In the Acid Strength & Electron Withdrawing Groups Chemical Demonstration Kit, the effect of electron-withdrawing groups on the strength of weak acids is shown by forming half-neutralized solutions of acids and comparing their pH values.
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Take the weak acid vinegar, substitute three chlorine atoms for the three hydrogen atoms and, remarkably, the new acid is 10,000 times more powerful! Acids vary greatly in their strength—their ability to ionize or produce H+ ions when dissolved in water. In this demonstration, the effect of electron-withdrawing groups on the strength of weak acids is shown by preparing half-neutralized solutions of a series of chlorinated acetic acids and then using indicators to compare their resulting pH values. A colorful lesson in recognizing and identifying trends or patterns in chemical properties. Excellent for reviewing pKa determinations! Teacher Demonstration Notes, along with both a reproducible student worksheet and answer sheet are included.
Concepts: Acid strength, weak acid, conjugate base, equilibrium constant, inductive effect.
Time Required: 20 minutes.
Materials Provided: Acetic acid solution, chloroacetic acid solution, trichloroacetic acid solution, sodium hydroxide solution, phenolphthalein indicator solution, orange IV indicator solution, bromphenol blue indicator solution, methyl red indicator solution, universal indicator solution, disposable Petri dishes, disposable pipets.
Correlation to Next Generation Science Standards (NGSS)
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Disciplinary Core Ideas
HS-PS1.B: Chemical Reactions
Cause and effect
Systems and system models
Scale, proportion, and quantity
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.