DATE: 1st November 2007.
CLASS: Lower six.
TIME: Two periods (2 hours).
UNIT: Ionic Equilibria.
TOPIC : Acid-Base titration.
| REFERENCES CAPE chemistry by Mr. Arnold Mark Samai & Mrs. Susan Maraj (2007). | ||||
| PREREQUISITES Knowledge: students should know that Titrations are used to determine the concentrations of unknowns. Acids and Bases are classified as strong and weak and give examples of each type. Acids and bases react to produce salts and water in a neutralization reaction. The point at which neutralization occurs can be identified using indicators. Skill: students should know how to Manipulate burettes, pipettes, and other pieces of apparatus to conduct titrations. Materials and Resources: For teacher OHP Indicators(Phenolphthalein, methyl orange, screened methyl orange & bromothymolblue ) Acids(1M HCl solution) Alkali(1M NaOH solution) For students A pH meter Solution of acids(0.05M H2SO4 , 0.1M CH3COOH) Alkali (0.1M NaOH solution, 0.5M solution of ammonium hydroxide). One 50ml Burette One 25ml Pipette Magnetic stirrer Computer with Microsoft excel Printer | ||||
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| CONCEPT: Volumetric analysis is a method of analysis which focuses on determining concentrations of unknown solutions. Titrations as they are popularly called uses indicators to identify the equivalence point in a neutralization reaction involving acids and bases .An alternative technique that can be used to identify the equivalence point in a titration is to monitor changes in the pH of the solution as the titration is conducted. | ||||
| OBJECTIVES
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| PROCESS SKILLS During this lesson, student will be engaged in: Identifying/formulating a problem Designing and Planning an experimental procedure Setting up and executing experimental work Observing and measuring Recording data and observations Interpreting and evaluating data and observations Communicating scientific ideas, observations and arguments Applying scientific ideas and methods to solve qualitative and quantitative problems Decision-making based on examination of evidence and arguments Extracting from available information data relevant to a particular situation | X X X X X X X | ||||||||||||||||||||||||||
| ACTIVITIES: Set Induction Introduction: Teacher sets up eight beakers, four with acids and four with bases. Teacher then introduces two drops of each indicator into an acid solution and a basic solution. This step is repeated for phenolphthalein, methyl orange, screened methyl orange and bromothymol blue. Students are kindly asked to observe the color changes that occur for each indicator and suggest plausible explanations for the changes in color. Teacher then elicits responses from students to account for the color changes (it is anticipated that students will raise the issue of changes in pH to account for the color changes, if not guiding questions will be used to help them). Development: Teacher explains that an alternative strategy that can be used to determine the end point or equivalence point of a titration is the measurement of the changes in pH of the solution as the titration is conducted. Teacher elicits response from the class as to the difference between end point and equivalence point in a titration. Teacher builds on their knowledge stating clearly the differences between the terms. Students are asked to suggest what changes in pH are likely to occur during a titration. Teacher further reiterates the changes that are likely to occur during a titration in a discussion format with students. Students are then placed into groups of four and given instructions (appendix 1) to carry out four titrations using 1. A strong acid and a strong base such as hydrochloric acid and sodium hydroxide. 2. A strong acid and a weak base such as Hydrochloric acid and ammonia solution. 3. A weak acid and a strong base such as Ethanoic acid and sodium hydroxide. 4. A weak acid and a weak base such as Ethanoic acid and ammonia solution. Teacher demonstrates the technique of performing this titration using the pH meter emphasizing how the pH probes are to be used, how to zero the instrument, setting up and altogether conducting the titration. Since this is a psychomotor skill that needs to be learnt the teacher will guide student’s efforts on handling and manipulation of the apparatus. Teacher uses guiding questions to assess students understanding of the procedure they are carrying out. On completion of the practical exercise the class proceeds to the computer room for the continuation of the class. Students are then asked to use Microsoft excel and construct tables to display their results and to construct four graphs of pH versus volume of alkali added. Teacher elicits response from the class as to how the equivalence point can be obtained from the graph. Teacher demonstrates this using an OHP slide. Using the graphs students are then asked to determine the equivalence point of each titration which includes: (1) the volume of alkali at the equivalence point (2) The pH range or value at the equivalence point. Teacher asks students to brainstorm as to the method used by scientists to select indicators for titration. It is anticipated that a link will be made between the equivalence point in a titration and the pH range of an indicator. Teacher presents chart showing different indicators and the range at which they work best. Students are then asked to select indicators that would be suitable for each of the titrations they performed today and justify their choice. At this point the class is engaged in discussion. Prior to this lesson the teacher ensures that the laboratory technician is available when this laboratory exercise is being conducted as well as all the chemicals and the laboratory apparatus is available including the Ph meter. Permission is granted to move over to the computer laboratory after one hour to carry out this class. The computer technical is also asked to be present to assist in any technical matters that may arise. Note that throughout this exercise there will be continuous questioning to assess students understanding of concepts. | |||||||||||||||||||||||||||
| CONSOLIDATION Teacher summarizes and recaps lesson. Teacher gives questions to class to be done individually. 1. Define the following terms.(objective 1) (a) end-point (b) equivalence point (c) indicator range State the relationship between the three terms above. 2. (a) Sketch the titration curve for the reaction between potassium hydroxide and benzoic acid. Suggest an indicator that will be effective in this titration giving reasons for your answer. (Objective 4). 3. Using the diagram below as a guide, outline the steps that can be taken to determine the equivalence point of a titration using changes in pH. Your steps should give details of the procedure that is used to conduct this type of titration. (objective 2) 4. Using your computer at home construct a suitable graph using Microsoft excel for the titration of phosphoric acid and sodium carbonate suggesting a suitable indicator and justifying your choice.(objective 3&4)
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| Teachers Reflection. | |||||||||||||||||||||||||||
National Educational Technology Standards for Students.
