The rate of a chemical reaction is obtained by dividing the amount of a chemical consumed in a reaction by the time in which the chemical is consumed. The rate at which a chemical reaction occurs is dependent on many variables, two of which are the concentration of a chemical in solution and the temperature at which the reaction occurs. In order to determine the effect of concentration and temperature on reaction rates, a chemist studied the following reaction sequence.HCHO + Na2SO3 + H2O → NaO3SOCH3 + NaOHReaction 1NaOH+ NaHSO3 → Na2SO3 + H2OReaction 2While Reaction 1 occurs slowly, Reaction 2 happens instantaneously. Both of the reactions occur in the same solution and base (NaOH) is built up in solution until all the NaHSO3 is used. Phenolphthalein, which is also in the solution, will cause the solution to turn pink when a measurable amount of base is present.Experiment 1:A standard solution of formaldehyde (HCHO) in water was prepared. This solution contained 0.3 mole of HCHO per liter of solution (0.3 M HCHO) and a small amount of phenolphthalein. This solution was diluted further with varying amounts of water and mixed with 10.0 mL of a 0.01 M solution of both Na2SO3 and NaHSO3 in water. The time required for pink color to appear is shown in Table 1.The fastest rate of reaction in Experiment 1 occurred when:
Correct answer: B. The concentration of NaOH was greatest in the reaction mixture
- A. The time for the color change to occur was greatest
- B. The concentration of NaOH was greatest in the reaction mixture
- C. The concentration of NaOH was smallest in the reaction mixture
- D. The concentration of HCHO was greatest in the reaction mixture
Explanation
The correct answer is Option B: The concentration of NaOH was greatest in the reaction mixture. The rate of reaction is indicated by the speed at which the pink color appears, which happens due to the presence of NaOH. A greater concentration of NaOH means the reaction progressed quickly, producing enough base to trigger the color change. Option A is incorrect because a longer time for color change suggests a slower reaction rate. Option C is incorrect as a smaller concentration of NaOH implies a slower reaction rate. Option D is incorrect because, although HCHO concentration impacts the reaction, the key indicator for reaction speed in this context is the NaOH concentration causing the color change.
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