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.If Reaction 1 was found to be exothermic (heat producing), How could Experiment 1 be modified to examine only the effect of concentration on reaction rate?

Correct answer: A. The concentration of HCHO could be increased

• A. The concentration of HCHO could be increased
• B. The concentration of formaldehyde could be used to determine the color change
• C. A different method could be used to set the solution to a constant temperature
• D. The temperature of the solution could be increased

Last updated 3 September 2026