- Creativity and Innovation.
Students demonstrate creative thinking, construct knowledge and develop innovative products using technology. Students:
a.) Apply existing knowledge to generate new ideas and products by using their prior knowledge of manipulating burettes and pipettes as well as their knowledge of acids and bases to carry out a new type of titration using a pH meter. In this way they are constructing new knowledge by using their prior knowledge as a scaffold to build and construct new knowledge by inquiry.
- Communication and Collaboration.
Students use digital media and environments to communicate and work collaboratively to support individual learning and contribute to the learning of others. Students:
a.) Interact, collaborate and publish with peers, a variety of digital environments and media. Students work in groups using a collaborative and interactive approach, in which they are better able to understand the issue that the lesson addresses. Negotiation of ideas takes place making learning meaningful.
3. Research and Information Fluency.
Students apply digital tools to gather, evaluate and use information. Students use planning strategies to guide inquiry. They organize their data in tables in excel, and analyze their data using graphical plots of pH verses volume of alkali.
5. Critical thinking, problem solving and decision making. Students use critical thinking skills to conduct research and make informed decisions using digital tools and resources. Students collect the data experimentally, analyze them using Microsoft excel and make decisions regarding suitable indicators for the titration.
6. Technology Operations and Concepts.
Students demonstrate a sound understanding of technology concepts, systems and operations. Students:
a. Understand and use technology systems.
b. Select and use applications effectively and productively.
Checklists for Applying the Technology Integration Planning Model.
Phase 1: Determining the “Relative Advantage” – Why use Technology?
----- Are there any topics or curriculum objectives I have difficulty teaching?
It is sometimes difficult to explain all the titration curves and why specific indicators work well for some titrations. Here I have discovered a new method of allowing the students the opportunity to decide for themselves. In a normal class without the use of ICT this process will take too long. Microsoft excel is an excellent tool that can be used to display data both in tabular form and in graphical form to show relationships immediately.
----- Do any of these instructional problem areas have technology – based solutions?
Yes as in this case where it is being used as a teaching aide which will also allow for the development of ICT skills. It will also allow disciplines to mix.
----- What is the relative advantage of the technology – based solutions?
The construction of the table and graph will allow me the opportunity to focus on whether student’s skills were good judging from the shape of the graphs obtained. It will also give students the opportunity to draw inferences and conclusions from experimental data.
----- Is the relative advantage sufficient to justify the effort involved in adopting these solutions?
Without any doubts or questions, I will definitely say ‘yes’.
Phase 2: Planning Assessments – What are Assessment Strategies?
----- What kinds of performances do I expect from students to show that they have learnt the topic?
I would expect students to become capable in the construction of graphs and more specifically to be able to use Microsoft excel in the future.
----- What is best way for me to assess students’ learning progress and products?
Assess the quality of graphs presented and focus on the meanings that students allocate to explain them.
----- Do the desired instruments exist or do I have to develop them?
They exist.
Phase 3: Planning Instruction – What are Appropriate Integration Strategies?
----- Will the instruction be single subject or interdisciplinary?
My instruction will be single subject. I will be looking at objective 6.1 from the Caribbean Secondary Examination Council (CSEC) Chemistry syllabus.
----- Should the instructional activities be individual, paired, small group, large group, whole class, or a combination of these?
The instructional activities will be small groups.
----- What sequence of activities should I teach?
(See lesson plan).
