Unit 4 Lesson 1 Chemical Reactions Essential Question: How are Chemical Reactions Modeled? Copyright © Houghton Mifflin Harcourt Publishing Company

Essential Question: How are chemical reactions modeled? Key Topics Chemical reactions Chemical equations Energy Reaction rates

Properties and Changes in Matter What are the signs of a chemical reaction? p214 Properties and Changes in Matter A chemical reaction is the process in which atoms are rearranged to produce new substances. During a chemical reaction, the bonds that hold atoms together may be formed or broken. The properties of the substances produced in a chemical reaction differ from the properties of the original substances.

Evidence for Chemical Reactions What are the signs of a chemical reaction? p214 Evidence for Chemical Reactions Chemical reactions involve two main kinds of changes that you can observe: Formation of new substances and Changes in Energy. Color Change Precipitate may form (a solid substance from 2 liquids) Production of gas (bubbles) Change in Odor Change in Energy: absorb or release energy Endothermic Exothermic

Exothermic vs. Endothermic What happens to energy during a chemical reaction? Exothermic vs. Endothermic P218-219 Exothermic (Heat Out) Releases Energy – usually in the form of heat, light, and/or sound Endothermic (Heat In) Requires an input of energy Energy is constantly added for a chemical reaction to occur Energy is absorbed from the surroundings, usually as heat - which makes it get colder. Total amount of energy does not change Energy cannot be created or destroyed Law of Conservation of Energy

Describing Chemical Reactions How are Chemical Reactions Modeled? p215 Describing Chemical Reactions Cellular phone messages make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

How are Chemical Reactions Modeled? Chemical Formulas p215 A chemical formula uses chemical symbols and numbers to represent a given substance. The chemical symbols in a chemical formula tell you what elements make up a substance. The numbers written below and to the right of chemical symbols are called subscripts. Subscripts tell you how many of each type of atom are in a molecule.

How are Chemical Reactions Modeled? p215 Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction.

Matter cannot be created nor destroyed, only changes forms How do Chemical equations show the law of conservation of mass? p216 Conservation of Mass The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. Matter cannot be created nor destroyed, only changes forms

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? p216 Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? p216 Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? p216 Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? p216 Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

Balancing Chemical Equations How do Chemical equations show the law of conservation of mass? Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

Balancing Act

2 2 White, round in shape, crystal, solid Clear, has strong odor, Bubbled, Fizzed, bottom of watch glass felt warmer White solid crystal Gas (formation of bubbles) Clear – some may have evaporated Exothermic because it got warmer 2 2

Liquid – may have evaporated White, solid, crystal Turned yellow to brown to black Gave off an odor, Boiled (Bubbled) Black, Solid Liquid – may have evaporated Endothermic – because you had to keep heating it for the reaction to continue 12 11

Blue transparent liquid Silver, malleable, shiny Bubbled formed, aluminum turned black,, temperature started to rise Grey transparent liquid Reddish solid Exothermic because the temperature rose 3 3

Relating Cause and Effect Causes Effect Increased rate of reaction

The Need for Speed What affects the rates of reactions? p220 The rate of a reaction describes how quickly the reaction occurs. For a reaction to occur, particles of the reactants must collide. Reaction rates are affected by how often the particles collide. Factors that affect reaction rates include: concentration surface area Temperature presence of a catalyst.

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What affects the rates of reactions? p220 At higher concentrations, there are more reactants in a given volume, so the particles are more likely to collide and react. The reaction rate is higher when reactant concentration is higher. Crushing or grinding solids increases their surface area and the reaction rate, as more reactant particles are exposed to one another. Reactions usually occur more quickly at higher temperatures. The reactant particles move more quickly, so they are more likely to collide and react. A catalyst is a substance that changes the rate of a chemical reaction without being used up or changed very much.

Relating Cause and Effect Causes Effect Increased rate of reaction

Relating Cause and Effect Causes Increase in surface area Effect Increase in temperature Increased rate of reaction Increase in concentration of reactants Use of a catalyst

Graphic Organizer Chemical reactions Activation energy can be are written as balanced with which require written using Activation energy

Endothermic reactions Graphic Organizer Chemical reactions can be are written as Endothermic reactions Exothermic reactions Equations balanced with which require written using Coefficients Activation energy Chemical formulas Arrows and plus signs

Atoms, Periodic Table, Bonding – TEST answers