Titrations
Name: Audrey Gonzalez
Date of lesson: Third Week of Acid Rain Unit Lesson
Length of lesson: Three 50 minute periods
Description of the class: Chemistry
Source of the lesson:
TEKS addressed:
(b) Introduction.
(1) In Chemistry, students conduct field and laboratory investigations, use scientific methods during investigations, and make informed decisions using critical thinking and scientific problem solving. Students study a variety of topics that include: characteristics of matter; energy transformations during physical and chemical changes; atomic structure; periodic table of elements; behavior of gases; bonding; nuclear fusion and nuclear fission; oxidation-reduction reactions; chemical equations; solutes; properties of solutions; acids and bases; and chemical reactions. Students will investigate how chemistry is an integral part of our daily lives.
(c) Knowledge and skills.
(2) Scientific processes. The student uses scientific methods during field and laboratory investigations. The student is expected to:
(B) collect data and make measurements with precision.
(14) Science concepts. The student knows the properties and behavior of acids and bases. The student is expected to:
(C) identify the characteristics of a neutralization reaction; and
A strong acid and strong base react to form water and a salt. If stoichiometric amounts react together, the strong acid and base will neutralize. An indicator is needed to show that the equivalence point has been reached. Students will engage in a week long lesson on neutralization and titration. They will be introduced to the concept with a titration simulation, and then they will perform an actual titration.
Students will be able to:
III. Resources, materials and supplies needed
Engage:
Titration Simulation
Titration
IV. Safety Considerations:
· Goggles must be worn at all times during titrations.
· Students need to notify teacher if spills occur so they can be clean up.
· Though they are dilute, acid and base may burn if come into contact with skin.
· Do not pour chemicals down the sink.
V. Supplementary material and handouts:
· Titration Simulation Instructions
· Titration Simulation Worksheet
· Determining Molarity
· Titration Lab
Five-E Organization
Teacher Does Probing Questions Student Does
| Engage: Learning experience(s) Day One Students will be introduced to the importance of indications for end points. Have baking soda in a 250 mL beaker and vinegar in a 50 mL beaker. In my hand I have some baking soda and vinegar. Pour the vinegar into the baking soda and walk around for the students to see. You are correct. When we mix vinegar and baking soda, bubbles are being produced. When the reaction is complete, it stops bubbling. Put down the vinegar and baking soda, and pick up a beaker with 25 mL dilute NaOH and methyl red and beaker with at least 25 mL HCl. I have beakers of HCl and NaOH. Let’s see what happens with this reaction. Pour HCl into NaOH. Solution should change from clear to red. |
Critical questions that will establish prior knowledge and create a need to know How do you know when a reaction is complete? What’s going to happen if I pour the vinegar into the beaker containing baking soda? How do we know for sure when it is over? Vinegar is what type of chemical? Baking soda is what type of chemical? How do we know when this reaction is complete. What just happened? Do you think the reaction is complete? |
Expected Student Responses/Misconceptions You can tell by looking at the reaction. If it stops bubbling or steaming it is finished. It’s going to react and start bubbling. When it stops bubbling and fizzing, the reaction will be over. It’s a weak acid- acetic acid It’s a base- sodium bicarbonate. Students may say a similar reaction of vinegar and baking soda will occur- when the bubbles stop. We can’t visibly tell. We would have to check the pH. When it’s neutral, the reaction is completed. When you mixed NaOH and HCl together, the solution changed color. Yes. |
| Explore: Have a brief introduction to neutralization and indicators. In the first reaction, we knew that the reaction was complete when it stopped bubbling and fizzing. In the second reaction, we saw the reaction was complete when it changed from clear to pinkish red. Some reactions must have some sort of sign that tells us when the reaction is complete. In this case we saw a color change. Correct. We have to perform calculations. The whole process of trying to figure out how much volume to use to react with another given volume of another solution is called a titration. There is a website that simulates titrations. We are going to the computer lab and have an introduction to titrations and neutralizations. If there are computers in the classroom, a computer lab is not need. Provide students with Titration Simulation Instruction and worksheet. Explain to students how to use the website. You will work in partners. Go to the link provided in your instructions. This will be preformed twice. You will notice that you have to select what you want to use in each section. You will select strong acid vs. strong base for section one. You then have the freedom to choose what you select for the other sections. Your goal for the simulation is to add the correct amount of acid/base in order to make the solution in the flask change color. This will require you to add acid/base by pushing the slider up or by pushing the dropwise button. You will need to do this slowly. Answer the worksheet questions as you do the simulation. |
Now that we know when a reaction is complete, how do we know how much acid or base we need to add to a solution? How did you know how much volume to add? Were all the volumes the same for the two trials? Once the color changed, what happened as you added more acid/base? When the color changed, were you moving the slider or where you using the dropwise button? How do you know if the volume you added was the exact amount for the reaction to be completed and cause the color to change? |
