Chemical Reactions. Unit 4

Transcription

1 Chemical Reactions Unit 4

2 Lesson 1: Chemical Bonds Unit 4: Reactions

3 Compounds Most substances around you are NOT elements. There are around 100 elements, but millions of different substances. Most substances are compounds. Remember, a compound is a substance made of atoms of two or more different elements. The atoms of different elements are held together in compounds by chemical bonds. Chemical bonds can hold atoms together in large networks or small groups. Chemical bonds help determine the properties of a compound.

4 Properties of Compounds The properties of a compound depend on: Which elements the compound contains How the atoms are arranged For example: the atoms of carbon and hydrogen can form many thousands of different compounds. Each compound has a certain number of carbon and hydrogen atoms arranged in a specific way. Some of these compounds include: Natural gas Parts of automobile gasoline Hard wax of candles Many plastics

5 Properties of Compounds The properties of a compound are very different from the properties of the elements that make them. Example: Water is made from two atoms of hydrogen bonded to one atom of oxygen. At room temperature, hydrogen and oxygen are both colorless, odorless gases. They remain gases down to extremely low temperatures. However, water is a liquid at temperatures up to 100 C and a solid below 0 C.

6 Chemical Formulas Remember that atoms of elements can be represented by their chemical symbols. A chemical formula uses these chemical symbols to represent the atoms of the elements and their ratios in a chemical compound. Carbon dioxide is a compound consisting of one atom of carbon attached by chemical bonds to two atoms of oxygen. Here is how you would write the chemical formula for carbon dioxide: Find the symbols for carbon and oxygen on the periodic table. Write these symbols side by side. To show that there are two oxygen atoms for every carbon atom, place the subscript 2 to the right of the oxygen atom s symbol. Because there is only one atom of carbon in carbon dioxide, you don t need a subscript for carbon. The subscript 1 is never used.

7 Carbon Dioxide CO 2 The chemical formula shows one carbon atoms bonded to two oxygen atoms.

8 Glucose (Sugar)

9 Same Elements, Different Compounds Vs.

10 Chemical Bonds Chemical bonds are the glue that holds the atoms of elements together in compounds. Chemical bonds are what make compounds more than just mixtures of atoms. Chemical bonds form when the electrons in the electron shell around two atoms interact. How the electrons interact determines the kind of chemical bond that is formed. Chemical bonds have a great effect on: Chemical and physical properties of compounds How different substances interact

11 Transfer of Electrons Remember an ion is formed when an atom gains or loses one or more electrons. Gaining an electron = negative ion Losing an electron = positive ion Ions typically form in pairs when one atom transfers one or more electrons to another atom. Group 1 metals commonly lose one electron to form positive ions. (1+) Group 2 metals commonly lose two electrons to form positive ions. (2+) Transition metals also lose (various amounts of) electrons to form positive ions. Group 17 nonmetals commonly gain one electron to form negative ions. (1-) Group 16 nonmetals commonly gain two electrons to form negative ions. (2-)

12 Ionic Bonds In general, ionic bonds form between metals and nonmetals. The metal loses the electron(s), so the nonmetal gains the electron(s). When an atom of an element from Group 1 comes near an atom of an element from Group 17, they form an ionic bond. An ionic bond is the force of attraction between positive and negative ions. Ionic bonds form between all nearby ions of opposite charge. These bonds make ionic compounds very stable and strong.

13 Names of Ionic Compounds The name of an ionic compound is based on the names of the ions it is made of. The name of a positive ion is the same as the name of the atoms from which it is formed. The name of a negative ion is formed by dropping the last part of the name of the atom and adding the suffix -ide. To name an ionic compound, the name of the positive ion is placed first, followed by the name of the negative ion. Example: Sodium Chloride Sodium is the positive ion; chlorine is the negative ion

14 Covalent Bonds Another way in which atoms can bond together is be sharing electrons. Nonmetal atoms usually form bonds with each other in this way. A pair of shared electrons between two atoms is called a covalent bond. In forming a covalent bond, neither atom gains or loses an electron, so no ions are formed.

