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Learn about the nature of chemical reactions, reaction types, balancing chemical equations, and rates of change in this comprehensive presentation. Explore the signs of chemical reactions, energy and reactions, and different types of reactions. Suitable for chemistry students and enthusiasts.
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The Nature of Chemical Reactions Chapter 6 Table of Contents Section 1 The Nature of Chemical Reactions Section 2 Reaction Types Section 3 Balancing Chemical Equations Section 4 Rates of Change
Section 1 The Nature of Chemical Reactions Chapter 6 Objectives • Recognizesome signs that a chemical reaction may be taking place. • Explain chemical changes in terms of the structure and motion of atoms and molecules. • Describe the differences between endothermic and exothermic reactions. • Identify situations involving chemical energy.
Section 1 The Nature of Chemical Reactions Chapter 6 Bellringer Methane, CH4, is an organic compound that is the principal component of natural gas. Many people burn methane when cooking or heating homes. The chemical reaction of methane burning is shown in several ways below. 1. What else besides carbon dioxide and water is produced in this reaction that makes methane useful for cooking and heating?
Section 1 The Nature of Chemical Reactions Chapter 6 Bellringer 2. Complete the table below with the number of atoms of each element before and after the reaction. 3. How does the number of atoms of each element on the left side of the equation compare to the number on the right? What law does this demonstrate? 4. Use your answer to item 1 and the law of conservation of energy to guess whether there is more energy stored in the bonds among the atoms before the reaction or among the bonds of the atoms after the reaction.
Section 1 The Nature of Chemical Reactions Chapter 6 Chemical Reactions Change Substances • Chemical reactions occur when substances undergo chemical changes to form new substances. • Production of gas and change of color are signs of chemical reactions. • Chemical reactions rearrange atoms. • A reactant is a substance or molecule that participates in a chemical reaction. • A product is a substance that forms in a chemical reaction.
Section 1 The Nature of Chemical Reactions Chapter 6 Chemical Reaction
Section 1 The Nature of Chemical Reactions Chapter 6 Signs of a Chemical Reaction
Section 1 The Nature of Chemical Reactions Chapter 6 Energy and Reactions • Energy must be added to break bonds. • Many forms of energy can be used to break bonds: • heat • electricity • sound • light • Forming bonds releases energy. • Example: When gasoline burns, energy in the form of heat and light is released as the products of the isooctane-oxygen reaction and other gasoline reactions form.
Section 1 The Nature of Chemical Reactions Chapter 6 Reaction Model
Section 1 The Nature of Chemical Reactions Chapter 6 Energy and Reactions, continued • Energy is conserved in chemical reactions. • Chemical energy is the energy released when a chemical compound reacts to produce new compounds. • The total energy that exists before the reaction is equal to the total energy of the products and their surroundings. • An exothermic reaction is a chemical reaction in which heat is released to the surroundings. • An endothermic reaction is a chemical reaction that absorbs heat.
Section 1 The Nature of Chemical Reactions Chapter 6 Energy and Reactions, continued • The graphs below represent the changes in chemical energy for an exothermic reaction and an endothermic reaction.
Section 2 Reaction Types Chapter 6 Objectives • Distinguishamong five general types of chemical reactions. • Predict the products of some reactions based on the reaction type. • Describe reactions that transfer or share electrons between molecules, atoms, or ions.
Section 2 Reaction Types Chapter 6 Bellringer There are thousands of ways that more than one hundred elements can combine with each other to form different substances. Just as the elements can be sorted into families, the many reactions the elements undergo can be classified as a few basic types. The types of reactions are classified based on whether they involve combining atoms or smaller molecules to make larger molecules (synthesis), breaking down larger molecules into atoms or smaller molecules (decomposition), or having atoms of one element replace the atoms of another element within a compound (single- or double-displacement).
Section 2 Reaction Types Chapter 6 Bellringer, continued 1. In which reaction model do three elements combine to make a compound? 2. In which reaction model is a complex substance broken down into simpler parts? 3. Identify the reaction model in which one element reacts with a compound, leaving behind another element and a new compound containing the first element. 4. In which reaction model do two compounds react to form two different compounds?
