My Acid Can Beat Up Your Acid! Strong and Weak Acids

Transcription

1 PRE-LAB: (Teacher signature) Look at your notes from and answer the following questions. 1. What ion is responsible for the properties of an acid? Give the name and formula? 2. How does an acid taste? 3. What determines how much damage an acid can cause? BACKGROUND Acids are compounds that donate hydrogen ions (H + ) in water. Acids (HA) can ionize or dissociate in the following ways: HA H + + A -1 Certain types of acids can be harmful when contact is made with them. Some acids will burn and cause open wounds when touched by the skin or eyes. If consumed they can burn the mouth, throat and stomach, deteriorate teeth and even turn the skin blue. In some cases they can cause death. Other acids will irritate the skin and cause rashes. The amount of damage caused by an acid is determined by how many hydrogen ions are present in solution. The more hydrogen ions present, the worse the effects. PROCEDURE PART A: CLASSIFYING ACIDS 1. Look at the following models in the Petri dishes. Answer the questions for each acid. Blue = H + Other color = A -1 ex. Br -1, ClO 2-1, PO 4-3. a. Petri Dish #1 = HNO 2 i. For this acid, what does the blue bead represent? Orange bead? 1

2 b. Petri dish # 2 = H 2 SO 4 i. Why are there two blue beads attached to each orange bead? ii. Do all the H + ionize from the A -1 ion at one time? How do you know? iii. For this acid, what ion does the blue bead represent? Orange bead? iv. Write the ionization reaction for this acid. + v. Record how many of the 10 acids molecules are whole molecules and how many acid molecules have broken into ions. Determine the % that are whole acids and % broken down into ions. c. Petri dish # 3 = HBr i. For this acid, what ion does the blue bead represent? Orange bead? + d. Petri dish # 4 = HF i. For this acid, what does the blue bead represent? Orange bead? 2

3 e Petri dish # 5 = H 2 CO 3 i. Why are there two blue beads attached to each orange bead? ii. Do all the H + ionize from the A -1 ion at one time? How do you know? iii. For this acid, what ion does the blue bead represent? Orange bead? iv. Write the ionization reaction for this acid. v. Record how many of the 10 acids molecules are whole molecules and how many acid molecules have broken into ions. Determine the % that are whole acids and % broken down into ions. f. Petri dish # 6 = HClO 4 i. For this acid, what ion does the blue bead represent? Green bead? + g. Petri dish # 7 = H 2 SO 3 i. Why are there two blue beads attached to each orange bead? ii. Do all the H + ionize from the A -1 ion at one time? iii. For this acid, what ion does the blue bead represent? Orange bead? iv. Write the ionization reaction for this acid. v. Record how many of the 10 acids molecules are whole molecules and how many acid molecules have broken into ions. Determine the % that are whole acids and % broken down into ions. 3

4 RESULTS AND ANALYSIS 1. Fill in the % ionization from Part A. Then look in your book (pg 532) at the table of strong and weak acids. Fill in the chart below. HNO 2 H 2 SO 4 HBr HF H 2 CO 3 HClO 4 H 2 SO 3 Name of acid % ionization Type of acid (Strong or Weak) 2. Looking at the data above, what is the relationship between the % ionization and a strong acid? 3. Looking at the data above, what is the relationship between % ionization and a weak acid? 4. (a)write a rule regarding strong acids and ionization, (b) write a rule regarding weak acids and ionization. a. b. 5. Look back at the acid ionization reactions, answer the following questions regarding these reactions: a. What type of acids have? c. What type of acids have? b. What does mean? d. What does mean? 4

5 PART B: WHAT HAPPENS WHEN WATER IS ADDED TO AN ACID? Canister number 1. At the lab table, look at the molecules/ions in the canister. Based on the ionization what type of acid is this? Explain why. 2. Place all the molecules/ions from the canister in the beaker. From the other canister, add half of the water beads to the beaker. By adding water, what have you done to the acid (think back to previous chapters)? 3. Does adding water change the % ionization of the acid? b. Is the acid strong or weak? 4. Add more water beads to the beaker. What type of acid is this now? Does this change the % ionization of the acid? 5. If the canister had the other type of acid (different from C-1), what would be the effect of water on the % ionization of this? ***Clean-up: Return the molecules to their correct canisters. PART C: RELATIONSHIP BETWEEN THE INDICATOR COLOR AND TYPE OF ACID. 1. Put on your goggles and aprons! 2. Fill one small test tube ¼ full with HCl. Do the same with HC 2 H 3 O 2. Add 2 drops of thymol blue to each test tube. Mix equally. Record on chart. 3. Repeat step 2 (new test tubes), then add equal amounts of water to each test tube. Record results. Thymol blue indicator colors: Large % ionization = pink Medium % ionization = orange Small % ionization = yellow Acid HCl HC 2 H 3 O 2 Type of acid Color of indicator Ionization (large, medium, small) Color of indicator (after water) Ionization (after water) 1. Compare the color of the solutions in the test tubes containing HCl (before and after adding water). Do the test tubes look the same? b. Why would they look different? 5

6 c. Now think about the % ionization of HCl in these two test tubes, are they the same? 2. Compare the color of the solutions in the test tubes containing HC 2 H 3 O 2 (before and after adding water). Do the test tubes look the same? b. Why would they look different? c. Now think about the % ionization of HC 2 H 3 O 2 in these two test tubes, are they the same? ***Clean- up: Pour the acid down the sink with water. ADDITIONAL QUESTIONS 1. What is the difference between a concentrated and a dilute acid? 2. A classmate states that H 2 CO 3 is a strong acid because it has two hydrogen ions to ionize. Explain why this is an incorrect statement. 3. A student dares you to drink either concentrated HC 2 H 3 O 2 or dilute HNO 3. Which one would be safer to drink? (Of course you shouldn t drink either of them.) Explain why? 4. What was the purpose/objective of this activity? a. b. 6