15 Covalent Bonds The shared electrons are attracted to both positively charged nuclei. The number of covalent bonds that an atom can form depends on the number of electrons that it has available for sharing. Example: Atoms of the Halogen family (Group 17) can contribute only one electron to a covalent bond. Example: Group 16 = 2 covalent bonds; Group 15 = 3 covalent bonds; Carbon & Silicon = 4 covalent bonds

16 Covalent Bonds A group of atoms held together by covalent bonds is called a molecule.

17 Covalent Bonds Sometimes atoms may share more than one pair of electrons with another atom. A double bond consists of four (two pairs of) shared electrons. A triple bond means that the atoms share six (three pairs of) electrons.

18 Covalent Bonds

19 Metallic Bonds Metal atoms bond together by sharing their electrons with one another. The atoms share the electrons equally in all directions, which allows the electrons to move easily among the atoms of the metal. The properties of metals are determined by metallic bonds. Good conductors because of the free movement of electrons Malleable because atoms can slide past one another

20 Checkpoint: Chemical Bonds 1. What is the difference between an ionic bond and a covalent bond? Ionic bond: One atom loses an electron, while the other atom gains the electron. Covalent bond: Electrons are shared between two atoms. Neither atom gains or loses an electron.

21 Lesson 2: Chemical Reactions Unit 4: Reactions

22 Chemical Reactions Remember that a chemical change occurs when substances change into one or more different (new) substances. A chemical reaction produces new substances by changing the way in which atoms are arranged. In a chemical reaction, bonds between atoms are broken and new bonds form between different atoms. This breaking and forming of bonds takes place when particles of the original materials collide with one another. After a chemical reaction, the new arrangements of atoms form different substances.

23 Reactants and Products Reactants are the substances present at the beginning of a chemical reaction. Example: In the burning of natural gas, methane (CH 4 ) and oxygen (O 2 ) are the reactants. CH 4 + O 2 à CO 2 + H 2 O Products are the substances formed by a chemical reaction. Example: In the burning of natural gas, carbon dioxide (CO 2 ) and water (H 2 O) are the products. CH 4 + O 2 à CO 2 + H 2 O Reactants and products can be elements or compounds, depending on the reaction taking place.

24 Reactants and Products During a chemical reaction, bonds between atoms in the reactants are broken and new bonds are formed in the products. Example: When natural gas is burned, bonds between the carbon and hydrogen atoms in methane are broken. Bonds between the oxygen atoms in the oxygen molecules are broken. New bonds are formed between carbon and oxygen in carbon dioxide gas and between hydrogen and oxygen in water vapor.

25 Evidence of Chemical Reactions Some chemical changes are easy to observe. Others are not as easy to see. If you observe two or more of these signs during a change, you most likely are observing a chemical change: Production of an odor Change in temperature Change in color Formation of a gas (bubbles) Formation of a precipitate (a solid)

26 Classification of Chemical Reactions Chemical reactions can be classified based on the ways the products are made: Synthesis Decomposition Combustion

27 Classification of Chemical Reactions: Synthesis In a synthesis reaction, a new compound is formed by the combination of simpler reactants. Example: nitrogen dioxide (NO 2 ), a part of smog, forms when nitrogen and oxygen combine in the air.

28 Classification of Chemical Reactions: Decomposition In a decomposition reaction, a reactant breaks down into simpler products. The products could be elements or other compounds. Decomposition can be thought of as being the reverse of synthesis. Example: water can be decomposed into its elements hydrogen and oxygen.

29 Classification of Chemical Reactions: Combustion In a combustion reaction, one reactant is always oxygen. Another reactant often contains carbon and hydrogen. The carbon and hydrogen atoms combine with oxygen, producing carbon dioxide and water. Example: The burning of methane is a combustion reaction.

