POGIL EXERCISE 12 How Can You Predict Chemical Reactions?

Transcription

1 RUN TIME: 60 MIN POGIL 12 Page 1 of 7 POGIL EXERCISE 12 How Can You Predict Chemical Reactions? Each member should assume his or her role at this time. The new manager takes charge of the POGIL folder and hands out the GRF and RRF to the appropriate members. The new recorder should record the names of the group members on the new GRF. Table 1. Group Member Role Assignments GROUP TYPE -> GROUPS OF THREE GROUPS OF FOUR MEMBER NO. -> Manager + + Reporter + + Recorder + + Reflector + + Technician Encourager + + SFUC + * + * OBSERVATION I. The power of any science is the ability to predict what will happen given the circumstances of the event. In chemistry this can involve the analysis of reactants to predict if a reaction will take place and what products can be formed. Sometimes just looking at a chemical equation is not enough to determine if a reaction took place. So we need other ways of looking at chemical equations. An alternative method is looking at the ionic and net ionic chemical equations. When soluble ionic compounds are placed in water, they disassociate (fall apart) into their respective ions. Equation 1 describes what happens when sodium sulfate is added to water. This process EQ1: Na2SO4 2 Na + + SO4-2 is called ionization. Only compounds made up of ions (Ionic compounds) are subjected to this process. Equation 1 is an example of a skeletal ionization equation. Insoluble ionic compounds as well as most covalent compounds do not ionize when placed in water. +5 MIN

2 RUN TIME: 60 MIN POGIL 12 Page 2 of 7 1. Write a primary ionization equation that describes what happens when each of the soluble ionic compounds below is placed in water. a. Ammonium sulfate (example): (NH4)2SO4 2 NH4 + + SO4-2 b. Calcium chloride: c. Potassium nitrate: d. Tin(II) chloride: e. Copper(II) iodide Equation 2 describes the reaction between sodium hydroxide and phosphoric acid. EQ2: NaOH + H3PO4 Na3PO4 + H2O 2. Name all the ionic compounds in Equation 2 and write their ionization equation like the example in 1a above. 3. Name all the covalent compounds in Equation MIN Submit your work to instructor for validation OBSERVATION II. When we combine all the primary ionization equations for each of the ionic compounds in the reaction and substitute them into Equation 2, we obtain the complete ionic skeletal equation for the reaction (Equation 3). NaOH H3PO4 Na3PO4 EQ3: Na + + OH H + + PO4-3 3 Na + + PO4-3 + H2O The ions that appear on both sides of the equation are known as spectator ions; the remaining ions are the ones that drive the chemical reaction to completion because they appear differently in the reactants than in the products. 4. Strike out the spectator ions in Equation 3 and write what s left in Equation 4. This equation is the net ionic skeletal equation. EQ4:

3 RUN TIME: 60 MIN POGIL 12 Page 3 of 7 5. Now, write the balanced Equation 4 in the space below by making sure that the same number of each element is on both sides of the equation. This is called the balanced net ionic equation. EQ4: 6. Using the balanced Equation 4, rewrite and balance Equation 2 in the space below. The recorder should submit answers to Items 4-6 to instructor for validation. Calcium chloride reacts with sodium phosphate to yield solid calcium phosphate and sodium chloride. 7. Write a skeletal chemical equation for this reaction in Equation 8 and double check the formulas with the instructor to make sure they are right. EQ8: 8. Next, using the Equation 8, write the complete skeletal ionic equation in the space designated Equation 9. EQ9: 9. Strike out the spectator ions in Equation Enter whatever is left in Equation 9 into the space labeled Equation 10; this is the skeletal net ionic equation. EQ10: The recorder should submit answers to Items 8 & 9 to instructor for validation. OBSERVATION III. As previously discussed a reaction goes to completion when reacting species appear differently on both sides of the chemical equation. In this exercise we will investigate four primary concepts that will enable us to predict whether a chemical reaction will take place and the products that will be formed. Here is a summary of the four concepts: +25 MIN

