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Chapter 21: Chemical Reactions Page 632 Section 1: Chemical Change/Conservation of mass Section 2: Chemical Equations Section 3: Classifying Chemical Reactions Section 4: Reactions and Energy Physical or Chemical Change? • Matter can undergo two kinds of changes— physical and chemical. • Physical changes in a substance affects only physical properties, such as its size and shape, or whether it is a solid, liquid or gas. • Chemical changes produce new substances that have properties different from those of the original substances. Physical or Chemical Change? • A process that produces chemical change is a chemical reaction. Evidence of Chemical Reaction (p462) Some indicators of chemical changes include: • color changes, • energy (such as heat or light) production or loss, • production of bubbles (which indicates the production of a gas), • precipitation of a new solid when two solutions are mixed. Chemical Changes Burning of gasoline, wood, or charcoal are examples of chemical changes. A chemical change is referred to as a chemical reaction. The burning of charcoal (carbon) in oxygen is a chemical reaction and can be expressed as the chemical equation carbon + oxygen carbon dioxide reactants product Chemical Equations • To describe a chemical reaction, you must know which substances react and which substances are formed in the reaction. • The substances that react are called reactants (ree AK tunts). • Reactants are the substances that exist before the reaction begins. • The substances that form as a result of the reaction are called the products. Using Words • One way you can describe a chemical reaction is with an equation that uses words to name the reactants and products. • Reactants are on the left side of an arrow, separated from each other by plus signs and products are on the right side of an arrow, separated from each other by plus signs.. Chemical Changes chemical equations may be expressed in words, but are most commonly expressed using chemical formulas. carbon + oxygen carbon dioxide C + O2 CO2 reactants product Symbols used in equations (P635) Symbol Meaning Produces or forms + Plus (s) Solid (l) Liquid (g) Gas (aq) Aqueous, substance dissolved in water Heat ∆ The reactants are heated Light The reactants are exposed to light Elec The reactants are exposed to electric current Conservation of Mass (P632) • According to the law of conservation of mass, the mass of the products must be the same as the mass of the reactants in that chemical reaction. • This principle was first stated by the French chemist Antoine Lavoisier (1743-1794). • He showed that chemical reactions are much like mathematical equations. • In math equations, the right and left sides of the questions are numerically equal. Conservation of Mass • Chemical equations are similar to math equations, the mass must be equal on the two sides. Example (P633) • 10 grams of solid Mercury (II) Oxide when heated produced 9.3 grams of liquid mercury and 0.7 grams of oxygen gas. • HgO Hg + O2 • 10.0 gm 9.3 gm + 0.7 gm • Mass Balanced Chemical Equation Law of Conservation of Mass Page 463 • When a chemical reaction takes place, the total mass of reactants equals the total mass of products. • If 18g of hydrogen react completely with 633 g of chlorine, how many grams of HCl are formed? The equation for the reaction is • H2 + Cl2 2 HCl 18g + 633g ______ g Assignment (p463:1,2 and 637:6) • In the following reaction, 24g of CH4 (methane) react completely with 96g of O2 (oxygen gas) to form 66g of CO2. How many grams of H2O are formed? CH4 + 2O2 CO2 + 2 H2O 24g + 96 g 66g + ? • In the following reaction, 54 g of Al react completely with 409.2 g of ZnCl2 to form 196.2 g of Zn metal. How many grams of AlCl3 are formed? 2 Al + 3 ZnCl2 3 Zn + 2 AlCl3 54g + 409.2g 196.2g + ? • In making soap, 890 g of fat react completely with 120 g of sodium hydroxide to produce 92 g of glycerin and soap. How much soap is produced? fat + sodium hydroxide glycerin + soap Conservation of Matter • Not only must the mass be equal on both sides but the type and number of atoms of each type must be equal. Balancing Chemical Equations • When you write the chemical equation for a reaction, you must observe the law of conservation of mass. • When you count the number of carbon, hydrogen, oxygen, and sodium atoms on each side of the arrow in the equation, you find equal numbers of each kind of atom. Balancing Chemical Equations • This means the equation is balanced and the law of conservation of mass is observed. Balancing Equations •HgO Hg + O2 Balancing Chemical Equations • Not all chemical equations are balanced so easily. Trial and error • The following unbalanced equation shows what happens when silver tarnishes. Count the Atoms • When balancing chemical equations, numbers are placed before the formulas as you did for Ag. • These are called coefficients. However, never change the subscripts written to the right of the atoms in a formula. • Changing these subscripts change the identity of the compound. How to Balance an Equation (p639) • Magnesium burns with such a brilliant white light that it is often used in emergency flares. Burning magnesium leaves a white powder called magnesium oxide. To write a balanced equation for this follow these 4 steps. • STEP 1 Write