30 The Rates of Chemical Reactions Most chemical reactions take place when particles of reactants collide with enough force to react. Chemical reactions can occur at different rates. Striking a match cause a very quick chemical reaction. The rusting of an iron nail may take months.

31 The Rates of Chemical Reactions However, the following factors can change the rates of chemical reactions: Concentration The number of particles present in a certain volume. A high concentration of reactants means a large number of particles that can collide and react. Example: Turning up the gas stove increases the concentration of methane molecules that can combine with oxygen in the air. The result is a bigger flames and a faster combustion reaction.

32 The Rates of Chemical Reactions However, the following factors can change the rates of chemical reactions: Surface area Suppose one of the reactants in a chemical reaction is a single large piece of material. Particles of the second reactant cannot get inside the large piece. They can react only with particles on the surface. To make the reaction go faster, the large piece of material could be broken into smaller pieces before the reaction starts. Breaking it into smaller pieces increase the surface area of the material, which increases the rate of the reaction.

33 The Rates of Chemical Reactions However, the following factors can change the rates of chemical reactions: Temperature The rate of a reaction can be increased by making the particles move faster. The result is that more collisions take place per second and occur with greater force. The way to make the particles move faster is to add energy to the reactants. Adding energy raises their temperature. Example: Many chemical reactions in cooking do not occur unless heat is added. Usually the reactions in cooking can be stopped by removing the food from the heat source.

34 The Rates of Chemical Reactions However, the following factors can change the rates of chemical reactions: Catalysts: The rate of a reaction can be changed chemically by adding a catalyst. A catalyst is a substance that increases the rate of a chemical reaction but is not itself consumed in the reaction. This means that after the reaction is complete, the catalyst remains unchanged. Catalysts are very important for many industrial and biological reactions. In fact, many chemical reactions would proceed slowly or not at all without catalysts.

35 Energy The Law of Conservation of Energy states that energy cannot be created or destroyed. It can only be transferred to a different type of energy. Energy is needed to break bonds in reactant molecules. Energy is released when bonds are formed in product molecules.

36 Exothermic Chemical Reactions Sometimes more energy is released when products form than is needed to break the bonds in the reactants. Then energy is released during the reaction. A reaction in which energy is released is called an exothermic reaction. This energy is often released as heat. Example: All common combustion reactions

37 Endothermic Chemical Reactions Sometimes more energy is required to break the bonds in the reactants than is released when the products form. Then energy must be added to the reaction. That is, the reaction absorbs energy. A reaction in which energy is absorbed is called an endothermic reaction. Endothermic reactions often produce a decrease in temperature.

38 Photosynthesis Probably the most important series of endothermic reactions on Earth is photosynthesis. Photosynthesis is the endothermic process by which light is absorbed and used to change carbon dioxide and water into oxygen and glucose. Unlike many other endothermic reactions, photosynthesis does not absorb energy as heat. The energy is stored in the glucose molecules, ready to be used by the plants when needed.

39 Photosynthesis

40 Law of Conservation of Mass The Law of Conservation of Mass states that in a chemical reaction, atoms are neither created nor destroyed. French chemist Antoine Lavoiseir concluded this law through work in the 1780s. In other words: All atoms present in the reactant are also present in the products.

41 Law of Conservation of Mass The total mass of reactants is equal to the total mass of products. Example: The mass of sodium plus the mass of chlorine that reacts with the sodium is equal to the mass of the product sodium chloride. Because atoms are only rearranged in a chemical reaction, there must be the same number of sodium atoms and chlorine atoms in the both the reactants and products.

42 Chemical Equations A chemical equation represents how atoms are rearranged in a chemical reaction. The atoms in the reactants are shown on the left side of the equation. The atoms in the products are shown on the right side of the equation. No atoms are created or destroyed. The number of atoms of each different element, therefore, must be the same on each side of the equation.

