Here's some important info Bender told us before the demo:
Phenolphthalein turns pink in a solution with [OH-] > 10-5 M.
MISSION? Determine whether the following equilibrium reaction is exothermic or endothermic in the forward direction
NH3(aq) + H2O(l) <-- --> NH4+(aq) + OH-(aq)
First we had to prepare a cold beaker with ice and a hot beaker on the hot plate at 80°C. After we added some ammonia, distilled water, and then phenolphthalein indicator into a two test tube, we placed them into the different water baths, and they changed colors to different pinks!
The result? Solution in hot water: light pink. Solution in cold water: hot pink
So is this solution exothermic(heat is reacted) or endothermic(heat is produced)?
If the solution because pinker in cold water, and since we know that OH- is making the solution pink, we know that the reaction is shifting right to produce more OH- in the absence of heat. So in the presence of heat, the reaction would shift left, which means heat is on the right, which makes the reaction exothermic! MYSTERY SOLVED!
Overall, "it was nice to actually see the reaction shift left/right, but it was a bit slow and not that colorful," said Michelle Chang. Even though this may not have been that colorful, it was a definite plus that we were able to see the reaction shifting and that we could apply things we previously learned to solve this demo :)
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