THREE CHEMICAL REACTIONS

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1 THREE CHEMICAL REACTIONS 1 NOTE: You are required to view the podcast entitled Decanting and Suction Filtration before coming to lab this week. Go to to view the podcast. You may have prelab quiz questions that are directly from the podcast. If your instructor believes that you have not watched the podcast, you may be sent home with a grade of zero! A chemical reaction is a change. One or more substances change to become different substances. For instance, when hydrogen gas (H2) reacts with oxygen gas (O2), water (H2O) is made. Two substances have combined to form one substance. In this case, the hydrogen and oxygen gases are called reactants and water is the product. Chemical reactions are often written as equations. The reaction between hydrogen and oxygen is written (in unbalanced form) as: H2 + O2 H2O [Eqn. 1] During chemical reactions, some chemical bonds are broken and new bonds are formed. The atoms are shuffled, re-organized. Atoms are not destroyed, however; nor are new atoms magically made. It is important to make sure that the number of each type of atom is the same on each side of the arrow in an equation. Equations are balanced with coefficients to show that matter is conserved. 2 H2 + O2 2 H2O [Eqn. 2] Equation 2 tells the reader that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. A hydrogen molecule consists of two hydrogen atoms bonded together by a single covalent bond (H H). An oxygen molecule has two oxygen atoms bonded together by a double bond (O=O). When the hydrogen and oxygen molecules react, these bonds are broken. New bonds are formed to make the water molecules (H O H). H H H H + O = O H O H H O H 2 H H bonds broken 1 O=O bond broken 4 new H O bonds formed A reaction such as this in which two different substances react to make one new substance is called a synthesis reaction. The opposite of synthesis reactions are decomposition reactions. They are easy to identify because they have only one reactant. To cause one substance to undergo a change in the absence of other chemical reactants requires an input of energy, usually in the form of heat or electricity. For example, when potassium chlorate is heated, potassium chloride and oxygen are produced. The bonds in potassium chlorate are broken, while new bonds are formed to make potassium chloride and oxygen. 2 KClO3 2 KCl + 3 O2 [Eqn. 3]

2 2 The symbol over the arrow is used in chemical equations to signify heat. Two other types of common reactions involve displacements. They are called single displacement and double displacement reactions. In a single displacement reaction, an element reacts with a compound to place a different element in its free state while forming a new compound: Mg + 2 HCl H2 + MgCl2 [Eqn. 4] element compound element compound The Mg has displaced H from the compound. In the end, H2 is a free element, and Mg is part of a compound. During a double displacement reaction, two ionic compounds exchange ions (monatomic or polyatomic) to become two new compounds. For example in Eqn. 5, Ag + and Na + ions are exchanged (displaced) to form the products. AgNO3 + NaCl AgCl + NaNO3 [Eqn. 5] Many reactions fall into the four categories mentioned so far: synthesis, decomposition, single displacement and double displacement. However, many chemical reactions do NOT fall into these specific categories. Numerous reactions are actually OXIDATION-REDUCTION (often called redox) reactions. Simply defined, oxidation-reduction reactions occur when a transfer of electrons happens between substances during bond breaking and making. In fact, three of the different types of reactions already defined are usually redox reactions: synthesis, decomposition and single displacement reactions. Double displacement reactions are NEVER redox reactions.

3 PURPOSE OF THIS LAB 3 In this lab you will perform three chemical reactions. In the first reaction you will react a sample of the element copper with nitric acid. Focus on what happens to the copper atoms. In this reaction, the Cu atoms will be converted to Cu 2+ ions and become part of a compound. The next reaction forms a different compound of copper. Finally, the last reaction will convert that compound into yet another different compound of copper. All of the reactions will be done in water. Water does not itself participate in the reaction; it simply provides a place for the reaction to occur. It is called the solvent. Substances which are dissolved in water are said to be "aqueous." They are denoted with the state symbol (aq) after their formulas in an equation. Remember the other state symbols, (s) for solid, (l) for liquid and (g) for gas. As you do these reactions, look for evidence that a change has occurred. Such evidence includes: color change temperature change formation of a precipitate (a solid that does not dissolve in the solvent) formation of a gas (seen as escaping bubbles) Also remember facts you already know when looking for evidence of a reaction. Elemental copper metal has a distinctive color. Other facts which you may not yet know, but which are helpful, are that an aqueous solution containing dissolved Cu 2+ ions has a blue color, and, that the gas NO2 has a brownish color to it. The three reactions you will do are as follows: 1. copper(s) + nitric acid(aq) copper (II) nitrate(aq) + nitrogen dioxide(g) + water(l) 2. copper (II) nitrate(aq) + sodium hydroxide(aq) copper (II) hydroxide(s) + sodium nitrate(aq) 3. copper (II) hydroxide(s) copper (II) oxide(s) + water(l) If no loss of copper occurs during these three reactions, all of the copper used in reaction 1 would be recovered as part of the product in reaction 3. However, a small amount of loss is unavoidable. Simply transferring a solution from one container to another will cause loss. Large losses occur when you have a spill or an accident. To measure the amount of these expected (and unexpected) losses, you will calculate your percent yield of copper (II) oxide. mass of copper (II) oxide recovered percent yield = x 100 theoretical mass of copper (II) oxide Keep in mind that the theoretical product yield (mass) in a chemical reaction can be calculated using dimensional analysis, following the common solution strategy

4 4 mass reactant ( in g) moles reactant moles product mass product (in g) The following work demonstrates the proper use of units to calculate the expected mass of oxygen gas from the decomposition of g of potassium chlorate (KClO 3 ). The balanced equation for this reaction is shown as equation 3 (found on page 1 of this experiment) g KClO3 1 mole KClO3 3 moles O g O2 =? g O g KClO3 2 moles KClO3 1 mole O2 What is the expected mass of the other product from this reaction? g KClO3

