IB 2022 May Chem SL IA Score 7

Chemistry Internal Assessment

Investigation of the effect of the NaOH concentration to the rate of phenolphthalein decomposition measured by a UV-Vis spectrophotometer

Investigation

Introduction and Rationale

During my IB Chemistry course, I came across acid deposition from chapter 8 and how acid rain can corrode buildings composed of concrete. 65% of buildings in Korea are made up of concrete and twice as much concrete is used in construction as all other building materials combined (Chen, 2001). Therefore, it would be extremely dangerous if the exposure to acid rain continues to corrode the building materials gradually. For instance, in Florida, Champlain Towers South apartment has collapsed due to continuous corrosion. Since I am currently living in a house made from concrete for exterior walls, I was worried that it could collapse if acid deposition is not controlled. I became curious if the corrosion level in a building could be estimated to reduce the damage. From my personal research on this area, I learned that phenolphthalein (C20H14O4), commonly used as an indicator for an acid-base titration, can be employed as an indirect indicator for the corrosion level. Phenolphthalein turns bright pink when corrosion happens, due to reaction with OH− ions (Gagg,2014). Although phenolphthalein can be a powerful tool to predict the corrosion level, I conjectured that if a color change occurs too slowly, it might lead to misinterpretation of data, as there may be a time lag between the corrosion level and the reaction indicated by the phenolphthalein. As the rate of reaction is dependent on the initial concentration of reactants, I wanted to investigate to what extent the change in the concentration of phenolphthalein affects the rate of color change. To study this, I came across a rate law from the IB Chemistry HL syllabus.

Research Question

How does change in the concentration of aqueous NaOH solution (0.025 M, 0.05 M, 0.1 M, 0.2 M, 0.3 M) affect the rate of reaction with phenolphthalein using a UV-Vis spectrophotometer?

Hypothesis

As the concentration of aqueous NaOH increases, the rate of reaction with phenolphthalein would increase and would be a first order manner. This is because the dissociation of NaOH is likely to be a first order. (Gagg, 2014)