HIGH SCHOOL SCIENCE. Physical Science 12: Chemical Reactions

Transcription

1 HIGH SCHOOL SCIENCE Physical Science 12: Chemical Reactions WILLMAR PUBLIC SCHOOL EDITION

2 CHAPTER 12 Chemical Reactions In this chapter you will: 1. Interpret and balance chemical equations. 2.Describe how to convert between moles and mass. 3. Classify chemical reactions as synthesis, decomposition, single-replacement, double-replacement, or combustion / endothermic and exothermic. 4.Describe oxidation-reduction reactions. 5. Describe the energy changes that take place and that energy is conserved during chemical reactions. 6.Describe & classify nuclear decay and radiation. 7. Identify sources of nuclear radiation and describe how nuclear radiation affects matter. 8.Describe methods of detecting nuclear radiation. 9.Describe half-life and how radioisotopes are used to estimate the age of materials. 10.Compare and contrast nuclear forces. 11.Describe nuclear fission and nuclear fusion.

3 SECTION 12.1 What are Chemical Reactions? OBJECTIVES: 1. Interpret and balance chemical equations. 2. Describe how to convert between moles and mass. Vocabulary: Does the term chemical reaction bring to mind a chemist mixing chemicals in a lab. Many chemical reactions take place in labs. However, most chemical reactions do not. Where do they occur? They happen in the world all around you. They even happen inside your own body. In fact, you are alive only because of the many chemical reactions that constantly take place inside your cells. A chemical reaction is a process in which some substances change into different substances. A useful description of a chemical reaction tells you the substances present before and after the reaction. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products. Chemical reactions may occur quickly or slowly. chemical reaction products law of conservation of mass mole reactants chemical equation molar mass Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right). Reactants Products 2

4 The arrow ( ) shows the direction in which the reaction occurs. In many reactions, the reaction also occurs in the opposite direction. This is represented with another arrow pointing in the opposite direction ( ). A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas. 2H2 + O2 2H2O This chemical equation represents a chemical reaction. In the reaction, two molecules of hydrogen gas combine one molecule of oxygen gas to produce two molecules of water. All chemical equations, like equations in math, must balance. This means that there must be the same number of each type of atom on both sides of the arrow. That s because matter is always conserved in a chemical reaction. This is the law of conservation of mass. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. In order to show that mass is conserved during a reaction, a chemical equation must be balanced. A coefficient is a number placed in front of a chemical symbol or formula that shows how many atoms or molecules of the substance are involved in the reaction. Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step Place coefficients, as needed, in front of the symbols or formulas to increase the number of atoms or molecules of the substances. Use the smallest coefficients possible. Warning! Never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. 3. Repeat steps 1 and 2 until the equation is balanced. Chemists need to practical units for counting things. Although you can describe a reaction in terms of atoms and molecules, these units are too small to be practical. Chemists use a counting unit called the mole to measure amounts of a substance. A mole (mol) is an amount of a substance that contains approximately 6.02 x 1023 particles of that substance. 3

5 A dozen eggs has a different mass than a dozen oranges. Similarly, a mole of carbon has a different mass than a mole of sulfur. The mass of one mole of a substance is called molar mass. For an element, the molar mass is the same as its atomic mass expressed in grams. For a compound, you can calculate the molar mass by adding up the atomic masses of its component atoms, and then express this sum in grams. Once you know the molar mass of a substance, you can convert moles into mass, or mass into moles. For either calculation, you need to express the molar mass as a conversion factor. The multiple either the moles or mass with the conversion factor. For example, the molar mass of CO2 is 44.0 grams. (44.0 g CO2)/(1 mol CO2) or (1 mol CO2)/( 44.0 gco2) Suppose you have 55 g of CO2. To calculate how many moles of CO2 you have, multiply the mass by the second conversion factor. (55g CO2)*(1 mol CO2)/(44.0 g CO2)= 1.25 mol CO2 Section Review: 1.What does a useful description of a chemical reaction tell you? 2.Do chemical reactants and products contain the same atoms in a chemical reaction? 3.How do the atoms form different combinations in the products from the reactants? 4.How are chemical reactions represented? 5.What do the arrows show? 6.Why must a chemical equation be balanced? 7.Write a balanced equation for the reaction between copper and oxygen to produce copper (II) oxide. Cu+ O2 CuO 8.Which of the following chemical equations is balanced? a) Zn+HCl ZnCl2 +H2 b) 2Zn+2HCl ZnCl2 +H2 c) 2Zn+HCl ZnCl2 +H2 d) Zn+2HCl ZnCl2 +H2 9.How do you determine molar mass for an element? 4