You can calculate it. I didn’t know. I had to guess. No; they were different. The volume of acid/base added when up, but nothing else happened. The solution did not change colors again. Slider. Dropwise button. If we used the dropwise button we know for sure since only a small amount was added at a time. If we used the slider then we don’t know for sure. |
| Explain: Day Two Congratulations, you performed a simulation titration! Titrations are a way of measuring the volume of one solution that is required to react with a given volume of another solution of known concentration, and usually involves an acid/base reaction. In our simulation, you should have noticed that near the bottom of the flask, the volume and concentration of the solution in the flask was given. This was the known concentration. By adding the solution in the burette until the solution changed color, we were able to determine the amount needed to do so, and we can calculate the concentration of that solution. In other words, if we have a beaker of acid or base with an unknown concentration, we could figure out that concentration by performing a titration. I have this jar full of NaOH. I don’t remember what the concentration is; I forgot to write it down on it. You need to help me figure it out by going through the steps of the simulation. Let’s use this known concentrated HCl to determine the concentration. We are using a strong base vs. strong acid. When a strong acid and base react, they neutralize. HCl + NaOH à NaCl + H20 Water and a salt are produced. We must add a chemical to determine when the reaction is complete. It is called an acid-base indicator. There are many different kinds of indicators. Their colors are sensitive to pH. All indicators have the same principle. They can exist in different forms. The color changes as the pH of a solution changes; their color is dependant upon whether they donate or accept protons. Phenolphthalein is a weak acid. When it has hydrogen attached to it, it is colorless. When it donates its protons, it is pink. If you add hydroxide ions to it, they remove the hydrogen ions and cause the indicator to turn pink. If you chose to use a base to fill the burette, the acid in the flask was colorless. As you added the base to the flask, the OH reacted with the indicator and the acid by pulling off the hydrogen. The base is called a titrant since you adding it to another solution. When the reaction was complete, the solution turned pink. There are many different indicators because they turn color when a certain pH is reached. Indicators have a transition interval where they have a pH range over which it changes color. Chemists have to determine which indicator to use by looking at the chemicals they use and matching up the equivalence point and its end point. The equivalence point is the point at which the two solutions used in a titration are present in stoichiometrically equivalent amounts, acid and base have reacted. The point where the indicator changes color is called the end point. Write equation on board. HCl + NaOH à NaCl + H20 For every mole of HCl, one more of NaOH react to produce a salt, NaCl and H20. Indicators that change color at the pH of which they react in this stoichiometric amount are used. As I mentioned earlier, a strong acid and strong base neutralize when combine together in stoichometrically amounts. We would need to use an indicator with an end point of pH 7. Not all indicators have equivalence point at pH = 7. Some change color at a lower pH than 7 such as Methyl orange. This is useful if we want to titrate a strong acid and weak base. The salt of this reaction is acidic. When we use a titration with phenolphthalein mixed with an acid, a base is added until the first appearance of a faint pink coloration that lasts for at least 15 seconds when the solution is swirled. This is the end point. At pH 7, phenolphthalein is clear, at pH 9 it is a light pinkish purple, and at pH 11 it is a hot pinkish purple. This is why we are able to use phenolphthalein as an indicator. |
What type of acid is HCl? What type of base is NaOH? What happens when a strong acid and strong base react? What is the final pH of a strong acid and strong base? How do we know when a reaction is complete? (What happened in our simulation?) What were some of the indicators you used in your simulation? Why do you think there are many different acid-base indicators? What do acids do in terms of protons? What do bases do? What do I mean by stoichiometrically equivalent amounts of acids and bases have reacted? Use the reaction of HCl and NaOH. What’s the importance of having the dropwise button on the website? |
Strong acid. Strong base. They produce water and salt. pH 7. It is neutral. It changes color. Methyl red, Bromothymol blue, methyl orange, and phenolphthalein. They are used for different acid and bases used. The donate protons. The accept protons. We learned last week, when a strong acid and base react, they form water so when HCl and NaOH combine, the form water and a salt. It takes one mole of HCl and NaOH to react completely. The slightest drop can cause the end point to be reached. |