5 PROCEDURE 5 1 COPPER + NITRIC ACID products 1. Place a watch glass on a balance. Press the ZERO key. The display should be g. Make a note of which balance you use. Use it for all mass measurements in this experiment. 2. Add copper turnings to the watch glass until the total is between gram. Record the actual mass you use. 3. Roll the copper into a compact ball and place it into a clean thick-walled 250-mL beaker. 4. SAFETY NOTE Handle nitric acid (HNO3) with care. Avoid breathing its vapors and avoid skin contact. If you do spill some on yourself, wash immediately with cold water. Notify your instructor. In the fume hood, carefully dispense 3.0 ml of nitric acid into the copper in the beaker by first holding the beaker under the tip of the dispenser. Then simply lift the handle gently until it stops and lower the handle all the way to deliver exactly 3.0 ml. Is there any evidence of a chemical reaction? Leave the beaker in the hood until the copper is dissolved completely. Gently swirl the beaker to make sure the copper is immersed in the acid. The reaction is over when NO MORE copper metal is visible. You can then take the beaker to your work area. 5. When the reaction is over, slowly add about 10 ml of deionized water to the 250-mL beaker to dilute the solution.

6 6 2 COPPER(II) NITRATE + SODIUM HYDROXIDE products 6. SAFETY NOTE Handle solutions of sodium hydroxide (NaOH) with care. Avoid contact with your skin. If you do spill some on yourself, wash immediately with cold water. Notify your instructor. Into a small beaker, measure out about 15 ml of 6 M sodium hydroxide. 7. While stirring the contents of the 250-mL beaker, add a small amount of the sodium hydroxide to it. You may see the formation of copper (II) hydroxide, a light blue precipitate. Keep adding sodium hydroxide until the solution becomes basic. You will know this by testing the solution with litmus paper. Dip a clean stirring rod into the solution. Touch the wet rod on a piece of red litmus paper. When litmus turns blue, the solution is basic. STOP adding sodium hydroxide when the solution becomes basic. NOTE: When testing this solution on litmus paper, don't be confused by the blue precipitate, which sometimes clings to the stirring rod. Some students mistake this blue solid for blue litmus. Focus on the WET part of the litmus paper to decide whether or not your solution is basic. 3 COPPER(II) HYDROXIDE products 8. Increase the volume of the solution from step 7 by adding about 100 ml of deionized water. 9. Place the beaker on a stirrer hot plate. Add a magnetic stirring bar and turn the knob to get the bar to stir the solution. Boil the solution until all of the copper (II) hydroxide has been converted into copper (II) oxide. You can determine when this has occurred by watching the colors. Copper (II) oxide is a black solid.

7 7 10. Obtain a filter flask, a porcelain Büchner funnel with rubber adapter, and a piece of filter paper to fit the funnel. Weigh the filter paper on the analytical balance to four decimal places. 11. Allow the black precipitate in your beaker to settle until the top liquid layer is clear. Decant most of the clear liquid into a separate beaker before filtering. Wet the filter paper in the Büchner funnel with a small amount of deionized water from a squeeze bottle just prior to filtering. 12. Filter the black solid from the mixture. Use a stream of deionized water from a squeeze bottle to rinse all traces of the black precipitate from your beaker into the funnel. SAVE the solid. The liquid, which should be clear, can be poured down the drain. While you are filtering, begin the next step. 13. Heat about ml of deionized water in a beaker on the hot plate. Wash the black solid on the filter paper with the hot deionized water. Allow the wash to drain through. SAVE the solid. The liquid from the washing can be poured down the drain. 14. Turn off the aspirator water flow. Carefully remove the Büchner funnel with the filter paper and place it into a clean 250 ml beaker. Allow the filter paper and solid to dry in your tray until the next laboratory period. At that time you will weigh the paper and product in order to determine the mass of the black copper (II) oxide.

8 8 THREE CHEMICAL REACTIONS Data Sheet Name Partner Mass of copper used (step 2) Mass of filter paper + product (step 14) Mass of filter paper (step 10) Mass of copper (II) oxide g g g g THREE CHEMICAL REACTIONS Post-lab Questions 1. Using your data, calculate how many moles of copper you used in reaction Look carefully at the three reactions you have performed. What is the mole ratio between copper in reaction 1 and the copper (II) oxide produced in reaction 3? Using that information, calculate the theoretical yield of copper (II) oxide using dimensional analysis. (Assume copper is the limiting reagent.) 3. Calculate the percent yield of copper (II) oxide. 4. Explain why you obtained a yield different than 100%. Give several possible reasons.

9 9 THREE CHEMICAL REACTIONS Pre-lab Assignment Name Date Section 1. Using chemical formulas, write out the equations of the three reactions you will do, then balance. (1) (2) (3) 2. Classify the reactions as synthesis, decomposition, single displacement or double displacement. Note: the first reaction is a redox reaction and does not fall into any one of the other four categories. (1) redox (2) (3) 3. During reaction 1, a brown gas is produced. What is its formula? 4. Reaction 2 involves combining a solution of sodium hydroxide with a solution of copper (II) nitrate. The sodium hydroxide solution is colorless. a. What color is the copper (II) nitrate solution? b. What species is responsible for this color? 5. How do you know when reaction 3 is complete? 6. A student started with g of copper. The mass of filter paper and copper (II) oxide product was found to be g, and that of the filter paper alone was g. Calculate the percent yield of copper (II) oxide. Last revised 9/21/2016 DN