element. mono- 1 Hexa- 6 di- 2 hepta- 7 tri- 3 octa- 8 tetra- 4 nona- 9 penta- 5 deca- 10

Chemical Formulas 2 In an ionic compound the name of metallic element is written first and –ide is added to the nonmetal.

Chemical Equations A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas. Reactant + Reactant → Product Reactants are the substances that undergo the change. Products are the new substances formed. C + O2 → CO2 Reactants → Product

2 + 3 = 5 Antoine Lavoiser determined that the mass of the products is always equal to the mass of the reactants. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. In order to show mass is conserved an chemical equation must be balanced.

Balanced Equations Equations need to be balanced. Matter can not be created nor destroyed. This means there needs to be the same number of atoms on each side of the equation. The equation below is balanced S8 + 12 O2→8 SO3

Balanced Equations Subscript Coefficients small numbers to the lower right of chemical symbols represent the number of atoms of each element in the molecule Coefficients large numbers in front of chemical formulas represent the number of molecules of the substance in the reaction

How Many Atoms? Using coefficients and subscripts to count atoms in equations: number of atoms = coefficient X subscript Example: How many of each type of atom are represented by: 2Al2(SO4)3 # of Al atoms = 2 X 2 = 4 # of S atoms = 2 X 1 X 3 = 6 # of O atoms = 2 X 4 X 3 = 24

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Section 3 Objectives Describe four types of chemical reactions. Classify a chemical equation as one of four types of chemical reactions.

Types of Reactions A synthesis reaction is a reaction in which two or more substances form a single substance. (Simple → Complex) A + B → AB

A decomposition reaction is when a compound breaks down into two or more simpler substances. (Complex → Simple) AB → A + B

Types of Reactions Single Displacement reaction in which one element takes the place of another element in a compound. (One person breaks up a couple and goes out with one of them. A + BC→ AC + B

Types of Reactions Double Displacement reaction is one in which two different compounds exchange positive ions and form two new compounds. AB + CD → AD + CB

Energy Changes Chemical energy the energy stored in the chemical bonds of a substance. Breaking a chemical bond requires energy. During a chemical reaction, energy is either Gained released

Section 4 Objectives Compare exothermic and endothermic reactions. Explain activation energy. Interpret an energy diagram. Describe five factors that affect the rate of a reaction.

Conservation of Energy Review The law of conservation of energy states neither mass or energy can be created or destroyed in a chemical reaction. Energy can be transferred from one object to another.

IN/OUT EXOTHERMIC Reaction the energy released as the products form is greater than the energy required to break the bonds. Reaction in which energy is RELEASED Exo = EXITS Examples: Fire Digestion Hand warmers

IN/OUT ENDOTHERMIC Reaction Examples: MORE energy is required to break the bonds in the reactants than is released by the formation of the products. Reaction in which energy is taken in Endo = ENTERS Examples: Alka-seltzer Ice packets Camera

Reaction Rate Reaction rates tell you how fast a reaction is going.

Factors That Affect Reaction Rates Temperature – Increasing temperature increases the reaction rate. Stirring – increases the exposure of the reactant increases the reaction rate.

Factors That Affect Reaction Rates Surface Area Increasing surface area increases the reaction rate The larger the surface area, the faster the reaction rate Concentration increasing concentration increases the reaction rate.

Factors That Affect Reaction Rates Catalyst a substance that increases the reaction rate without being used up in the reaction. A substance that speeds up a reaction without being permanently changed. NOT A REACTANT!! Ex: Enzymes speed up reactions in your body.

Get Started Many reactions occur at room temperature, but some need a “little help” to get started. You may need to add heat (Lighting a candle) or pressure (Diesel engine) Start a reaction

Activation Energy ACTIVATION ENERGY Energy needed to START a reaction. This is represented by an activation curve on a graph.

Reaction Rate & Catalyst A catalyst works by decreasing the amount of activation energy required for the reaction to occur. Remember, reactions require activation energy catalyst simply lowers the amount of energy. You can speed up a reaction (rxn) rate by: Increasing Temperature Increase Surface Area Increase Concentration Using a catalyst A catalyst is a substance that speeds up a reaction without being part of the reaction.

Catalyst Continued Without Catalyst With Catalyst A catalyst is neither a reactant or a product When you finish the reaction all the catalyst is still there – none was used or changed in the reaction. Without Catalyst With Catalyst

Inhibit An inhibitor is a substance that slows down or stopping a reaction may sometimes be useful. Examples: Penicillin Preservatives Fungicide

Section 1 Review 1. Use the following terms in the same sentence: chemical reaction and precipitate. 2. Most chemical reactions: A. have starting substances that collide with each other. B. do not break bonds. C. do not rearrange atoms. D. cannot be seen. 3. If the chemical properties of a substance have not changed, has a chemical reaction occurred? 4. Steam is escaping from a teapot. Is this a chemical reaction? Explain.

Section 2 Review 1. Make the following statement TRUE: A chemical formula describes a chemical reaction 2. Make the following statement TRUE: The substances formed from a chemical reaction are reactants. A. have starting substances that collide with each other. B. do not break bonds. C. do not rearrange atoms. D. cannot be seen. 3. The correct chemical formula for carbon tetrachloride is: A. CCl3 B. C3Cl C. CCl D. CCl4

Section 3 Review 1. In your own words, write a definition for each of the following terms: synthesis reaction and decomposition reaction. 2. What type of reaction does the following equation represent? FeS + 2HCl -> FeCl2 +H2S A. synthesis reaction B. double-displacement reaction C. single-replacement reaction D. decomposition reaction 3. Describe the difference between single- and double-displacement reactions. 4. Write the balanced equation in which potassium iodide, KI, reacts with chlorine to form potassium chloride, KCl, and iodine.

Section 4 Review 1. Make the following TRUE: An exothermic reaction absorbs energy. 2. Make the following TRUE: The rate of a reaction can be increased by adding an inhibitor. 3. Which of the following will not increase the rate of a reaction? A. adding a catalyst B. increasing the temperature of the reaction C. decreasing the concentration of reactants D. grinding a solid into powder 4. How does the concentration of a solution affect the rate of the reaction?