6 Section Review Continued: 10. How do you determine molar mass for a compound? 11. How do you convert moles into mass? 5

7 SECTION 12.2 Types of Reactions OBJECTIVES: 1. Classify chemical reactions as synthesis, decomposition, single-replacement, doublereplacement, or combustion / endothermic and exothermic. 2. Describe oxidation-reduction reactions. 3. Describe the energy changes that take place and that energy is conserved during chemical reactions. Vocabulary: synthesis reaction single replacement reaction double replacement reaction oxidation-reduction reaction decomposition reaction combustion oxidation reduction The type of reactant or the number of reactants and products often classifies reactions. Some general types of chemical reactions are synthesis, decomposition, single replacement, double replacement, and combustion. Type of Reaction Types of Chemical Reactions General Equation Example Synthesis A+Bè C 2Na + Cl 2 è 2NaCl Decomposition ABè A+B 2H 2 O è 2H 2 + O 2 Single Replacement Double Replacement Combustion A+BCè B+AC 2K+2H 2 Oè 2KOH+H 2 AB+CDè AD+CB fuel + oxygen è carbon dioxide + water NaCl+AgF è NaF+AgCl CH 4 +2O 2 è CO 2 +2H 2 O A synthesis reaction occurs when two or more reactants combine to form a single product. An example of a synthesis reaction is the combination of sodium (Na) and chlorine (Cl) to produce sodium chloride (NaCl). This reaction is represented by the chemical equation: chemical energy exothermic reaction 2Na + Cl2 2NaCl endothermic reaction law of conservation of energy activation energy Sodium is a highly reactive metal, and chlorine is a poisonous gas. The compound they synthesize has very different properties. Sodium chloride is commonly called table salt, 6

8 which is neither reactive nor poisonous. In fact, salt is a necessary component of the human diet. A decomposition reaction occurs when one reactant breaks down into two or more products. Carbonic acid (H2CO3) is an ingredient in soft drinks. A decomposition reaction takes place when carbonic acid breaks down to produce water (H2O) and carbon dioxide (CO2). This occurs when you open a can of soft drink and some of the carbon dioxide fizzes out. The equation for this reaction is: H2CO3 H2O + CO2 Another decomposition reaction occurs when water (H2O) breaks down to produce hydrogen (H2) and oxygen (O2) gases. The equation for this reaction is: 2H2O 2H2 + O2 This happens when an electric current passes through the water. A replacement reaction occurs when elements switch places in compounds. A single replacement reaction occurs when one element replaces another in a single compound. An example of a single replacement reaction occurs when potassium (K) reacts with water (H2O). A colorless solid compound named potassium hydroxide (KOH) forms, and hydrogen gas (H2) is set free. The equation for the reaction is: 2K + H2O 2KOH + H2 In this reaction, a potassium ion replaces one of the hydrogen atoms in each molecule of water. Potassium is a highly reactive group 1 alkali metal, so its reaction with water is explosive. A double replacement reaction occurs when two ionic compounds exchange ions.an example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This reaction is represented by the equation: NaCl+AgF NaF+AgCl During the reaction, chloride and fluoride ions change places, so two new compounds are formed in the products: sodium fluoride (NaF) and silver chloride (AgCl). A combustion reaction occurs when a substance reacts quickly with oxygen (O2). Combustion is commonly called burning, and the substance that burns is usually referred to as fuel. The fuel that burns in a combustion reaction contains compounds called hydrocarbons. Hydrocarbons are compounds that contain only carbon (C) and hydrogen (H). The charcoal pictured above consists of hydrocarbons. So do fossil fuels such as natural gas. The main component of natural gas is the hydrocarbon called methane (CH4). The combustion of methane is represented by the equation: CH4 +2O2 CO2 +2H2O 7

