Numerous studies show that marine life is largely affected by global warming and ocean acidification. Calcium carbonate (CaCO3) is the main component of shells of marine organisms and coral’s skeleton1. Stony corals are foundations of reef structures, formed when each stony coral organism, or polyp, secretes a skeleton of calcium carbonate. Calcium carbonate provides both protection through the formation of shells while also allowing the creation of coral reefs which are essential to providing healthy and nourishing habitats for many marine organisms2. Ocean acidification is a process by which CO2 gets absorbed from the atmosphere into the water, increasing the concentration of H+ ions in the water through a series of chemical reactions and thereby lowering its pH (see Equation 1). Due to ocean acidification, some sea creatures have more difficulty forming shells, including coral polyps.
Equation 1: Ocean acidification equilibrium
Additionally, the degradation of calcium carbonate can be observed in the effects of acid rain on sculptures or monuments made from limestone or marble. Normal rain has an acidity of approximately a pH of 5.6 while acid rain usually has a pH between 4.2 and 4.43 due to a higher concentration of carbonic acid.
There are many other factors which have been researched to have an effect of the degradation of calcium carbonate, such as sunlight, temperature, various chemicals in the runoff from coastal industrial sources. I was particularly interested in investigating whether water current also had some effect on the degradation of calcium carbonate, and the extent of this effect compared to a change in pH. My initial hypothesis was that current strength would have some effect on the degradation of calcium carbonate due to an increase in collisions, and therefore reactions taking place, but that it would be less significant than the effect of the pH change.
It is important to understand how and why calcium carbonate degrades. More acidic oceans are not only caused by the increase in CO2 in our atmosphere, but also by numerous chemicals released in the runoff from coastal industries. The increase in sea level due to global warming also affects the current strength and dynamics around coral reefs and marine organisms, which also could possibly affect them in adverse ways.
Collision Theory and the Degradation of CaCO3
Collision theory explains how chemical reactions occur and why reaction rates differ for different reactions4, through calculating the theoretical percentage of successful collisions.
Equation 2: Rate constant
The collision frequency (Z) increases the rate of a reaction the larger it becomes (see Equation 2). This factor is based on the idea that if a system contains na type-a molecules per unit volume and nb type-b molecules per unit volume, then the number of a-b collisions per unit time in that volume will be proportional to the product nanb5 This means that the larger the concentration of either of the molecules taking place in the reaction, the larger the value of the collision factor and therefore, the higher the rate of the reaction. In Equation 2, it can be seen that temperature also affects the rate of a reaction, and therefore this variable was controlled during the experiment by conducting the experiment out of direct sunlight and at the same time every day.
Equation 3: Collision frequency
In equation 3, it can be seen that temperature, a measure of kinetic energy, influences the collision frequency. As a molecule’s velocity increases, it’s kinetic energy increases and therefore its temperature increases as well. When a molecule moves at a faster speed, it is more likely to collide in a certain time than a molecule which moves at a slower rate.
Degradation of CaCO3
The degradation of calcium carbonate by hydrochloric acid is a spontaneous reaction which produces CO2, H2O and CaCl2 (see Equation 4). The fact that water is produced in this reaction shows that the solution will become more dilute if no hydrochloric acid is added over time, and therefore the rate of reaction would decrease according to collision theory as the concentration of H+ ions in will decrease.
Equation 4: Reaction between Calcium carbonate and Hydrochloric acid
Collision theory demonstrates that an increase in H+ ions results in a higher collision frequency and therefore a higher reaction rate. The higher the reaction rate, the more CaCO3 will have been used and degraded. Secondly, though not explicitly states, the higher the velocity of H+ ions, the more frequently they will react with the calcium carbonate in the system. Therefore, it could be hypothesized that increasing the current strength in a closed container where this reaction takes places would increase the degradation of calcium carbonate.
1 Bryan Walsh. “Ocean Acidification Will Make Climate Change Worse.” Time, Time, 26 Aug. 2013, science.time.com/2013/08/26/ocean-acidification-will-make-climate-change-worse/.
2 “Global Warming’s Evil Twin: Ocean Acidification.” Climate Reality, 21 June 2016, www.climaterealityproject.org/blog/global-warming-ocean-acidification.
3 “What Is Acid Rain?” EPA, Environmental Protection Agency, 1 Mar. 2017, www.epa.gov/acidrain/what-acid-rain.
4 IUPAC, Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”) (1997). Online corrected version: (2006–) “collision theory”.
5 “Collision Frequency.” Wikipedia, Wikimedia Foundation, 6 Nov. 2017, en.wikipedia.org/wiki/Collision_frequency.