Physical Changes and Chemical Reactions Gezahegn Chaka, Ph.D., and Sudha Madhugiri, Ph.D., Collin College Department of Chemistry Objectives Introduction To observe physical and chemical changes. To identify physical and chemical changes. To write balanced chemical equations to represent these physical and chemical changes. In a physical change the appearance of a substance changes but its composition and identity remains unaltered. Examples of physical changes include freezing of water to form ice, dissolving of sugar in a hot cup of tea, and the filing of metal to produce powder or filings. There are a number of evidences that suggest whether a physical change has taken place or not. Some of the evidences are changes in state or dissolution of a solid. In a chemical reaction a change in the composition and identity of a substance occurs. The observation of chemical changes (chemical reactions) and the proper descriptions of these changes are important concerns of chemistry. One method of describing a physical or a chemical change is with a chemical equation. For chemical equations, symbols written in parenthesis can be used to indicate the changes in the physical state of substances. In such equations, (aq) means in aqueous solution, (s) is for solid, (l) is for liquid, and (g) is for gas. For chemical changes, the chemical equation is a statement that represents the reacting species (the reactants) and those produced (the products) with formulas and symbols. A proper chemical equation is based on chemical evidence. Simply writing an equation does not mean the reaction takes place. Also to be in agreement with the law of conservation of mass, the equation must be balanced. Only balanced equations can be used in chemical calculations. To balance an equation, coefficients are used. Subscripts are NEVER changed to balance an equation.
Predicting the Products of Chemical Reactions The most difficult step is to predict the products of a chemical reaction. Rather than trying to memorize each and every chemical reaction, it is helpful to try and understand a few trends. A large number of inorganic chemical reactions may be classified into one of the four general categories: combination, decomposition, single displacement, and double displacement reactions. Combination Reactions: Two or more elements combine to form a new chemical compound or simple compounds combine to form one product. A + B AB Example: SO3 (g) + H2O(l) H2SO4 (aq) Decomposition Reactions: A compound splits itself up into two or more elements or compounds. A compound often decomposes after addition of energy (heat, light, or electricity) or by way of a catalyst. The products of a decomposition reaction are often small, stable molecules such as carbon dioxide, water, and oxygen gas. In any case, you must account for each type of atom since no atoms will be created or destroyed. AB A + B Example: 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) Single Displacement Reactions: Single displacement reactions involve the reaction of an element with a compound and one element takes the place of a different element in another reacting compound. A + BC AC + B Example: Fe(s) + CuSO4 (aq) FeSO4 (aq) + Cu(s) If a metal is used, it will only form cations, and so it must displace the cation in a single-displacement reaction. 2
Double Displacement Reactions: Two species in the reactants exchange places. Cations (positive ions) can only be paired with anions (negative ions). Two cations will not pair up; neither will two anions. In double displacement reactions, therefore, the cations and anions just switch. When an insoluble ionic compound is formed as a product, double displacement reactions can also be called as Precipitation Reactions. The solid formed is called a precipitate. AB + CD AD + CB Example: Pb(NO3)2 (aq) + 2 KI (aq) 2KNO3 (aq) + PbI2 (s) --- ( 1) For double displacement reactions, you should be able to write molecular, ionic and net ionic equations. Molecular Equation: The equation written as shown in equation (1) is called a molecular equation. Ionic Equation: In an ionic equation, the soluble ionic compounds are dissociated. Molecular substances, solids and liquids are not split up and are written as they were in the molecular equation. Example: Pb 2+ (aq) + 2 NO3 - (aq) + 2K + (aq) + 2I - (aq) 2K + (aq) + 2 NO3 - (aq) + PbI2 (s) ------ (2) Net Ionic Equation: Eliminate the spectator ions (which do not participate in the reaction) from the ionic equation (2) and write what is left over (3). Pb 2+ (aq) + 2 NO3 - (aq) + 2K + (aq) + 2I - (aq) 2K + (aq) + 2 NO3 - (aq) + PbI2 (s) Pb 2+ (aq) + 2I - (aq) PbI2 (s) ------ (3) 3
Acid-Base Neutralizations: These are a type of double displacement reactions where the reactants are acids and bases and the products formed will contain a salt and water. Example: Molecular Equation HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) acid base salt water In some of the acid base reactions, you might not visibly see any change happening. But look for other ways to identify the reaction as a physical or a chemical change. Molecular, ionic and net ionic equations can be also be written for acid-base neutralizations. Ionic Equation: H + (aq)+cl - (aq) Na(aq)+OH - (aq) Na + (aq)+cl - (aq)+h2o (l) Net Ionic Equation: H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) Na + (aq) + Cl - (aq) + H2O (l) H + (aq) + OH - (aq) H2O (l) Note that for a strong acid strong base reaction, the net ionic equation will be formation of water. Identifying a Chemical