formula and symbols to describe the reaction. Remember that oxygen is one of the diatomic gases. BrINClHOF • Solid Magnesium metal + oxygen Magnesium oxide • Mg(s) + O2(g) MgO(s) • STEP 2 Count the atoms in reactants and products. • Mg • O Balancing Continued • Step 3 Choose coefficients that balance the equation. • NEVER CHANGE SUBSCRIPTS • ____ Mg(s) + ____ O2(g) ____ MgO • STEP 4 Recheck the number of each type of atom on each side of the equation. Practice (page 640) • When lithium metal is treated with water, hydrogen gas and lithium hydroxide are produced. • STEP 1 Write the formulas and symbols • STEP 2 Count the atoms in reactants and products • STEP 3 Choose coefficients that balance the equation. Begin with atoms that occur only once, then polyatomic ions, then O and H • STEP 4 Recheck You Practice • Iron metal reacts with Chlorine gas to form solid Iron(III)Chloride. • Write the formula for each reactant and product. • Iron Metal • Chlorine gas • Iron(III)Chloride • Write the equation • Balance the equation Chapter 21 Assignments • • • • • • • • • • Practice Problems (test extra credit) Conservation of Matter Problems Writing equations worksheets Note taking worksheet Chapter Review page 656: 11-19, 29, 30 Thursday 4/28 Types of reactions Friday 4/29 Energy and Chemical Reactions Monday 5/2 Chapter Review Tuesday 5/3 Lab – John Adams Hall Wednesday 5/4 Test Chapter 21 Section 3 Classifying Chemical Reactions Page 641 Section 3: Classifying Chemical Reactions (4 or 5 classes) • Synthesis (composition) – Combustion • Decomposition – Electrolysis • Single Displacement • Double Displacement • You can’t learn these by going over them in class once! You must study them! Synthesis Reactions • In a synthesis reaction two or more substances combine to form a new compound. • Synthesis reactions are also known as composition reactions. • Synthesis reactions may be represented by the general equation Substance-1 + substance-2 new substance A + B AB Synthesis Reactions • Metals react with oxygen to form metal oxides. 2 Mg(s) + O2(g) 2 MgO(s) 04m15vd1 Combustion Reactions Page 641 (subclass) •In a combustion reaction a substance combines with oxygen, releasing a large amount of energy in the forms of heat and light. carbon + oxygen carbon dioxide + heat C(s) + O2(g) CO2(g) + heat Combustion (cont) • The burning of hydrocarbons such as methane, propane, and gasoline are examples of combustion. • Combustion of a hydrocarbon always produces carbon dioxide and water. • Propane burns to form carbon dioxide and water. • C3H8 + O2 CO2 + H2O Hydrocarbons • Methane CH4 • Ethane C 2H 4 • Propane C 3H 8 • Butane C4H10 Decomposition Reactions • In a decomposition reaction a single compound reacts to form two or more simpler substances. • It is the opposite of a synthesis reaction. • Usually requires energy (heat, light, electricity) • Decomposition reactions may be represented by the general equation Substance-1 substance-2 + substance-3 AB A + B Decomposition Reactions • There are many types of decomposition reactions. • The decomposition of a substance by an electric current is called electrolysis. • elec 2 H2O(l) 2 H2(g) + O2(g) Decomposition Reactions • Metal carbonates decompose with heat to yield a metal oxide and carbon dioxide gas. heat CaCO3(s) CaO(s) + CO2(g) Single-Displacement Reactions • Displacement reactions are also called replacement reactions. • In a single-displacement reaction one element replaces a similar element in a compound. • Single-replacements reactions may be represented by the general equations metal + compoundnew compound + different metal or Y + XB YB + X Single-Displacement Reactions • Hydrogen in water may be displaced by a metal to produce hydrogen and a basic solution. 2Na(s) + 2 H2O(aq) 2 NaOH(aq) + H2(g) 02m17vd1 What Replaces Activity Series (P643) • An element will replace any element below it on the activity series. • • • • • • • • • • • • • Lithium Potassium Calcium Sodium Aluminum Zinc Iron Tin Lead Hydrogen Copper Silver Gold Li K Ca Na Al Zn Fe Sn Pb H Cu Ag Au Double-Displacement Reactions • Displacement reactions are also called replacement reactions. • In a double-displacement reaction the ions of two compounds exchange places in an aqueous solution to form two new compounds. • Double-replacements reactions may be represented by the general equation AB(aq) + CD(aq) AD(s)+ CB(aq) Double-Displacement Reactions The “driving force” is the formation of an insoluble compound — a precipitate. Lead nitrate + potassium iodide potassium nitrate + lead iodide Pb(NO3)2(aq) + 2 K I(aq) 2 KNO3(aq) +PbI2(s) Double Displacement • Ba(NO3)2(aq) + K2 SO4(aq) BaSO4(s) +2 KNO3(aq) • This type reaction takes place only when one of the products formed is insoluable in water. Reaction Types (P641-643) Combustion A +O2 CO2 + ? + heat Synthesis or composition (2 or more to 1) A + B AB Decomposition AB A + B Single Displacement A + BC AC + B Double Displacement AB + CD AD + CB Energy in Chemical Reactions Section 4 page 646 • Often, energy is released or absorbed during a chemical reaction. • For example, energy of a welding torch is released when hydrogen and oxygen combine to form water. Chemical Energy Released • Where does this energy come from? • In reactions that release energy, the products are more stable, and their bonds have less energy