4 RUN TIME: 60 MIN POGIL 12 Page 4 of 7 You know that a reaction has taken place if a. A solid is formed. b. A gas is produced. c. Water is formed. d. Oxidation and reduction take place. OBSERVATION IV. A reaction has taken place when a solid is produced. This usually happens when some ions bind to each other in such a way that water can t separate them. This makes them insoluble. When encountered in the lab, the solid that forms is called a precipitate. To use this indicator you will have to have a good idea of what compounds are likely to be soluble and which ones are likely to be insoluble. Table I is what you will need to refer to in this exercise and MEMORIZE for the rest of the course. (There is a similar list in Table 5.4 (P. 194) of your test; however, the use of the following solubility rules are recommend for maximum grade in this course): Table II. Common Solubility Rules Formula of SOLUBILITY RULE Target Ion(s) A. Soluble [EXCEPTIONS] 1. All Group IA (alkali metal) salts Li +, Na + 2. All ammonium salts 3. All acetates 4. All nitrate salts 5. Chlorides, bromides and iodides [EXCEPT HALIDES OF sliver, mercury, and lead] [ ] 6. Most sulfates [EXCEPT SULFATES OF calcium, strontium, barium, and lead]. [ ] B. Insoluble [EXCEPTIONS] 1. Phosphates [EXCEPT Group IA and ammonium cations.]` 2. Carbonates [EXCEPT Group IA and ammonium cations.] 3. Hydroxides [EXCEPT Group IA and ammonium cations and hydroxides of strontium, barium and calcium.] 4. Sulfides [EXCEPT Group IA, IIA and ammonium sulfides]. 11. Please fill in the second column of Table II with the symbol of the ion(s) mentioned in the rules in Column 1 as soluble or insoluble and exceptions when indicated by brackets. +30 MIN

5 RUN TIME: 60 MIN POGIL 12 Page 5 of Indicate whether each compound below is soluble (S) or insoluble (I). a. (NH4) 3PO4 f. KOH k. Fe2S3 b. BaCO3 g. Ca(OH)2 l. MgS c. AgI h. Pb(C2H3O2)4 m. BBr3 d. ZnCl2 i. Hg2(NO3)2 n. CsCl e. SrSO4 j. CuSO4 o. MnCl4 Consider the proposed reaction in Equation 11 EQ11: 3 Pb(NO3)4 + 4 BCl3 4 B(NO3)3 + 3 PbCl4 13. List the compounds in Equation 11 that are soluble and the appropriate rule: 14. List any compound Equation 11 that is not soluble and the appropriate rule: 15. Has a net reaction taken place? Give a reason for your answer: 16. A solution of sodium chloride was added to a solution of ammonium sulfate. When the water was removed and analyzed to additional compounds were identified: ammonium chloride and sodium sulfate. Write a skeletal chemical equation to describe this reaction (Equation 12). EQ12: 17. List the compounds in Equation 12 that are soluble: 18. List any compound in Equation 12 that is not soluble: 19. Has a net reaction taken place? Give a reason for your answer: +50 MIN

6 RUN TIME: 60 MIN POGIL 12 Page 6 of 7 OBSERVATION V. You know that a reaction has taken place if a gas is formed in the reaction. Common reactions of this type involve the production of carbon dioxide, hydrogen, and oxygen. 20. When one adds acetic acid (CH3COOH) to sodium carbonate an immediate formation of bubbles is observed. The bubbles are carbon dioxide. Analysis shows that the other product is sodium acetate. Write a skeletal equation to describe this chemical reaction (Equation 13) EQ13: 21. Did a true chemical reaction take place? What is the explanation of your answer? OBSERVATION VI. You know that a reaction has taken place when water is a product of the reaction. This is because the water that is formed comes from a hydrogen ion (H + ) and a hydroxide ion (OH - ). Since under most environmental conditions water can not break apart (it is a covalent bond that s formed), the reaction goes to completion. Most of the reactions that produce water are acid-base reactions. The other product of these reactions is a salt consisting of the cation from the base and the anion from the acid. 22. Potassium hydroxide reacts with sulfurous acid to yield potassium sulfite and water. Write a primary chemical equation to describe this reaction (Equation 14). EQ14: 23. Write a complete skeletal ionic equation for this reaction as Equation 15 and strike out the spectator ions. Then write the net skeletal ionic equation in Equation 16 and finally write the complete balanced ionic equation for Equation 17. EQ15: EQ16: EQ17: +50 MIN

7 RUN TIME: 60 MIN POGIL 12 Page 7 of Did a true chemical reaction take place? What is the explanation of your answer? 24. Write a balanced chemical equation for the reaction of potassium hydroxide and phosphorous acid in space labeled Equation 18. EQ18: 25. Write a balanced chemical equation for the reaction of calcium hydroxide and potassium acetate in the space labeled Equation 19. EQ19: 26. Did a reaction take place? How do you know? OBSERVATION VII. You know that a reaction has taken place when you detect oxidation and reduction in the reaction. This is subject will be addressed in detail in a latter POGIL lesson. EXERCISE END. Managers should collect the GRF and RRF, staple them together, and place in the back of the left pocket of the folder. The folder should be closed and left on the table. NOTIFY INSTRUCTOR WHEN FINISHED. WAIT FOR INSTRUCTIONS. +60 MIN