Section 2 Reaction Types Chapter 6 Classifying Reactions • A synthesis reaction is a reaction in which two or more substances combine to form a new compound. • Synthesis reactions have the following general form: A + B → AB • Example: In the following synthesis reaction, the metal sodium reacts with chlorine gas to form sodium chloride, or table salt. • 2Na + Cl2 → 2NaCl
Section 2 Reaction Types Chapter 6 Synthesis Reaction
Section 2 Reaction Types Chapter 6 Classifying Reactions, continued • A decomposition reaction is a reaction in a single compound breaks down to form two or more simpler substances. • Decomposition reactions have the following general form: AB → A + B • Example: The following shows the decomposition of water. • 2H2O → 2H2 + O2 • Electrolysis is the process in which an electric current is used to produce a chemical reaction, such as the decomposition of water.
Section 2 Reaction Types Chapter 6 Decomposition Reaction
Section 2 Reaction Types Chapter 6 Electrolysis
Section 2 Reaction Types Chapter 6 Classifying Reactions, continued • A combustion reaction is the oxidation reaction of an organic compound, in which heat is released. • Combustion reactions use oxygen as a reactant. • Water is a common product of combustion reactions. • In combustion the products depend on the amount of oxygen available for the reaction.
Section 2 Reaction Types Chapter 6 Combustion Reaction
Section 2 Reaction Types Chapter 6 Classifying Reactions, continued • A single-displacement reaction is a reaction in which one element or radical takes the place of another element or radical in the compound. • Single-displacement reactions have the following general form: AX+B → BX + A • Example: The single-displacement reaction between copper(II) chloride and aluminum is shown as follows. 3CuCl2 + 2Al → 2AlCl3 + 3Cu
Section 2 Reaction Types Chapter 6 Single Displacement
Section 2 Reaction Types Chapter 6 Classifying Reactions, continued • A double-displacement reaction is a reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between two compounds. • Double-displacement reactions have the following general form: AX+BY → AY + BX • Example: The double-displacement reaction that forms lead chromate is as follows. Pb(NO3)2 + K2CrO4 → PbCrO4 + 2KNO3
Section 2 Reaction Types Chapter 6 Double Displacement Reaction
Section 2 Reaction Types Chapter 6 Electrons and Chemical Reactions • An oxidation-reduction reaction is any chemical change in which one species gains electrons and another species loses electrons. • Oxidation-reduction reactions are often called redox reactions for short. • Substances that accept electrons in a redox reaction are said to be reduced. • Substances that give up electrons in a redox reaction are said to be oxidized. • A radical is an organic group that has one or more electrons available for bonding. • Polymerization reactions can occur when radicals are formed.
Section 2 Reaction Types Chapter 6 Redox Reactions
Section 3 Balancing Chemical Equations Chapter 6 Objectives • Demonstratehow to balance chemical equations. • Interpret chemical equations to determine the relative number of moles of reactants needed and moles of products formed. • Explain how the law of definite proportions allows for predictions about reaction amounts. • Identify mole ratios in a balanced chemical equation. • Calculate the relative masses of reactants and products from a chemical equation.
Section 3 Balancing Chemical Equations Chapter 6 Bellringer You have already used scientific shorthand by writing symbols for elements and formulas for compounds. You can use these formulas to write chemical equations that summarize what happens during a chemical reaction and how much of each substance is involved. Examine the reaction model for the water synthesis reaction shown on the next slide, and answer the items that follow.
Section 3 Balancing Chemical Equations Chapter 6 Bellringer, continued 1. What is the difference between reaction models A and B? 2. Why is reaction model A not fully complete as written? (Hint: Consider how many atoms of each element exist before and after the reaction.) 3. A friend tells you that an easier way to make sure the same number of atoms are on both sides of the equation is to change the subscript on the product so that it is H2O2 instead of H2O. What’s wrong with this reasoning? (Hint: If you did this, would it still be a synthesis reaction for water?)
Section 3 Balancing Chemical Equations Chapter 6 Describing Reactions • One way to record the products and reactants of a reaction is to write a word equation. • Example: methane + oxygen → carbon dioxide + water • A chemical equation is a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products. • In a chemical equation, such as the one above, the reactants, which are on the left-hand side of the arrow, form the products, which are on the right-hand side.
Section 3 Balancing Chemical Equations Chapter 6 Describing Reactions • When the number of atoms of reactants matches the number of atoms of products, then the chemical equation is said to be balanced. • Balancing equations follows the law of conservation of mass. • You cannot balance chemical equations by changing chemical formulas themselves, because that would change the substances involved. • To balance chemical equations, numbers called coefficients must be placed in front of the chemical formulas.