43 Chemical Equations In order to write a chemical equation, you need to know: The reactants and products in the reaction The atomic symbols and chemical formulas of the reactants and products in the reaction The direction of the reaction C + O 2 à CO 2 Carbon reacts with oxygen to yield carbon dioxide.

44 Chemical Equations Chemical reactions follow the Law of Conservation of Mass. The equations show this equality in terms of atoms. The same number of atoms of each element must appear on both sides of a chemical equation. You must balance the equation to make the number of atoms equal on each side of the equation.

45 Balancing Chemical Equations The numbers in front of the chemical formulas are called coefficients. Coefficients indicate how many molecules take part in the reaction. If there is no coefficient, then only one molecule of that type takes part in the reaction. Remember a subscript indicates the number of atoms of an element in a molecule.

46 Coefficients Vs. Subscripts Coefficient 3NaCl 3 molecules of NaCl = NaCl NaCl NaCl Subscript H 2 O 2 atoms of hydrogen in 1 molecule of H 2 O = H H O 3 H 2 O 3 molecules of H2O (with 2 atoms of hydrogen in each molecule) = H 2 O H 2 O H 2 O = HHO HHO HHO = 6 atoms of H 3 atoms of O

47 Balancing Chemical Equations CH 4 + 2O 2 à CO H 2 O H 4 = 4 atoms of hydrogen We need 4 atoms of oxygen C = 1 H = 4 O = 4 Need 4 atoms of hydrogen Now we have 4 atoms of oxygen C = 1 H = 4 O = 4

48 Lesson 3: Solutions Unit 4: Reactions

49 Solutions A solution is a mixture of two or more substances that is identical throughout. If you stir sand into a glass of water, you can identify the sand as a separate substance that falls to the bottom of the glass. Sand in water is a mixture, but not a solution. If you stir sugar into a glass of water, you cannot identify the sugar as a separate substance. Therefore, sugar in water is a common solution. Many solutions have certain properties that make us call them acids or bases.

50 Acids Acids are found in many foods, such as orange juice, tomatoes, and vinegar. They taste slightly sour when dissolved in water and produce a burning or itchy feeling on the skin. Strong acids should never be tasted or touched. These solutions are used in manufacturing and are dangerous chemicals.

51 Bases Bases are the chemical opposite of acids. They tend to taste bitter, rather than sour, and often feel slippery to the touch. Bases are also found in common products around the home, including soap, ammonia, and antacids. Strong bases, like the lye used for unclogging drains, are also dangerous chemicals.

52 Acids, Bases, and Ions Generally, a compound that is an acid or a base acts as an acid or a base only when it is dissolved in water. In a water-based solution, these compounds produce ions. For example, if a hydrogen atom, which consists of 1 proton and 1 electron, loses its electron, it becomes a hydrogen ion. It has a positive charge.

53 Acids An acid is a substance that can donate a hydrogen ion (a proton) to another substance. For example, when the compound HCl is dissolved in water, the compound separates into hydrogen ions (H + ) and chloride ions (Cl - ). The hydrogen ions are free to react with other substances, so the solution is called an acid. When hydrogen chloride is dissolved in water, the solution is called hydrochloric acid. l H 2 O l

54 Bases A base is a substance that can accept a hydrogen ion from another substance. For example, when the compound NaOH is dissolved in water, the compound separates into sodium ions (Na + ) and hydroxide ions (OH - ). The hydroxide ions are free to accept protons from either substances, so the solution is a base. The solution that results when NaOH is dissolved in water is called sodium hydroxide. H 2 O

55 Atomic Level On an atomic level, the difference between acids and bases is that acids donate protons and bases accept protons. When a proton a hydrogen ion from an acid is accepted by a hydroxide ion from a base, the two ions join together and form a molecule of water. This simple transfer of protons between substances is involved in many useful and important chemical reactions.

