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Should we delete Le Chatelier's principle from our chemistry curriculum?
In Hong Kong, 12 experienced chemistry teachers participated in a research study. They had difficulties in solving the following equilibrium problem:
The reaction N2(g) + 3H2(g) ⇌ 2NH3(g) is at equilibrium in a reactor fitted with a movable piston. The amounts of N2, H2, and NH3 in the equilibrium mixture are 0.510 mol, 0.197 mol, and 0.204 mol, respectively. The total volume of the gaseous mixture is 1.00 L. Predict the direction of the shift in equilibrium position if 0.140 mol of N2 gas is suddenly added to the equilibrium system at constant temperature and pressure. Clearly show your calculations.
Why did the 12 teachers fail to solve this problem? The difficulties encountered by the teachers can be found in the journal article below:
Cheung, D. (2009). Using think-aloud protocols to investigate secondary school chemistry teachers’ misconceptions about chemical equilibrium. Chemistry Education Research and Practice, 10(2), 97-108. (http://www.rsc.org/cerp)
In another study, 33 Hong Kong chemistry teachers had difficulties in solving the following three problems:
Problem #1. The reaction CS2(g) + 4H2(g) ⇌ CH4(g) + 2H2S(g) is at equilibrium in a reactor fitted with a movable piston. If a small amount of CS2(g) is suddenly added to the equilibrium mixture at constant temperature and pressure, what will happen to the number of CH4(g) molecules when equilibrium is re-established? Give reasons for your answer.
Problem #2. The reaction CO(g) + 2H2(g) ⇌ CH3OH(g) is at equilibrium in a reactor fitted with a movable piston. What will happen if some argon gas is added to the equilibrium mixture at constant pressure and temperature? Give reasons for your answer.
Problem #3. The reaction N2O4(g) ⇌ 2NO2(g) is at equilibrium in a syringe. If the volume is decreased at constant temperature by moving the plunger, will the concentration of NO2(g) be higher or lower than the original concentration when equilibrium is re-established? Give reasons for your answer.
Of the 33 chemistry teachers, only three teachers understood that the addition of more CS2 gas at constant pressure and temperature can shift the equilibrium system CS2(g) + 4H2(g) ⇌ CH4(g) + 2H2S(g) to the reactant rather than the product side. Only four teachers understood that the addition of argon gas at constant pressure and temperature will decrease the concentration of every gas in the CO(g) + 2H2(g)⇌ CH3OH(g) system and change its position of equilibrium. Only two teachers applied the equilibrium law successfully to predict that when the volume of the equilibrium system N2O4(g) ⇌ 2NO2(g) is decreased at constant temperature, the concentration of NO2 gas will increase in the new equilibrium state. Details of the research study can be found in the journal article below:
Cheung, D. (2009). The adverse effects of Le Chatelier’s principle on teacher understanding of chemical equilibrium. Journal of Chemical Education, 86(4), 514-518. (http://www.JCE.DivCHED.org)
Further Reading and Additional Teaching-Learning Materials:
Test Construction Support System for Chemistry Teachers (Version 2.0)
Stand-alone Application for Teachers
Web-based Application for Students
Cheung, D. (2006). TCSS: A new computer system for developing formative assessments. School Science Review, 88(322), 61-70.
Cheung, D. (2006). A test construction support system for chemistry teachers. Journal of Chemical Education, 83(9), 1399-1402.
Ten examples of inquiry-based chemistry experiments
Cheung, D. (2005). Investigating toothpastes through inquiry-based practical work. Science Activities, 42(3), 31-37.
Cheung, D. (2006). The carbon dioxide problem. Education in Chemistry, 43(2), 46-48.
Cheung, D. (2006). Learning about lithium through inquiry-based practical work. Hong Kong Science Teachers’ Journal, 23(1), 1-8.
Cheung, D. (2007). Investigating activation energies. Education in Chemistry, 44(1), 23-26.
Cheung, D. (2008). Facilitating chemistry teachers to implement inquiry-based laboratory work. International Journal of Science and Mathematics Education, 6(1), 107-130.
Some useful laboratory manuals
1. Bauer, R. D., Birk, J. P., & Sawyer, D. J. (2008). Laboratory inquiry in chemistry (3rd ed.). Belmont, CA: Brooks/Cole.
2. Cooper, M. M. (2003). Cooperative chemistry laboratory manual (2nd ed.). Boston: McGraw-Hill.
3. Kerner, N. K. & Lamba, R. S. (2008). Guided inquiry experiments for general chemistry: Practical problems and applications. Hoboken, NJ: John Wiley & Sons, Inc.
4. Lechtanski, V. L. (2000). Inquiry-based experiments in chemistry. Washington, DC: American Chemical Society/Oxford University.
An example of criteria for assessing written laboratory reports
Teaching chemistry through the jigsaw strategy
What is jigsaw strategy? (PDF file, 319 KB)
Examples of topics and subtopics for secondary school chemistry
1. Thalidomide: A controversial chiral drug (PDF file, 114 KB)
2. Methanal (PDF file, 52 KB)
3. Uses of EDTA (PDF file, 146 KB)
4. Uses of lithium (in Chinese) (PDF file, 156 KB)
5. Diamond (in Chinese) (PDF file, 576 KB)
Cheung, D. (2001). School-based assessment in public examinations: Identifying the concerns of teachers. Education Journal, 29(2), 105-123.
Cheung, D. (2002). Refining a stage model for studying teacher concerns about educational innovations. Australian Journal of Education, 46(3), 305-322.
Cheung, D. & Yip, D. Y. (2003). School-based assessment of chemistry practical work: Exploring some directions for improvement. Education Journal, 31(1), 133-152.
Cheung, D. & Yip, D. Y. (2004). How science teachers' concerns about school-based assessment of practical work vary with time: The Hong Kong experience. Research in Science and Technological Education, 22(2), 153-169.
Links to school-based assessment schemes in the UK
More hints on improving teaching and learning of chemistry
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Last updated 8 Sept 2009 The Chinese University of Hong Kong