Observing Chemical Change Introduction to Chemistry Chapter 5

Observing Chemical Change Introduction to Chemistry Chapter 5 Lesson 1 By the end of this lesson, the student should be able to: 1. Explain how changes in matter can be described. 2. Identify ways to tell that a chemical reaction has occurred. DCI-MS-PS 1. B. 1 Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants.

Chemistry in the Kitchen Planet Diary Teen chef Fatoumata Dembele knows that chemical reactions are an important part of cooking great food. In fact, Fatoumata is so skilled at using chemistry in the kitchen that she won an award for her recipes. � Fatoumata knows that to prepare some foods, such as eggs, adding heat is required. Other foods, such as gelatin, need to have heat removed to taste best. Fatoumata says you have to keep a close eye on food while it’s cooking. A good chef always pays attention to signs of change. � For example, when you cook meat, the color is what tells you when it’s ready. A raw steak is red, but a medium steak should be dark brown on the outside and pink in the center. Fatoumata prefers her steak well done. She knows it’s ready when the meat is brown all the way through. For chefs like Fatoumata, there is one particular property that matters the most. It’s the taste! �

Describing Changes in Matter � Matter is often described by its characteristics, or properties, and how it changes. The two kinds of properties of matter are physical properties and chemical properties. � A physical property is a characteristic of a substance that can be observed without changing the substance into another substance. The temperature for melting a solid metal is a physical property. Color, texture, density, and conductivity are physical properties of matter.

Describing Changes in Matter � A chemical property is a characteristic of a substance that describes its ability to change into another substance. To observe the chemical properties of a substance, it must change or be changed into another substance. A chemical property can be a material’s flammability or its ability to rust or tarnish. � Changes in matter can be described in terms of physical changes and chemical changes. A physical change is any change that alters the form or appearance of the substance but does not change it into another substance.

Describing Changes in Matter � A change in matter that produces one or more new substances is a chemical change or chemical reaction. In a chemical change, the atoms rearrange to form new substances. When a substance undergoes a chemical change, it results in different physical properties as well. � Substances that undergo chemical changes are called reactants. The new substances that form are the products. Chemical changes occur when existing bonds break and new bonds form. New substances are produced.

Chemical Reactions �Chemical reactions involve changes in properties and changes in energy that you can observe. One way to detect chemical reactions is to observe changes in physical properties of materials. �For instance, formation of a precipitate, gas production, and a color change are possible evidence that a chemical reaction has taken place.

Chemical Reactions A chemical reaction occurs when atom bonds break and new bonds form. Breaking bonds requires energy, while forming bonds releases energy. � In an exothermic reaction, energy released as the products form is greater than the energy required to break the bonds of the reactants. � The energy is usually released as heat. In an endothermic reaction, more energy is required to break the bonds of the reactants released by the formation of the products. When energy is absorbed, the surroundings become cooler. �

Physical or Chemical Change Physical Change � When you cut an apple in half, you cause a physical change in matter. A physical change is any change that alters the form or appearance of a substance but does not change it into another substance. � Some of the physical properties of the material may be altered, but the chemical composition remains the same. Bending, crushing, and cutting are all physical changes. Changes in the state of matter, such as melting, freezing, and boiling, are also physical changes. �

Physical or Chemical Change � When you leave a cut avocado out in the air, it turns brown because it goes through a chemical change. In a chemical change, or chemical reaction, the atoms rearrange to form new substances. Oxygen in the air reacts with compounds in the avocado to form new compounds. � When a substance undergoes a chemical change, it results in different physical properties as well. Burning and rusting are both chemical changes. A substance that undergoes a chemical change is called a reactant; the new substance that forms is the product. �

Describing Chemical Reactions Objective: By the end of this lesson, the student should be able to: 1. Identify the information included in a chemical equation. 2. Explain how mass is conserved during a chemical reaction. 3. Identify three categories of chemical reactions. � Performance Expectation: DCI-MS-PS 1. B. 1 Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants. �

Lifesaving Reactions What moves faster than 300 km/h, inflates in less than a second, and saves lives? An airbag, of course! Did you know that the “air” in an airbag is made by a chemical reaction? A compound called sodium azide (Na. N 3) breaks down into sodium metal (Na) and nitrogen gas (N 2). The nitrogen gas fills the airbag and cushions the passengers in an accident. � It’s important that the correct amount of sodium azide is used. The mass of sodium azide in the airbag before the collision will equal the mass of sodium and nitrogen that is made by the reaction. If too little or too much nitrogen gas is made, the airbag will not inflate properly. �

What Information Does a Chemical Equation Contain? � � � A chemical equation is a way to show a chemical reaction, using formulas and other symbols instead of words. The formula of a compound identifies the elements in the compound and the ratio in which the atoms or ions combine. A chemical equation tells you the substances you start with (reactants) in a reaction and the substances that are formed at the end (products). The formulas for the reactants are written on the left, followed by an arrow, which is read as “yields. ” The formulas for the products are written to the right of the arrow. Plus signs are used to separate two or more reactants or products.

How Is Mass Conserved During a Chemical Reaction? � � � � In a chemical reaction, all of the atoms present at the start of the reaction are present at the end of the reaction. Atoms are neither created nor destroyed. They are rearranged to form one or more new substances. This principle is called the law of conservation of mass. All chemical reactions must obey this law. If a reaction takes place in an open system, matter can enter from or escape to the surroundings. In a closed system, matter does not enter or leave.