----- Have I allowed students enough time to get used to materials before beginning a graded activity?
No, but in a subsequent School Based Assessment exercise they will be tested allowing a few days for practice.
----- Have I built in demonstrations of the skills students will need to use both the equipment and the specific software?
Since Information and communication Technology is compulsory at the CSEC level at my school coupled with the fact that Microsoft excel is part of that course, there was no need to do demonstrations.
Phase 4: Logistics – How do I Prepare the Classroom Environment and Instructional Materials?
----- How many computers and copies of software will be needed to carry out the activities?
I will need 10 computers Microsoft excel to carry out my instructions.
-----How many computers and copies of software are available?
The school has 40 computers and copies of Microsoft excel and all of them are available.
----- Over what time period and for how long will technology resources be needed?
The technology resources will not be needed for the full lesson - 60 minutes. .
----- Do I need to schedule time in a lab or media centre?
I will need to schedule my 60 minutes in the computer lab.
----- If demonstrations and learning stations are to be used, will projection devices or large – screen monitors be needed?
Yes.
----- What other equipment, software, media and resources will be needed?
Printers will be needed.
----- Have I checked out the legality of the uses I want to make?
I checked the Information Technology (IT) teachers for availability of the computer lab and then checked the principal for permission to keep the overhead projector in the computer lab for 60 minutes.
----- Have I provided for students’ privacy and safety?
Yes, I ensured that the internet sites are only for students under the age of 18 years. The other sites are blocked off.
----- Have I become familiar with troubleshooting procedures specific to the piece of hardware or software package being used?
Yes, with the help of the IT teachers I was able to familiarize myself with the equipment.
----- Have I built in time to test run an equipment setup before students arrive?
Yes, I feel this is a very important area. Before the lesson, I need to check all computers to ensure that the internet is working, the Microsoft excel is properly installed, ink in the printer cartridges and the proper functioning of the overhead projector.
----- Have I built in time to back up important files? Have I trained students to back up theirs?
Yes, students who have their pen drives are told to use them as back ups. Students know how to use their pen drives so there is no need for me to train them on this.
----- Do I have the original program disks/discs handy to reinstall them if necessary?
The lesson will be taught in the computer lab and if I need to reinstall any programs, the IT technician will be there to assist.
----- Do I have a back up plan if I cannot use the resources as I had planned?
Yes I have a power point presentation ready to go.
These are the charts to be used by students to determine which Indicator is suitable for each titration.
pH range of the indicator.
Colour change and end-point of some indicators
| Indicator | Colour change | pH at end point |
| Methyl orange Litmus Bromothymol blue Phenolphthalein | Orange à yellow Red à blue Yellow à blue Colourless à pink | 3.7 6.5 7 9.1 |
Appendix 1: Instructions for practical exercises to be given to each student
Titration Curves of
Strong and Weak Acids and Bases
In this experiment you will react the following combinations of strong and weak acids and bases:
· Hydrochloric acid, HCl (strong acid), with sodium hydroxide, NaOH (strong base)
· Hydrochloric acid, HCl (strong acid), with ammonia, NH3 (weak base)
· Acetic acid, HC2H3O2 (weak acid), with sodium hydroxide, NaOH (strong base)
· Acetic acid, HC2H3O2 (weak acid), with ammonia, NH3 (weak base)
A pH electrode will be placed in one of the acid solutions. A solution of one of the bases will slowly drip from a burette into the acid solution at a constant rate. As base is added to the acid, you should see a gradual change in pH until the solution gets close to the equivalence point. At the equivalence point, equal numbers of moles of acid and base have been added. Near the equivalence point, a rapid change in pH occurs. Beyond the equivalence point, where more base has been added than acid, you should again observe more gradual changes in pH. A titration curve is normally a plot of pH versus volume of titrant. In this experiment, however, we will monitor and plot pH versus time, and assume that time is proportional to volume of base. The volume being delivered by the burette per unit time should be nearly constant.
One objective of this lab is to observe differences in shapes of titration curves when various strengths of acids and bases are combined. You will also learn about the function and selection of appropriate acid-base indicators in this experiment. In order to do several other experiments in this lab manual, you need to be able to interpret the shape of a titration curve.
Figure 1
MATERIALS
LabPro system 0.10 M NaOH
TI Graphing Calculator 0.10 M NH3
Vernier pH Amplifier and pH Electrode 0.10 M HCl
TI-Graph Link (optional) 0.10 M HC2H3O2
magnetic stirrer (if available) 50-mL burette
stirring bar ring stand
250-mL beaker 2 utility clamps
phenolphthalein indicator distilled water
wash bottle
PROCEDURE
1. Obtain and wear goggles.
2. Place 8 ml of 0.1 M HCl solution into a 250-mL beaker. Add about 100 ml of distilled water. Add 3 drops of phenolphthalein acid-base indicator. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin.