| Extend/Elaborate: Now that understand how titrations work, we can calculate the molarity. We just learned that a strong acid and strong base must react stoichiometrically. The first step of a titration is to write out the balanced equation. Refer back the NaOH and HCl reaction equation. The simulation showed us that we had 25 mL of 0.1912 M HCl. We want to know the concentration of NaOH so we need to convert the moles of HCl to moles of NaOH. As you can see from this equation, one mole of NaOH reacts with one mole of HCl. This information is very important. Calculate the number of moles. Molarity = mol/ L. We need to multiply the volume by the molarity to get moles. When we do this, the liters cancels out and we are left with mol. 0.1912 M X .0025L = 0.00478 mol. We have a one to one ratio. 0.00478 mol HCl x 1 mol NaOH / 1 mol HCl. The moles of HCl cancel each other out and you are left with moles of NaOH. Determine the molarity of trials one and two from the titration simulations. Pass out Titration Lab. Have students do pre-lab for homework and finish up calculations from titration simulation if needed. The pre-lab includes purpose, materials, procedure, and data tables. |
We know how many moles of HCl we have, but how can we find the moles of NaOH? How many moles will NaOH contain in the reaction? How many moles of HCl do we have? How many moles of NaOH do we have? What’s the molarity of NaOH? How did you figure that out? |
We need to use the mole ratio from the equation to convert from moles of HCl to moles of NaOH. It will contain the same amount as HCl since it’s a one to one mole. 0.00478 mol The same as HCl, 0.00478 mol. We know the mol by calculating it from HCl. We know the volume from the amount of titrant needed. We divide mol by volume to get molairty. |
| Evaluation: Day Three Students will perform an actual titration. Their lab- write up and calculations will be used as the evaluation. They must include an analysis on neutralization and titrations to explain their lab. Students will be given instructions on how to perform the titration prior to the actual titration. Goggles must be worn at all times when performing the titrations. Remember how one drop can cause the end point to be reached. Remember to go very slowly. Keep swirling the flask. If the titrant splashes on the sides of the flask, swirl it around so that it goes in the solution and the reaction can take place with those drops of titrant. |
Why must you rinse the burette walls with the appropriate solutions? Why is it necessary to go back and forth by adding HCl and NaOH? What color changes are we expecting? Why are we using phenolphthalein? What is happening in this titration? How do you determine molarity? How would passing the equivalence point have an affect on anything? |
We need to clean the sides of the walls. They may have other chemicals that could affect the results. We want to find the exact point where the pH changes and neutralization has occurred. It’s colorless in acid solution and pink in basic. It will turn clear when we add enough HCl is added to make it acidic. We need an indicator to let us known when the equivalence point has been reached. Phenolphthalein’s end point is close the equivalence point which is neutral of this titration. HCl is causing NaOH in the flask to neutralize. This is when the number of moles of HCl equals the number of moles of NaOH. Subtract the final and initial volume of both HCl and NaOH to find the volumes used. Multiply molarity of HCl by volume to get mol. Use the mol ratio to find mol of NaOH. Divide mole NaOH by Vol NaOH to find molarity. Average the trials to find avg molarity. The pH would no longer be neutral. It would acidic or basic depending if you’re adding acid or base. |
Titration Simulation Instructions:
1) Go to the website:
2) In Select Type of Reaction in number one, click on Strong Acid vs. Strong Base.
3) Choose what you want to fill the burette with for number two: acid or base. Fill out the Titration Simulation Worksheet as you choose your own settings.
4) Select the type of acid and base you want to use for section 3 if Selection the Acid and Base.
5) Select the Indicator type for section 4.
6) Move the slider up to add volume of the acid/base you choose.
7) Push the red dropwise button to add a drop of acid/base.
8) Watch what happens in the flask as you add acid/base.
9) Once there is a color change, write out the total volume used. After recording the volume, watch what happens as you add more acid/base.
10) When finished recording, press the red reset button and repeat steps 1-9.
11) After selecting strong Acid vs. Strong Base twice, select Weak Acid vs. Strong Base and repeat steps 1-9.
Name:___________________________________________
Partner’s Name: _________________________________________
Titration Simulation Worksheet
Trial 1
1) Type of reaction selected ___________________________________________
2) What was the burette filled with? ______________________________________
3) What acid was selected? __________________________
4) What base was selected? _____________________________
5) Total volume used: _________________________
6) What happened as you added more acid/base? ____________________________
Trial 2
7) Type of reaction selected ___________________________________________
8) What was the burette filled with? ______________________________________
9) What acid was selected? __________________________
10) What base was selected? _____________________________
11) Total volume used: _________________________
12) What happened as you added more acid/base? ____________________________
Name:___________________________________________
Determining Molarity
1. Determine the Molarity of the Titrant in Trial One. Show all steps.
| Name of Titrant |
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| Volume of Titrant |
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| Name of solution in flask |
|
| Volume of flask |
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| Molarity of flask |
|
| Molarity of Titrant |
2. Determine the Molarity of the Titrant in Trial Two. Show all steps.
| Name of Titrant |
|
| Volume of Titrant |
|
| Name of solution in flask |
|
| Volume of flask |
|
| Molarity of flask |
|
| Molarity of Titrant |
Titration Lab
Determine the Molarity of NaOH by using 0.500 M HCl.
Materials:
Procedure: