Chemical and Biomolecular Engineering

Official blog of the Lehigh University Chemical Engineers

Equilibrium Reactions — Chem Lab

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In a previous Chem 31 Lab, we studied systems that form equilibrium reactions and the affect Le Chatelier’s Principle has on those reactions. Le Chatelier’s Principle basically states: when a stress is added to a system at equilibrium (alter concentration, pressure, temperature, volume), the system will then proceed to a new equilibrium state. One method in determining the direction of the shift is qualitative analysis. By observing a color change or formation of a precipitate (solid), one can determine if a system shifted toward the products or reactants. After observing and determining the change in equilibrium, the reasoning behind the shift can be resolved. In the lab, we completed various reactions and observed what we saw.

The first was an acid-base reaction which exemplified common ion effects. Due to the toxicity, this had to be done through a YouTube video. HCl (strong acid) and NaOH (strong base) are added to a mixture of chromate and dichromate. Here is the chemical equation : chromate-dichromate eqn

Adding HCl increases the H+ concentration causing the solution to be orange, shifting right, away from the increase. Adding NaOH increases the concentration of OH- ions which bond with H+ to produce water. This increases the H2O concentration and shifts the equilibrium away from the addition, causing the solution to become yellow.

The next reaction involved saturated Sodium Chloride (NaCl —  table salt). Saturated simply means that a solution cannot dissolve any more liquid, some solid remains in the otherwise liquid solution. We added HCl and then water to the solution after removing the dissolved salt from solid. Adding HCl increases the concentration of Cl- pushing the equilibrium to form solid NaCl, so a white precipitate appeared, the salt. Next, by adding H2O the volume increases; an increase in volume causes a shift to the side with my moles (ie. the right side). This causes the precipitate (salt) to dissolve. Na(aq) + Cl(aq) means that the ions are dissolved in water, no solid salt appears.


Saturated Salt Solution

Saturated Salt Solution

It’s really interesting to visually experience what happens in these reactions. Obviously it’s important to be able to write-out on paper what will occur and why, but there is no substitute for physically doing something. One of the best parts of Chem 31 is the lab experiments parallel the information taught in lectures. After talking about Le Chatelier’s Principle in class, we were able to explore tangible evidence of what was written on the chalk board. I thoroughly enjoy chemistry and chemical engineering because it provides an opportunity to work with my hands and truly understand the dynamics behind everyday processes in the world.

We did two other reactions in the lab and I’ll share them in my next post!


Author: Ben Dunmire

I am a sophomore Chemical Engineering major at Lehigh. On campus, I am the president of Club Baseball and a member of AIChE and NSCS. Outside of school, I enjoy the outdoors, fishing, and any type of athletic activity. You can reach me at

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