3. Place the beaker onto a magnetic stirrer and add a small stirring bar.
4. Prepare the pH system for data collection.
· Plug the pH amplifier into the adapter cable in Channel 1 of the LabPro. The pH electrode is already connected to the pH amplifier.
5. Use a utility clamp to suspend a pH electrode on a ring stand as shown in Figure 1. Situate the pH electrode in the HCl solution and adjust its position toward the outside of the beaker so that it is not struck by the stirring bar.
6. Obtain a 50-mL burette and rinse the burette with a few ml of the 0.1 M NaOH solution. Fill the burette to about the 0-mL mark. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
7. Turn on the calculator. Start the APPS program and proceed to the DATAMATE.
8. Set up the calculator for data collection.
· Select SETUP from the MAIN MENU.
· Arrow to MODE and press ENTER.
· Select EVENTS WITH ENTRY.
· Press OK. Select ADVANCED, then change the graph settings.
· Enter “0” as the minimum pH (Ymin).
· Enter “14” as the maximum ph (Ymax).
· Enter “1” as the pH increment (Yscl).
· Select OK to return to the main menu.
9. You are now ready to begin monitoring data.
· Press START to begin data collection. Carefully open the burette stopcock and add 1 ml of the NaOH. When the pH stabilizes, press ENTER. Type in 1 and press ENTER.
· Add another ml of NaOH. When the pH stabilizes, press ENTER. Type in 2 and press ENTER.
· Continue in the same manner until the pH doesn’t change when 1 ml is added. Press STO to end.
10. Watch to see if the phenolphthalein changes color before, at the same time, or after the rapid change in pH at the equivalence point. If phenolphthalein is a suitable indicator for this reaction, it should change from clear to red at about the same time as the jump in pH occurs. In your data table, record the elapsed time when the phenolphthalein color change occurs.
11. When you have finished collecting data, turn the burette stopcock to stop the flow of NaOH titrant. Examine the data points along the curve. As you move the cursor right or left, the ml of NaOH added (X) and pH (Y) values of each data point are displayed below the graph. Determine the approximate ml for the equivalence point; that is, for the biggest jump in pH in the steep vertical region of the curve. Record this time in the data table. Rinse the pH electrode and return it to the electrode storage solution. Dispose of the beaker contents as directed by your teacher. Clean and dry the 250-mL beaker for the next trial.
(Optional)
12. Use the TI-Graph Link cable and program to transfer the graph of pH vs. time to a Macintosh or IBM-compatible computer. Print a copy of the graph. Label the trial number on the paper copy of the graph.
(The next steps will take much time and are optional.)
13. Repeat the procedure using NaOH titrant and acetic acid solution, HC2H3O2. CAUTION: Handle the solutions with care. You do not need to refill the burette. Add 8 ml of 0.10 M HC2H3O2 solution to the 250-mL beaker. Add about 100 ml of distilled water and 3 drops of phenolphthalein to the beaker. Rinse the electrode and position it in the acid solution as you did in Step 5. Press 
, then select YES to repeat the data collection. Use the same Y-axis settings as in Part I. Repeat Steps 10-13 of the procedure.
14. Repeat the procedure using NH3 titrant and HCl solution. CAUTION: Handle the solutions with care. Drain the remaining NaOH from the burette and dispose of it as directed by your teacher. Rinse the 50-mL burette with a few ml of the 0.1 M NH3 solution. Fill the burette with NH3 solution to about the 0-mL mark. Add 8 ml of 0.10 M HCl solution to the 250-mL beaker. Add about 100 ml of distilled water and 3 drops of phenolphthalein to the beaker. Rinse the electrode and position it in the acid solution as you did in Step 5. Press , then select YES to repeat the data collection. Use the same Y-axis settings as in Part I. Repeat Steps 10-13 of the procedure.
15. Repeat the procedure using NH3 titrant and HC2H3O2 solution. CAUTION: Handle the solutions with care. You do not need to refill the burette. Add 8 ml of 0.10 M HC2H3O2 solution to the 250-mL beaker. Add about 100 ml of distilled water and 3 drops of phenolphthalein to the beaker. Rinse the electrode and position it in the acid solution as you did in Step 5. Press , then select YES to repeat the data collection. Use the same Y-axis settings as in Part I. Repeat Steps 10-13 of the procedure.
16. When you are finished, rinse the pH electrode with distilled water and return it to the electrode storage solution.
This procedure was adapted for the textbook: CAPE Chemistry by Arnold Mark Samai & Susan Maraj (2007).
Students' product
These are the graphs that are to be generated during the class from students’ experimental data.
| Strong Acid / Strong Alkali | |||||||||||
|
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| Volume of alkali | pH | ||||||||||
| 0 | 1 | ||||||||||
| 5 | 1.1 | ||||||||||
| 10 | 1.17 | ||||||||||
| 15 | 1.25 | ||||||||||
| 20 | 1.3 | ||||||||||
| 24.9 | 2.5 | ||||||||||
| 25.1 | 10 | ||||||||||
| 30 | 11.6 | ||||||||||
| 35 | 11.9 | ||||||||||
| 40 | 12.2 | ||||||||||
| 45 | 12.2 | ||||||||||
| 50 | 12.3 | ||||||||||
| Weak Acid / Strong Alkali |
| ||||||||||
| Volume of alkali | pH | ||||||||||
| 0 | 3 | ||||||||||
| 5 | 3.2 | ||||||||||
| 10 | 3.5 | ||||||||||
| 15 | 3.9 | ||||||||||
| 20 | 4.5 | ||||||||||
| 24.9 | 6 | ||||||||||
| 25.1 | 10 | ||||||||||
| 30 | 11.6 | ||||||||||
| 35 | 11.9 | ||||||||||
| 40 | 12.2 | ||||||||||
| 45 | 12.2 | ||||||||||
| 50 | 12.3 | ||||||||||
| | Strong Acid / Weak Alkali | ||||||||||
|
| |||||||||||
| Volume of alkali | pH | ||||||||||
| 0 | 1 | ||||||||||
| 5 | 1.1 | ||||||||||
| 10 | 1.17 | ||||||||||
| 15 | 1.25 | ||||||||||
| 20 | 1.3 | ||||||||||
| 24.9 | 2.5 | ||||||||||
| 25.1 | 9 | ||||||||||
| 30 | 10.8 | ||||||||||
| 35 | 11.3 | ||||||||||
| 40 | 11.6 | ||||||||||
| 45 | 11.9 | ||||||||||
| 50 | 12.2 | ||||||||||
| Weak Acid / Weak Alkali |
| ||||||||||
| Volume of alkali | pH | ||||||||||
| 0 | 3 | ||||||||||
| 5 | 3.2 | ||||||||||
| 10 | 3.5 | ||||||||||
| 15 | 3.9 | ||||||||||
| 20 | 4.5 | ||||||||||
| 24.9 | 6 | ||||||||||
| 25.1 | 9 | ||||||||||
| 30 | 10.8 | ||||||||||
| 35 | 11.3 | ||||||||||
| 40 | 11.6 | ||||||||||
| 45 | 11.9 | ||||||||||
| 50 | 12.2 | ||||||||||
These are the answers to the assessment questions.
Define the following terms:
End-point
Equivalence point
State the relationship between the three terms above.
The end point is a signal that marks the completion of the reaction in the titration. When an indicator is used in a titration, it is the point at which the colour of the indicator changes in the titration. The end point must coincide with the equivalence point in the titration. The equivalence point is the point where the number of moles of acid equals the number of moles of base in the titration.
3. (a) Sketch the titration curve for the reaction between potassium hydroxide and benzoic
acid. Suggest an indicator that will not be effective in this titration giving reasons
for your answer.
3. Using the diagram below as a guide, outline the steps that can be taken to determine the equivalence point of a titration using changes in pH. Your steps should give details of the procedure that is used to conduct this type of titration.
pH recordings
Set up the apparatus as shown in the diagram.
Ensure that the probe is always immersed in distilled water.
Zero the instrument with de-ionized water at 250C.
Do not use probe as a stirrer and always use a magnetic stirrer.
Record all readings to 1 decimal place.
Ensure apparatus is clamped firmly.
4. Using your computer at home construct a suitable graph using Microsoft excel for the titration of phosphoric acid and sodium carbonate suggesting a suitable indicator and justifying your choice.
| Strong Acid / Weak Alkali | |||||||||
|
| |||||||||
| Volume of alkali | pH | ||||||||
| 0 | 1 | ||||||||
| 5 | 1.2 | ||||||||
| 10 | 1.19 | ||||||||
| 15 | 1.3 | ||||||||
| 20 | 1.35 | ||||||||
| 24.9 | 2.7 | ||||||||
| 25.1 | 9.5 | ||||||||
| 30 | 10.4 | ||||||||
| 35 | 11.7 | ||||||||
| 40 | 11.9 | ||||||||
| 45 | 11.9 | ||||||||
| 50 | 12.4 | ||||||||
The End
