Chemical Reactions

1. Chemical Reactions Preview Section 1 Forming New Substances Section 2 Chemical Formulas and Equations Section 3 Types of Chemical Reactions Section 4 Energy and Rates of Chemical Reactions Concept Mapping

2. Section 1 Forming New Substances Bellringer Write an answer to the following question in your science journal: What do you think baking bread, launching the space shuttle, and digesting food have in common? Can you think of three other unlikely processes that are similarly linked?

3. Section 1 Forming New Substances 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.

4. Section 1 Forming New Substances Chemical Reactions • Achemical reactionis a process in which one or more substances change to make one or more new substances. • The chemical and physical properties of the new substances differ from those of the original substances.

5. Section 1 Forming New Substances Chemical Reactions, continued • Signs of Chemical Reactions include color changes, gas formation, the formation of aprecipitate(a solid substance formed in a solution), and energy given off as light, thermal energy, or electrical energy. • A Change of PropertiesThe most important sign of a chemical reaction is the formation of new substances that have different properties.

6. Section 1 Forming New Substances Bonds: Holding Molecules Together • A chemical bond is an attraction that holds atoms together in a molecule. • Breaking and Making BondsIf molecules bump into each other with enough energy, the chemical bonds in the molecules break. The atoms then rearrange, and new bonds form to make new substances.

7. Section 1 Forming New Substances

8. Section 1 Forming New Substances Bonds: Holding Molecules Together, continued • New Bonds, New SubstancesWhenever new substances form, their properties differ from the properties of the starting substances. • For example, sodium is a violently reactive metal and chlorine is a greenish poisonous gas. The two elements combine to form a white solid called sodium chloride, or table salt.

9. Section 2 Chemical Formulas and Equations Bellringer Write the following chemical symbols in yourscience journal. Then, try to write the names of the elements matching the symbols. Check your answers with the periodic table in your book. H O C Na N Cl K Mg Ca Al Au F Cu Fe

10. Section 2 Chemical Formulas and Equations 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.

11. Section 2 Chemical Formulas and Equations Chemical Formulas • Achemical formulais a combination of chemical symbols and numbers to represent a substance. A chemical formula shows how many atoms of each kind are present in a molecule.

12. Section 2 Chemical Formulas and Equations Chemical Formula Click below to watch the Visual Concept. Visual Concept

13. Section 2 Chemical Formulas and Equations Chemical Formulas, continued • Writing Formulas for Covalent Compounds You can use the name of a covalent compound to write its chemical formula. The names of covalent compounds use prefixes. Each prefix represents a number, as shown below.

14. Section 2 Chemical Formulas and Equations Chemical Formulas, continued • Writing Formulas for Ionic CompoundsTo write the formula for an ionic compound, make sure the compound’s charge is 0. • The formula must have subscripts that cause the charges of the ions to cancel out.

15. Section 2 Chemical Formulas and Equations

16. Section 2 Chemical Formulas and Equations Chemical Equations • Describing Reactions by Using EquationsAchemical equationuses chemical symbols and formulas as a shortcut to describe a chemical reaction. • From Reactants to ProductsThe starting materials in a reaction arereactants.The substances formed from a reaction areproducts.

17. Section 2 Chemical Formulas and Equations

18. Section 2 Chemical Formulas and Equations Chemical Equations, continued • The Importance of AccuracyThe symbol or formula for each substance in a chemical equation must be written correctly or it will not correctly describe the reaction. Some formulas and symbols can be confused.

19. Section 2 Chemical Formulas and Equations Chemical Equations, continued • The Reason Equations Must Be BalancedAtoms are never lost or gained in a chemical reaction, they are just rearranged. Every atom in the reactants becomes part of the products. • When writing a chemical equation, make sure the number of atoms of each element in the reactants equals the number of atoms of those same elements in the products. This is called balancing the equation.

20. Section 2 Chemical Formulas and Equations Chemical Equations, continued • In the 1700s, French chemist Antoine Lavoisier found that the total mass of the reactants was always the same as the total mass of the products. • Lavoisier’s work led to the law of conservation of mass,which states that mass is neither created nor destroyed in ordinary chemical and physical changes.

21. Section 2 Chemical Formulas and Equations Chemical Equations, continued • How to Balance an EquationTo balance an equation, you must use coefficients. A coefficient is a number that is placed in front of a chemical symbol or formula. • For an equation to be balanced, all atoms must be counted. So, you multiply the subscript of each element in a formula by the formula’s coefficient. • The next slide shows how to use coefficients to balance an equation.

22. Section 2 Chemical Formulas and Equations

23. Section 3 Types of Chemical Reactions Bellringer Do you think that products of a reaction are always more complex than the reactants? Could products be simpler than the reactants? Use what you have learned about chemical reactions to write a short explanation for your answers in your science journal.