9 Natural gas is a fuel that is commonly used in home furnaces and gas stoves. As scientists learned more about the structure of the atom, they found different ways to describe how reactions take place. The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms. A reaction in which electrons are transferred from one reactant to another is called oxidation-reduction reaction, or redox reaction. A type of synthesis reaction in which a metal combines with oxygen, traditionally have been classified as oxidations. However, today we say that any process in which an element loses electrons during a chemical reaction is called oxidation. The process in which an element gains electrons during a chemical reaction is called reduction. Oxidation and reduction always occur together. When one element loses electrons, another must gain electrons. All chemical reactions involve energy. Chemical energy is the energy stored in the chemical bonds of a substance. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions. In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light. All combustion reactions are exothermic reactions. During combustion, a substance burns as it combines with oxygen, releasing energy in the form of heat and light. An endothermic reaction is a chemical reaction in which more energy is needed to break bonds in the reactants than is released when new bonds form in the products. A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going. One of the most important series of endothermic reactions is photosynthesis. The energy needed for photosynthesis comes from light. Chemical reactions also need energy to be activated. They require a certain amount of energy just to get started. This energy is called activation energy. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas won t occur without the spark of energy to begin the reaction. You have probably used activation energy to start a chemical reaction. For example, if you ve ever struck a match to light it, then you provided the activation energy needed to start a combustion reaction. When you struck the match on the box, the friction started the match head burning. Combustion is exothermic. Once a match starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, the match won t burst into flames on its own. Whether a chemical reaction absorbs or releases energy, there is no overall change in the amount of energy during the 8

10 reaction. That s because energy cannot be created or destroyed. This is the law of conservation of energy. Section Review: 1.Identify these reactions as synthesis, decomposition, single replacement, double replacement, or combustion. a)pb(no3)2 + 2HCl b)2c2h6 + 7O2 PbCl2 + 2HNO3 4CO2 + 6H2O c)ca + 2HCl d)2so2 + O2 a)caco3 CaCl2 + H2 2 SO3 CaO + CO2 2. Why are oxidation and reduction always together? 3. Contrast exothermic and endothermic chemical reactions. 4. Give an example when you have used activation energy to start a chemical reaction. 5. Why is the amount of energy the same before and after a chemical reaction? 9

11 SECTION 12.3 Nuclear Chemistry OBJECTIVES: 1. Describe & classify nuclear decay and radiation. 2. Identify sources of nuclear radiation and describe how nuclear radiation affects matter. 3. Describe methods of detecting nuclear radiation. 4. Describe half-life and how radioisotopes are used to estimate the age of materials. 5. Compare and contrast nuclear forces. 6. Describe nuclear fission and nuclear fusion. Vocabulary: radioactivity radioactive isotope alpha particle gamma ray half-life weak nuclear force nuclear fission nuclear fusion radiation nuclear radiation beta particle background radiation radioactive dating strong nuclear force uncontrolled nuclear fission For an atom of one element to change into a different element, the number of protons in its nucleus must change. That s because each element has a unique number of protons. For example, lead atoms always have 82 protons, and gold atoms always have 79 protons. Alchemists, who lived during the Middle Ages, were people who strived to turn lead into gold. They tried all sorts of chemical reactions involving lead, but they were never able to produce gold. Today, scientists know that one element cannot be changed into another by chemical processes. Radioactivity is the ability of an atom to emit, or give off, charged particles and energy from its nucleus. The charged particles and energy are called by the general term radiation. Only unstable nuclei emit radiation. They are unstable because they have too much energy, too many protons, or an unstable ratio of protons to neutrons. For example, all elements with more than 83 protons such as uranium, radium, and polonium have unstable nuclei. They are called radioactive isotope or radioisotope. The nuclei of these elements must lose protons to become more stable. When they do, they become different elements. Radioactivity was discovered in 1896 by Antoine Henri Becquerel when he found that uranium leaves an image like an X-ray on a photographic plate. Besides uranium, radioactive elements include radium and polonium, both of which were discovered by Marie Curie. Scientists can detect a radioactive substance by measuring the nuclear radiation it gives off. Nuclear radiation is charged 10