56 Testing for Acids One safe way to test for an acid is to place a few drops of a solution on a compound that contains a carbonate (CO 3 ). For example, limestone is a rock that contains calcium carbonate (CaCO 3 ). When an acid touches a piece of limestone, a reaction occurs that produces carbon dioxide gas. Acids also react with most metals. The reaction produces hydrogen gas, which you can see as bubbles. The most common way to identify is their ability to change colors of certain compounds known as acid-base indicators. One common indicator is litmus, which is often prepared on slips of paper. When a drop of acid is placed on litmus paper, the paper turns red.

57 Testing for Bases Bases feel soapy or slippery because they react acidic molecules in your skin called fatty acids. This is actually how soap is made mixing a base (usually sodium hydroxide) with fatty acids produces soap. So, when a base touches your skin, the combination of the base with your own fatty acids actually makes a small amount of soap. Bases also change the colors of acid-base indicators. When a drop of base is placed on litmus paper, the paper turns blue.

58 Strengths of Acids Strong acids break apart completely into ions. Example: Hydrogen chloride (HCl) dissolves in water to form hydrochloric acid. It breaks down into hydrogen ions and chloride ions. No hydrogen chloride remains in the solution. Because all the hydrogen chloride forms separate ions, hydrochloric acid is a strong acid. Weak acids do not form many ions in a solution. Example: Acetic acid (HC 2 H 3 O 2 ) is the acid in vinegar. When it dissolves in water, only about 1% of the acetic acid breaks up into hydrogen ions and acetate ions. The other 99% of the acetic acid remains unchanged. Therefore, it is a weak acid.

59 Strengths of Bases Bases can also be strong or weak. When sodium hydroxide (NaOH) dissolves in water, it forms sodium ions (Na + ) and hydroxide ions (OH - ). None of the original NaOH remains in the solution, so sodium hydroxide is a strong base. When ammonia (NH3) dissolves in water, only about 1% of the ammonia reacts with water to form OH - ions. The other 99% remains unchanged. Therefore, ammonia is a weak base.

60 Measuring Acidity The acidity of a solution depends on the concentration of H+ ions in the solution. This concentration is often measured on the ph scale. A ph scale represents a high H + concentration with a low number (acid) and a low H + concentration with a high number (base). A neutral compound is a substance that is neither an acid or a base.

61 Measuring Acidity A ph scale usually has a range from Numbers below 7 indicate acidic solutions. A concentrated strong acid has a low ph value. Numbers above 7 indicate basic solutions. A concentrated strong base has a high ph value. Neutral compounds like pure water have a ph of 7.

62 ph Scale

63 ph Scale

64 Checkpoint: Solutions 1. Explain how to read a ph scale. 0-6 = acidic solution (high H + concentration) 7 = neutral 8-14 = basic solution (low H + concentration)

65 Lesson 4: Chemistry of Living Things Unit 4: Reactions

66 Chemistry of Living Things Chemical reactions happen everywhere, including in living things. Example: Exothermic reactions provide us with the energy we need for our nerves to work and acids to break down food in our stomachs. Biochemistry is the study of the substances and processes occurring in living organisms.

67 Composition of Living Things Close to 100 elements occur naturally. Of these, 25 are found in living things. The six most common elements in living things are: Carbon Hydrogen Oxygen Nitrogen Phosphorus Sulfur

68 Other Elements Other elements are also important to the proper functioning of organisms. These elements include: Sodium & potassium ions key role in the transmission of nerve impulses between your brain and all parts of the body Calcium ions essential for muscle contractions and the clotting of blood Fluorine forms a salt with calcium, which makes teeth stronger Iron helps carry oxygen in the blood to body cells Chlorine combines with hydrogen to create acid in the stomach to break down food Magnesium muscle contractions

69 Water and Salt The human body relies on many compounds. Many of these have very complicated molecular structures. However, two of the most important compounds, water and salt, have very simple chemical structures. They are important because of the types of bonds they contain.

70 Water Remember that water is a covalent bond. These bonds make water an excellent solvent. Our body cells contain mostly water, so many substances can dissolve inside cells. This allows many chemical reactions to take place as well. Substances (including blood) can easily pass in solution from the outside of the cell to the inside of the cell.

