It is common in chemistry lab to use diluted solution of HCl(aq). Concentrated H
ID: 523349 • Letter: I
Question
It is common in chemistry lab to use diluted solution of HCl(aq). Concentrated HCl(12M) is diluted with water to make 1 M solutions. HCl completely ionizes in water by the following reaction: H_2O + HCl rightarrow H_3O^+ + Cl^- Delta H = -75.4 kJ/mol a) To safely prepare about 1Lof a 1 M HCl solution 83 mL of concentrated HCl is slowly added to 1 L of water. What is the final temperature of the diluted solutions if both solutions are initially at 20 degree C. Assume the HCl solutions have the same density and heat capacity as water. Assume any heat loss to the surrounding is negligible. b) In safety training is it heavily stressed that acid must always be added to water when preparing diluted HCl solutions. Why is this important? What would happen if instead you slowly added 1L of water to 83 mL of concentration HCl? Explain, providing a sample calculation if possible.Explanation / Answer
Part a)
We know,
q = mCT
where q = heat lost or gained by the system
If the reaction is exothermic then q = -H = 75.4 kJ mol-1
The heat liberated by dilution of HCl is taken up by water resulting in warming of water
m = mass of water = 1.0 g L-1X 1 L = 1 g
C = specific heat capacity of water = 4.18 J g-1 0C-1
T = q / mC
= 75.4 kJ mol-1 / 1 g• 4.18 J g-1 0C-1
= 18 0C
T2- T1 = 18 0C
T1 = 20 0C
T2 = 18 0C + 20 0C
= 38 0C
b) The heat liberated by dilution of HCl is taken up by HCl as there is limited amount of water in the container
m = mass of HCl = 1.49 g L-1 X 0.083 L = 0.1237 g (density of conc HCl is 1.49 g L-1)
T = q / mC
= 75.4 kJ mol-1 / 0.1237 g• 4.18 J g-1 0C-1
= 146 0C
T2- T1 = 146 0C
T1 = 20 0C
T2 = 146 0C + 20 0C
= 166 0C
Thus we can see the reaction is highly exothermic and the temperature change is dramatic. This causes boiling of the concentrated acid and the droplets of this acid will be thrown in air. This might cause corrosion of the fume hood and might corrode the skin of the chemist causing severe acid burns. Hence acid should be poured in water for obtaining dilute acidic solutions (as seen in the calculations of part a, there solution was slightly warmed up) and not conversely.
FYI: Just a word of caution, in case of acid burns one must not rinse the affected area with cold water but should rinse it with warm water. The reason is that cold water diminishes the skin pores and affects removal of acid causing further damage.
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