than those of the reactants. • The extra energy is released in various forms—light, sound, and heat. Energy Absorbed • In reactions that absorb energy, the reactants are more stable, and their bonds have less energy than those of the products. Energy Absorbed • As you have seen, reactions can release or absorb energy of several kinds, including electricity, light, sound, and heat. • Electrical energy is needed to break water into its components. • Electrolysis Energy Absorbed/Released • Endothermic (en doh THUR mihk) reactions absorb heat energy. • Exothermic (ek soh THUR mihk) reactions release heat energy. Heat Released • Burning (combustion) is an exothermic chemical reaction in which a substance combines with oxygen to produce heat along with light, carbon dioxide, and water. Heat Absorbed • An example of an endothermic physical process that absorbs heat energy is the cold pack shown. • The heavy plastic cold pack holds ammonium nitrate and water. • The two substances are separated by a plastic divider. • When you squeeze the bag, you break the divider so that the ammonium nitrate dissolves in the water. • The dissolving process absorbs heat energy. Energy in the Equation • The word energy often is written in equations as either a reactant or a product. • Energy written as a reactant helps you think of energy as a necessary ingredient for the reaction to take place. Endothermic • Similarly, in the equation for an exothermic reaction, the word energy often is written along with the products. • Energy = heat, light, electricity, sound Reaction Rate • A rate tells you how much something changes over a given period of time. • Chemical reactions have rates, too. • The rate of reaction tells how fast a reaction occurs after it has started. • To find the rate of a reaction, you can measure either how quickly one of the reactions is consumed or how quickly one of the products is created. Slowing Down Reaction • A substance that slows down a chemical reaction is called an inhibitor. • And inhibitor makes the formation of a certain amount of product take longer. • Rust inhibitors. Fruit fresh (lemon juice) • Some inhibitors completely stop reactions. Speeding Up Reactions • A catalyst is a substance that speeds up a chemical reaction. • Catalysts do not appear in chemical equations, because they are not changed permanently or used up. Catalytic Converters • Catalysts are used in the exhaust systems of cars and trucks to aid fuel combustion. • Catalysts speed the reactions that change incompletely burned substances that are harmful, such as carbon monoxide, into less harmful substances, such as carbon dioxide. Chapter 21 Schedule • Monday --4-12 – Balancing Chemical Equations • Tuesday – 4-13 – Reaction Types • Wednesday -- 4-14 – Chemical Reactions and Energy • Thursday – 4/15 – Lab-Evidence for Chemical Reactions – Chapter Review P656-657:11-19,23,25,26,29,30 • Friday – 4/16 – Notetaking worksheets • Monday 4/19 – Chapter review • Tuesday 4/20 – Chapter 21 Test Rapid Release • Sometimes energy is released rapidly. • For example, charcoal lighter fluid combines with oxygen in the air and produces enough heat to ignite a charcoal fire within a few minutes. Slow Release • Other materials also combine with oxygen but release heat so slowly that you cannot see or feel it happen. • This is the case when iron combines with oxygen in the air to form rust. How Fast? • Fireworks explode in rapid succession on a summer night. • Old copper pennies darken slowly while they lie forgotten in a drawer. How Fast? • Not all chemical reactions take place at the same rate. • Some reactions, such as fireworks or lighting a campfire, need help to get going. • Others seem to start on their own. Activation Energy— Starting a Reaction • Before a reaction can start, molecules of the reactants have to bump into each other, or collide. • The collision must be strong enough. • This means the reactants must smash into each other with a certain amount of energy. • To start any chemical reaction, a minimum amount of energy is needed. • This energy is called the activation energy of the reaction. Temperature Changes Rate • You can keep the food you buy at the store from spoiling so quickly by putting it in refrigerator or freezer. • Food spoiling is a chemical reaction. • Lowering the temperature of the food slows the rate of this reaction. Temperature Affects Rate • Most chemical reactions speed up when temperature increases. • Molecules collide more frequently at higher temperatures that at lower temperatures. • This means they are more likely to react. Concentration Affects Rate • The closer reactant atoms and molecules are to each other, the greater the chance of collisions between them and the faster the reaction rate. • The amount of substance present in a certain volume is called the concentration of that substance. • If you increase the concentration, you increase the number of particles of a substance per unit of volume. Surface Area Affects Rate • The exposed surface area of reactant particles also affects how fast the reaction can occur. • Only the atoms or molecules in the outer layer of the reactant material can touch the other reactants and react. • To start a campfire use wood cut into fine sticks called kindling wood because it is has more surface area but less weight.