CHEM101: Diagnose the Acidity/Alkalinity of Chemical Substances - “Acid Test” Chemistry Assessment Answers

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Chemistry Assignment: Diagnose the Acidity/Alkalinity of Chemical Substances - “Acid Test”

INTRODUCTION Indicators are dyes that change colour depending on the pH of the solution. Some dyes occur naturally, For example, keen gardeners have noted that hydrangeas flowers are blue when grown in acidic soils, but in basic soils turn out pink. Another example is cyanidin, one of the pigments responsible for the stunning colours seen in red cabbage. These indicator molecules are usually weak acids that absorb different wavelengths of light depending on their protonation state. This property can be used to diagnose the acidity/alkalinity of chemical substances, literally providing an “acid test”. Task: Q1. (a) Which of your solutions was the most acidic and what was its estimated pH? (b) What is the corresponding hydronium ion concentration, [H3O]+? Q2. The key to the colour change seen in indicator molecules is they can exist in different forms at different pH. Three different forms of the cyanidin indicator molecule labelled X, Y and Z are shown below. Which form do you think is present at acidic pH, which one in neutral solutions, and which one in alkaline solutions? Briefly justify your answers, based on the availability of protons at different pH levels.   (a) acidic solutions ________ (b) neutral solutions ________ (c) alkaline solutions ___________ Q3. Based on your observations in the experiments of PART 3, steps 1-3, can the indicator molecule reversibly change from one form to another and back again? Explain how you can tell. Q4. The main component of vinegar is acetic acid, CH3COOH. Write a balanced chemical equation for the dissociation of CH3COOH in water, including the phase state of each chemical species. Q5. Based on your chemical equation in the previous question, write an expression for corresponding equilibrium constant for acid dissociation, Ka. (hint: which one of the species is NOT included in the expression?) Q6. The equilibrium constant for acid dissociation of acetic acid, Ka 1.8x10-5. What does this tell us about the favoured direction of the equilibrium (i.e. does it favour reactants or products)? Q7. Is acetic acid a weak or strong acid? Justify your answer? Q8. Write a balanced chemical equation for the neutralization of acetic acid (CH3COOH) with sodium bicarbonate (NaHCO3), including the phase state of each chemical species.