24. Section 3 Types of Chemical Reactions Objectives • Describe four types of chemical reactions. • Classify a chemical equation as one of four types of chemical reactions.

25. Section 3 Types of Chemical Reactions Synthesis Reactions • Asynthesis reactionis a reaction in which two or more substances combine to form one new compound. • For example, a synthesis reaction takes place when sodium reacts with chlorine to produce sodium chloride, which you know as table salt.

26. Section 3 Types of Chemical Reactions Decomposition Reactions • Adecomposition reactionis a reaction in which a single compound breaks down to form two or more simpler substances. • Decomposition is the reverse of synthesis.

27. Section 3 Types of Chemical Reactions Single-Displacement Reactions • Asingle-displacement reactionis a reaction in which an element replaces another element that is part of a compound. The products of single-displacement reactions are a new compound and a different element. • Reactivity of ElementsIn a single-displacement reaction, a more reactive element can displace a less reactive element in a compound.

28. Section 3 Types of Chemical Reactions Double-Displacement Reactions • Adouble-displacement reactionis a reaction in which ions from two compounds exchange places. One of the products of this type of reaction is often a gas or a precipitate. • The next slide shows models of each of the four types of chemical reactions.

29. Section 3 Types of Chemical Reactions

30. Section 4 Energy and Rates of Chemical Reactions Bellringer Now that you know a little about chemical reactions, think about the many chemical reactions that take place around you every day. Describe your “favorite” chemical reaction. Use your imagination. Remember how diverse chemical reactions really are. How do you think energy is involved in the reaction? Record your thoughts in your science journal.

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

32. Section 4 Energy and Rates of Chemical Reactions Reactions and Energy • By comparing the chemical energy of the reactants with the chemical energy of the products, you can decide if energy is released or absorbed in the reaction. • Exothermic Reactionsare reactions in which energy is released. Energy can be released as light, electrical energy, or thermal energy.

33. Section 4 Energy and Rates of Chemical Reactions Reactions and Energy, continued • Endothermic Reactionsare reactions in which energy is taken in. • The Law of Conservation of Energystates that energy cannot be created or destroyed. The energy released in exothermic reactions was first stored in the chemical bonds of the reactants. And the energy taken in during endothermic reactions is stored in the products.

34. Section 4 Energy and Rates of Chemical Reactions Rates of Reactions • The speed at which new particles form is called the rate of a reaction. • Activation Energyis the smallest amount of energy that molecules need to react. A chemical reaction needs a boost of energy greater than or equal to the activation energy before the reaction can start.

35. Section 4 Energy and Rates of Chemical Reactions Rates of Reactions, continued • Sources of Activation EnergyFriction is one source of activation energy. In a match, friction provides the energy needed to break the bonds in the reactants and allow new bonds to form. • An electric spark in a car’s engine is another source of activation energy. This spark begins the burning of gasoline. • Light can also be a source of activation energy for a reaction.

36. Section 4 Energy and Rates of Chemical Reactions Activation Energy and Chemical Reactions Click below to watch the Visual Concept. Visual Concept

37. Section 4 Energy and Rates of Chemical Reactions Factors Affecting Rates of Reactions • TemperatureA higher temperature causes a faster rate of reaction. At high temperatures, particles of reactants move quickly and collide hard and often. • At low temperatures, particles move slowly and collide less often.

38. Section 4 Energy and Rates of Chemical Reactions Factors Affecting Rates of Reactions, continued • Concentration In general, a high concentration of reactants causes a fast rate of reaction. Concentration is a measure of the amount of one substance when it is dissolved in another substance. • When concentration is high, there are many reactant particles in a given volume. So, there is little distance between particles and the particles collide more often and react faster.

39. Section 4 Energy and Rates of Chemical Reactions Factors Affecting Rates of Reactions, continued • Surface Area is the amount of exposed surface of a substance. Increasing the surface area of solid reactants increases the rate of a reaction. • Inhibitorsare substances that slow down or stop a chemical reaction. The rate of a reaction decreases in the presence of an inhibitor.

40. Section 4 Energy and Rates of Chemical Reactions Factors Affecting Rates of Reactions, continued • Catalysts are substances that speed up a reaction without being permanently changed. Because it is not changed, a catalyst is not a reactant. • A catalyst lowers the activation energy of a reaction, which allows the reaction to happen more quickly. • Catalysts called enzymes speed up most reactions in your body.

41. Chemical Reactions Concept Mapping Use the terms below to complete the concept map on the next slide. products coefficients reactants chemical reactions chemical equations chemical formulas

42. Chemical Reactions

43. Chemical Reactions