Formic acid has reducibility, which is not difficult to understand theoretically. But in the experiment of reducing copper hydroxide with it, it is not sure to do the experiment in teaching, and it can not achieve the expected effect. Therefore, we should improve the experimental conditions to achieve better results.
1. Reagent and preparation
5% CuSO4 solution: 7.4g CuSO4 · 5H2O was dissolved in 93.6ml water,
10% NaOH solution: 11.1g NaOH mixed with 100ml solution
Formic acid concentrated solution: ～ 80% (original)
2. Operation and phenomenon
For the convenience of observation and comparison, the reaction was carried out in a U-tube (or a test tube) and heated in a water bath (90-95 ℃). First, pour about 20-30 ml 110% NaOH solution into the U-shaped tube to make the liquid level about 5 cm away from the tube orifice. Then, carefully drop 3-5 drops of 5% CuSO4 solution from the tube orifice (I). There is Cu (OH) 2 Precipitation on the liquid level. Then slowly inject 8-10 drops of concentrated formic acid solution. The precipitation dissolves and the liquid layer is blue (do not shake). After heating the U-shaped tube in a water bath (90-95 ℃) for about 2 minutes, a thin layer of earth red chaotic liquid (containing Cu2O) appeared at the interface between the blue liquid layer and the colorless liquid layer. As the reaction proceeds, the turquoise chaotic liquid layer expands, and after about 5 minutes, all the blue liquid layers turn into Turquoise turbid liquid.
Then 3-5 drops of 5% CuSO4 solution were dropped into the nozzle (II), and immediately the Black Gel (Cu (OH) 2 formed in hot alkali) was decomposed into CuO precipitate, and then 2-3 drops of formic acid solution were dropped. Soon, a soil red turbid layer appeared on the liquid surface, and some black gels floated on the liquid surface and gradually turned into soil red.
The results show that the turbidity layer containing Cu2O is earthy red, one is that the amount of reduction product (Cu2O) is less, the other is that it is mixed with CuO2, but it does not affect the good effect of the experiment.