Change: There are many clues an observer might note that would suggest a chemical change is taking place. These could be the formation of a gas or a precipitate, a color change, the disappearance of a solid, or the warming or cooling of a reaction mixture caused by the evolution or absorption of heat. A splint test is used to identify gases produced in a chemical reaction. A wood splint is ignited and then extinguished. While the splint is still glowing, it is inserted into the test tube where the reaction has occurred. If the gas produced by the reaction is oxygen the splint will momentarily glow more brightly and may even ignite. If hydrogen gas is present, there will be an audible pop as the glowing splint is inserted into the test tube. If the splint is completely extinguished, then the gas produced must be a nonflammable gas such as CO2, N2, or Cl2. 4
Problems Observe the changes that occur in each of the following tests and classify the change as a physical change or a chemical change. Represent the changes in correctly balanced chemical equations. What evidence suggests that a physical or a chemical reaction took place in each procedure? Chemicals Needed copper(ii) sulfate silver nitrate iron magnesium zinc potassium chlorate hydrochloric acid sulfuric Acid manganese(iv)oxide sodium carbonate calcium carbonate iodine sodium hydroxide Procedures GOGGLES MUST BE WORN WHILE WORKING IN THE LABORATORY Test 1: Place about 10 ml of the blue copper(ii)sulfate solution in a small beaker. Dip a piece of steel wool, which is composed mainly of elemental iron into the blue copper(ii)sulfate solution by holding it in the solution with tongs for about 30 seconds and then take it out. Examine the coating on the steel wool. Record your observation. Now put the steel wool in the solution and let it remain for at least 15 minutes while you perform the other procedures. Record your observation. Test 2: Working in the fume-hood, place a small piece of zinc in a test tube. Add enough hydrochloric acid to just cover it. Perform a splint test on any gases that are produced. Record your observation. 5
Test 3. Working in the fume-hood, place on a watch glass a few drops of the liquid from the test tube in Test 2. Place the watch glass on wire gauze and gently heat until the water evaporates. What is the residue (compound) on the watch glass? Record your observation. Test 4: Working in the fume-hood, dropwise, add several drops of dilute hydrochloric acid (HCl) to a test tube containing a tiny amount of calcium carbonate. Perform a splint test on any gases that are produced. Record your observation. Test 5: Working in the fume-hood, ignite a small strip of magnesium ribbon by holding it with tongs in the flame of the burner. Do not look directly at the burning magnesium flame. Record your observation. Test 6: Mix a small quantity of solid potassium chlorate with solid manganese(iv)oxide in a test tube and attach the test tube to a stand with a clamp at a 45º angle. Working in the fume-hood, heat the content of the test tube over the Bunsen burner flame. Perform a splint test on any gases that are produced. Record your observation. (Note: MnO2 is a catalyst in this reaction and so does not need to be part of the chemical equation. A catalyst is a reagent that speeds up the chemical reaction without being consumed). Test 7: Take a small quantity of solid calcium carbonate in a test tube and attach the test tube to a stand with a clamp at a 45º angle. Working in the fume-hood, heat the content of the test tube using the flame from the burner. Perform a splint test on any gases that are produced. Record your observation. Test 8: Place 10 drops of sodium carbonate solution in a test tube. Add 5 drops of silver nitrate solution to it. Record your observation. 6
Test 9: Place 5 drops of 2.0M aqueous sulfuric acid in a test tube and add 2 drops of phenolphthalein indicator. Then add 10 drops of aqueous 2.0 M sodium hydroxide solution to it. Record the observation. Test 10: Working in the fume-hood, support a 150 ml clean and dry beaker with a wire gauze on a ring stand. Place 0.5 g of iodine in the beaker. Place an evaporating dish on top of the beaker. Place a small amount of ice in the evaporating dish. Heat the iodine in the 150 ml beaker very gently with low Bunsen burner flame. Observe the process inside the beaker and at the bottom of the evaporating dish. Record your observation. Critical Data to Include in Your Lab Report: For every reaction, write your detailed observations, write the balanced chemical equations, and classify the process as a physical or a chemical change and identify the type of reaction. If performed, explain the results of the splint test. For double replacements and acid-base reactions, write the molecular, ionic and net ionic equations also. References 1. T. G. Greco; L. H. Rickard; G. S. Weiss; Eds.; Experiments in general chemistry: Principles and Modern Applications. Ninth edition, Pearson Prentice Hall, 2007. 2. C. H. Henrickson; L. C. Byrd; N. W. Hunter; A Laboratory for General, Organic, and Biochemistry, Fifth Edition, McGraw Hill, 2007. 3. Southeastern Louisiana University; Department of Chemistry and Physics, Chemical Reactions and Chemical Equations, Chemistry 103, spring 2006. 7
Test # Observations Balanced Equation Physical or Chemical Reaction Type Splint Test
For Double Displacement and Acid-Base reactions: (Duplicate for each test that qualifies for these type of reactions) Test #: Molecular Equation: Ionic Equation: Net Ionic Equation: 9