71 Salt Salt (NaCl) is another useful compound for the body. Remember that salt is an ionic bond. When ionic compounds dissolve in water, they separate into positive and negative ions. These ions are in solution in the blood and cells. Salt separates into Na + and Cl - ions. Separated ions allow a solution to conduct electric current. This is how the body sends nerve impulses. Sodium ions also help to keep the amount of water constant in the cells of body tissues. They regulate the amount of water that passes in and out of the cells. Chloride ions combine with hydrogen to make hydrochloric acid, which helps our body digest food.

72 Carbon Although carbon is not the most abundant element in living things, it is the most important. Carbon has the ability to bond with other atoms in different ways. Organic compounds are compounds based on carbon. All living things (organisms) are organic because they contain carbon. However, not all substances made from carbon are found in living things. Examples: diamonds, graphite entirely made of carbon, but not organic A compound that is not considered organic is called an inorganic compound. This includes all compounds that do not contain carbon. But it also includes some carbon-containing compounds that are not found in living things.

73 Carbon Millions of different carbon-based molecules exist. Carbon-based molecules can have many different structures. One important structure has molecules shaped like long chains. Another structure has molecules shaped like rings.

74 Carbon Chains Unlike atoms of other elements, carbon atoms can bond to each other to form very long chains. One carbon chain might contain hundreds of carbon atoms. A carbon chain can be straight or branched. In a branched carbon chain, other carbon atoms, or even other carbon chains, can bond to carbon atoms in the main carbon chain. Straight chains and branched chains are both results of carbon s ability to form four bonds. A very large carbon-based molecule made of repeating units is called a polymer. Each unit of a polymer is known as a monomer. A polymer can be thousands of atoms long.

75 Isomers There s another reason why there are so many carbon-based molecules. Carbon can form different molecules with the same atoms. The atoms in these molecules are in different places, therefore they have different structures. Because the atoms are arranged differently, they are actually two different substances. Compounds that contain the same atoms, but in different places, are called isomers.

76 Building Blocks of Life The organic molecules in living things fall into four major groups: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates include sugars and starches. They are found in goods, like pasta and bread. Lipids are fats or oils. Proteins are necessary for many functions in the body, including the formation of muscle tissue. Nucleic acids are the molecules that carry the genetic code for all living things.

77 Carbohydrates Carbohydrates are a type of molecule made up of subunits of sugars, and are used for energy and structure. They include sugars, starches, and cellulose. They contain three elements: carbon, hydrogen, and oxygen. Two main functions: Source of chemical energy for cells in many living things Part of the structural materials of plants

78 Lipids Lipids are a type of molecule made up of subunits of fatty acids. Lipids are found in fats, oils, and waxes, and are used for structure and to store energy. Most lipids are made of carbon, hydrogen, and oxygen. Animals store chemical energy in fat; plants store chemical energy in oils. Some lipids are important parts of cell structure.

79 Proteins Proteins are one of the many types of molecules made up of chains of amino acid subunits. They control the chemical activity of a cell and support growth and repair. Proteins contain carbon, hydrogen, oxygen, nitrogen, sulfur, and other elements. There are at least 100,000 proteins in your body, each with a different structure that gives it a specific function: Structural materials Control chemical reactions Transport substances within cells Part of the immune system and protect you from infections

80 Enzymes Some proteins that curl up into a shape of a ball are enzymes. Enzymes are a catalyst for a chemical reaction in living things. They are necessary for many chemical reactions in your body. Without enzymes, these reactions would occur too slowly to keep you alive.

81 Nucleic Acids Nucleic Acids are a type of molecule that is part of the genetic material of a cell, and is needed to make proteins. Examples: DNA and RNA They are made of carbon, hydrogen, oxygen, nitrogen, and phosphorus. Each of the cells in your body contains a complete set of nucleic acids. This means that each cell has all the instructions necessary for making any protein in your body.