Section 3 Balancing Chemical Equations Chapter 6 Law of Conservation of Mass
Section 3 Balancing Chemical Equations Chapter 6 Describing Reactions, continued • When the numbers of atoms for each element are the same on each side, the equation is balanced, as shown below.
Section 3 Balancing Chemical Equations Chapter 6 Reading a Chemical Equation
Section 3 Balancing Chemical Equations Chapter 6 Chemical Equation
Section 3 Balancing Chemical Equations Chapter 6 Balancing a Chemical Equation by Inspection
Section 3 Balancing Chemical Equations Chapter 6 Math Skills Balancing Chemical Equations Write the equation that describes the burning of magnesium in air to form magnesium oxide. 1. Identify the reactants and products. Magnesium and oxygen gas are the reactants that form the product, magnesium oxide. 2. Write a word equation for the reaction. magnesium + oxygen → magnesium oxide.
Section 3 Balancing Chemical Equations Chapter 6 Math Skills, continued 3. Write the equation using formulas for the elements and compounds in the word equation. Remember that some gaseous elements, like oxygen, are molecules, not atoms. Oxygen in air is O2, not O. Mg + O2 → MgO 4. Balance the equation one element at a time. The same number of each kind of atom must appear on both sides. So far, there is one atom of magnesium on each side of the equation. But there are two oxygen atoms on the left and only one on the right.
Section 3 Balancing Chemical Equations Chapter 6 Math Skills, continued 4. Balance the equation one element at a time, continued To balance the number of oxygen atoms, you need to double the amount of magnesium oxide: Mg + O2 → 2MgO This equation gives you two magnesium atoms on the right and only one on the left. So you need to double the amount of magnesium on the left, as follows. 2Mg + O2 → 2MgO
Section 3 Balancing Chemical Equations Chapter 6 Math Skills, continued 4. Balance the equation one element at a time, continued 2Mg + O2 → 2MgO Now the equation is balanced. It has an equal number of each type of atom on both sides.
Section 3 Balancing Chemical Equations Chapter 6 Determining Mole Ratios • The law of definite proportions states that a compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compound is formed. • Because the law of definite proportions holds true for all chemical substances in all reactions, mole ratios can be derived from balanced equations. • Mole ratiois the relative number of moles of the substances required to produce a given amount of product in a chemical reaction.
Section 3 Balancing Chemical Equations Chapter 6 Law of Definite Proportions
Section 3 Balancing Chemical Equations Chapter 6 Determining Mole Ratios, continued • The mole ratio for any reaction comes from the balanced chemical equation. • Example: The equation for the electrolysis of water shows that the mole ratio for H2O:H2:O2 is 2:2:1. • 2H2O → 2H2 + [1]O2 • If you know the mole ratios of the substances in a reaction, you can find the relative masses of the substances required to react completely. • Relative masses can be found by multiplying the molecular mass of each substance by the mole ratio from the balanced equation.
Section 4 Rates of Change Chapter 6 Objectives • Describethe factors affecting reaction rates. • Explain the effect a catalyst has on a chemical reaction. • Explain chemical equilibrium in terms of equal forward and reverse reaction rates. • Apply Le Châtelier’s principle to predict the effect of changes in concentration, temperature, and pressure in an equilibrium process.
Section 4 Rates of Change Chapter 6 Bellringer Not all reactions happen at the same speed. Atoms, ions, and molecules can only interact when they are in close contact with each other. Below is a sample of zinc arranged in three different ways.
Section 4 Rates of Change Chapter 6 Bellringer, continued 1. In the reaction Zn + 2HCl → ZnCl2 + H2, which sample do you think would react the fastest? Why? 2. When you want to start a bonfire, why do you use many small sticks as kindling to start the larger logs? 3. Which do you think will react faster with hydrochloric acid, HCl–atoms of liquid zinc at its melting point or atoms of solid zinc at its melting point? (Hint: Which situation allows more contact among the particles?)
Section 4 Rates of Change Chapter 6 Factors Affecting Reaction Rates • For any reaction to occur, the particles of the reactants must collide with one another. Therefore, whatever will help particles collide with one another will speed up the reaction rate. • Most reactions go faster at higher temperatures. • Greater surface area speeds up reactions. • Concentrated solutions react faster. • Reactions are faster at higher pressure. • Massive, bulky molecules react slower.