How Is Mass Conserved During a Chemical Reaction? � To obey the law of conservation of mass, a chemical equation must be balanced by showing the same number of atoms of each element on both sides of the equation. � To begin, write the correct chemical formulas for both the reactants and the products, count the number of atoms of each element on each side of the equation, use coefficients, which are numbers placed in front of a chemical formula in an equation, and then check that the equation is balanced.

What Are Three Types of Chemical Reactions? � Three types of chemical reactions are synthesis, decomposition, and replacement. � In a synthesis reaction, two or more elements or compounds combine to form a more complex substance. � A decomposition reaction occurs when compounds break down into simpler products. � In a replacement reaction, one element replaces another element in a compound (single replacement) or two elements in different compounds trade places (double replacement).

What Are Three Types of Chemical Reactions?

Balancing Equations Introduction to Balancing Equations � Chemists use chemical equations to describe a reaction. � To do so accurately, a chemical equation must show the same number of each type of atom on both sides of the equation. � Chemists say an equation is balanced when it accurately represents conservation of mass. � https: //www. pearsonrealize. com/community/program/b 5 aea 82 b-ad 2 c-3447 -b 122 -4 b 953 e 6 a 7 cba/18/tier/3 bbae 27 e-4 e 71 -3393 ac 93 -e 88047 fe 1 e 01/16/lesson/a 89 d 4 a 3 f-f 3 da-323 c-be 582 d 8 ceed 23534/16/content/87 d 4862 d-000 e-3 f 8 b-a 26 d 95 ec 11 ae 5185/17 �

Conservation of Matter Chemical Change A change in matter that produces one or more new substances is a chemical change, or a chemical reaction. In one type of chemical change, a single substance changes to one or more other substances. In other chemical changes, two or more substances combine to form different substances. � Methane Combustion When substances such as methane burn in air, the chemical change that occurs is called combustion. When methane burns, each methane molecule rapidly combines with two oxygen molecules. � Products of a Chemical Change One carbon dioxide gas molecule and two water vapor molecules are made from the combustion reaction. � https: //www. pearsonrealize. com/community/program/b 5 aea 82 b-ad 2 c-3447 b 122 -4 b 953 e 6 a 7 cba/18/tier/3 bbae 27 e-4 e 71 -3393 -ac 93 e 88047 fe 1 e 01/16/lesson/a 89 d 4 a 3 f-f 3 da-323 c-be 582 d 8 ceed 23534/16/content/c 87 d 6163 -a 906 -3547 -9113 -a 0 ec 843 a 59 d 1/17 �

Controlling Chemical Reactions Objective: By the end of this lesson, the student should be able to: 1. Explain how activation energy is related to chemical reactions. 2. Identify factors that affect the rate of a chemical reaction. � Performance Expectations: DCI-MS-PS 1. A. 6 The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter. � DCI-MS-PS 1. B. 3 Some chemical reactions release energy, others store energy. �

Up in Flames � � � � On May 6, 1937, the Hindenburg airship was consumed by flames while landing at Lakehurst Naval Air Station in New Jersey. Thirty-five of the 97 people on board and one person on the ground died in the accident. The cause of the fire, which destroyed the ship within one minute, is still unknown. Most people now believe that an electrical spark started the fire. However, several theories have been proposed to explain what caught fire first. One theory states that the spark ignited the flammable hydrogen inside the blimp. Another theory proposes that the paint on the fabric covering the ship caught fire first, setting off a reaction in the hydrogen.

How Do Reactions Get Started? � � � � Chemical reactions need energy to break the chemical bonds of the reactants so that the atoms can be rearranged to form the new bonds of the products. Activation energy is the minimum amount of energy needed to start a chemical reaction. All chemical reactions need a certain amount of activation energy to get started. Usually, once a few molecules react, the rest will follow. The first few reactions provide the activation energy for more molecules to react. On an energy graph, activation energy is the top of the energy curve. Both endothermic and exothermic reactions require activation energy.

What Affects the Rate of a Chemical Reaction? � Chemical reactions do not all occur at the same rate. � A particular reaction can occur at different rates depending on the conditions. � A chemical reaction will happen faster if there are more reactant particles, if the reactant particles react more quickly, or if the reactant particles react with more energy.

What Affects the Rate of a Chemical Reaction? � Factors that can affect rates of reactions include surface area, temperature, concentration, and the presence of catalysts and inhibitors. � Factors that increase the rate of a chemical reaction include increases of surface area, where reactant particles react more quickly; temperature, when reactant particles react with more energy; and concentration, where more reactant particles react. � Concentration refers to the amount of a substance in a given volume.

What Affects the Rate of a Chemical Reaction? � A catalyst increases the reaction rate by lowering the activation energy needed. � A catalyst is not permanently changed by a reaction and is not considered a reactant. � Biological catalysts found in the human body are called enzymes. � A substance used to decrease the rate of a chemical reaction is called an inhibitor.

Activation Energy � https: //www. pearsonrealize. com/community/program/b 5 aea 82 b-ad 2 c-3447 b 122 -4 b 953 e 6 a 7 cba/18/tier/3 bbae 27 e-4 e 71 -3393 -ac 93 e 88047 fe 1 e 01/16/lesson/32 cda 605 -1 dd 7 -39 ff-b 028 e 113 e 94 fbe 55/16/content/20 a 2 e 628 -90 e 0 -3257 -9 c 1 d-33 d 5 ec 55575 c/17