12 particles and energy that are emitted from the nuclei of radioisotopes. Common types of nuclear radiation include alpha particles, beta particles, and gamma rays. An alpha particle is a positively charged particle made up of two protons and two neutrons. Alpha particles are the least penetrating type of nuclear radiation. Most alpha particles travel no more than a few centimeters in air, and can be stopped by a sheet of paper or clothing. A beta particle is an electron emitted by an unstable nucleus. Due to their smaller mass and faster speed, beta particles are more penetrating than alpha particles. Beta particles pass through paper, but can be stopped by a thin sheet of metal. Not all nuclear radiation consists of charged particles. A gamma ray is a penetrating ray of energy emitted by an unstable nucleus. Gamma radiation has no mass or charge. Gamma rays are much more penetrating than either alpha particles or beta particles. It can take several centimeters of lead or several meters of concrete to stop gamma radiation. You may not know it but you are exposed to nuclear radiation every day. Most of this is background radiation, or nuclear radiation that occurs naturally in the environment. Radioisotopes in air, water, rocks, plants, and animals all contribute to background radiation. Cosmic rays also contribute to background radiation. When nuclear radiation exceeds background levels, it can damage the cells and tissues of your body. When cells are exposed to nuclear radiation, the bonds holding together protein and DNA molecules may break. As these molecules change, the cell may no longer function properly. Although you cannot see, hear, or feel the radioactivity around you, scientific instruments can measure nuclear radiation. Devices that are used to detect nuclear radiation include Geiger counters and film badges. Despite its dangers, radioactivity has several uses. For example, it can be used to determine the ages of ancient rocks and fossils. It can also be used as a source of power to generate electricity. Radioactivity can even be used to diagnose and treat diseases, including cancer. A radioisotope decays and changes to a different element at a constant rate. The rate is measured in a unit called the halflife. Half-life is the length of time it takes for half of a given amount of the radioisotope to decay. This rate is always the same for a given radioisotope, regardless of temperature, pressure, or other conditions outside the nuclei of its atoms. Different radioisotopes may vary greatly in their rate of decay. That s because they vary in how unstable their nuclei are. The more unstable the nuclei, the faster they break down. The age of a rock or other specimen can be estimated from the remaining amount of a radioisotope it contains and the radioisotope s known rate of decay, or half-life. This method 11

13 Radioac've Isotope Rubidium- 87 Uranium- 238 Potassium- 40 Uranium- 235 Carbon- 14 Thorium- 234 Radon- 222 Half- Life 48.8 Billion Years Billion Years 1.26 Billion Years Million Years 5730 Years 24.1 Days 3.82 Days of dating specimens is called radioactive dating. Radioisotopes with longer half-lives are used to date older specimens, and those with shorter half-lives are used to date younger ones. Carbon-14 dating is used to date specimens younger than about 60,000 years old. It is commonly used to date fossils of living things and human artifacts. What holds the nucleus together? Remember that the protons in the nucleus are positively charged, so they tend to repel one another. The strong nuclear force is a powerful attractive force that binds protons and neutrons together in the nucleus. The range over which the strong nuclear forces acts is approximately equal to the diameter of a proton. Although this force acts over only extremely short distance, it is 100 times stronger than the electric force of repulsion at these distances. The other powerful force in the nucleus is the weak nuclear force. As the name implies, the weak force is weaker in strength than the strong nuclear force. The weak nuclear force is an attractive force that acts only over a short range and affects all particles, not just protons and neutrons. Einstein s equation, E = mc2, shows that matter and energy are two forms of the same thing. It also shows that there is a tremendous amount of energy (E) in a small mass (m) of matter. In nuclear reactions, matter changes to energy, but the total amount of mass and energy together does not change. Nuclear fission is the splitting of the nucleus of an atom into two smaller nuclei. This type of reaction releases a great deal of energy from a very small amount of matter. It begins when the nucleus of a radioactive atom gains a neutron. In uncontrolled nuclear fission, one fission reaction starts a chain reaction, in which neutrons produced in one reaction cause other reactions, which cause more reactions, and so on. Energy released by nuclear fission is used to produce electrical energy in nuclear power plants. Production of nuclear energy doesn t produce air pollution but it poses the risk of accidents that release harmful radiation. In nuclear fusion, two or more small nuclei combine to form a single larger nucleus, a neutron, and a tremendous amount of energy. Nuclear fusion of hydrogen to form 12

14 helium occurs naturally in the sun and other stars. It takes place only at extremely high temperatures. Scientists are searching for ways to create controlled nuclear fusion reactions in order to produce safe nuclear power. Fusion involves only harmless, plentiful elements but requires extremely high temperatures. Section Review: 1.Why are some nuclei unstable? 2.List the types of nuclear radiation from least penetrating to most penetrating. 3.What contributes to background radiation? 4.How can radiation damage cells? 5.What devices are used to detect nuclear radiation? 6.How can radioactivity be useful? 7.Why do some radioisotopes decay at a faster rate? 8.Which radioactive isotope would be used to date a rock from about 10,000 years ago? 9.What holds the nucleus together? 10.What changes in a nuclear reaction? 11.What is uncontrolled nuclear fission? 12.What is an advantage of nuclear fission? 13.What are advantages and disadvantages to nuclear fusion? 13