Why does this reaction happen?

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In summary, the reaction between Mg2+ and Ca(OH)2 to form Mg(OH)2 and Ca2+ is favored by thermodynamics due to an increase in entropy. This is because the solubility of Mg(OH)2 is much lower than that of Ca(OH)2, and the formation of Mg(OH)2 releases more free energy than breaking apart Ca(OH)2. Therefore, when both substances are present, the reaction will proceed in a way that produces the less soluble salt (Mg(OH)2) as the main product.
  • #1
jaumzaum
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For we to obtain metallic magnesium from sea water we can precipitate it in the form of magnesium hydroxide if we add calcium hydroxide:

Mg2+ + Ca(OH)2 → Mg(OH)2 + Ca2+

Why does this reaction happen?
 
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  • #2
Have you checked the usual suspects? Such as being favored thermodynamically or an increase in entropy?
 
  • #3
Woopydalan said:
Have you checked the usual suspects? Such as being favored thermodynamically or an increase in entropy?
Hi Woopydalan.
Actually I would like a more qualitative explanation, not a quantitative one. If you had no numbers like the entalpy/entropy/gibbs energy of the substances, how would you explain that a reaction like this would happen?
 
  • #4
The qualitative answer is that it is favored by thermodynamics to happen or there is an increase in entropy. Whether or not you had the values at your disposal would not really matter. That is how you would explain it happening. I know you are looking for another answer, but simply that is the reason reactions occur.

It's like asking ''Why is a banana yellow?'' the answer would be ''It doesn't absorb wavelengths between 570-590 nm'' but then you say ''well what if I don't know the wavelength of yellow light, then how do I explain it?'' then my response would follow ''It doesn't absorb yellow light.''
 
  • #5
Try looking up the solubility (specifically, the solubility product constant Ksp) for each of these substances.
 
  • #6
Ygggdrasil said:
Try looking up the solubility (specifically, the solubility product constant Ksp) for each of these substances.

I thought the solubility could have something to do with it, but I still cannot explain it. Actually the calcium hydroxide is much more soluble than the magnesium one.

Solubility of magnesium hydroxide is 1.4mg/L while solubility of calcium hydroxide is 1.73g/L at 20 °C
The Ksp are 1.5 10-11 for magnesium and 4.68 10-6 for calcium

How can I explain it now, knowing that Mg(OH)2 is much less soluble than Ca(OH)2
 
  • #7
Calculate concentrations of Ca2+ and OH- in the saturated solution of Ca(OH)2.

Introduce any magnesium salt in such an amount concentration of Mg2+ becomes the same as concentration of Ca2+.

What will happen now?
 
  • #8
Borek said:
Calculate concentrations of Ca2+ and OH- in the saturated solution of Ca(OH)2.

Introduce any magnesium salt in such an amount concentration of Mg2+ becomes the same as concentration of Ca2+.

What will happen now?

Actually when I was trying to find the concentration of Ca2+ I've got another doubt.

When I try to find the Ksp from the solubility, I get an absurd result.

solubility of calcium hydroxide is 1.85g/L at 0°C, that is 2.5 10-2M
Ca(OH)2 → Ca2+ + 2OH-
2.5 10-2-----0-------0------
---0-----2.5 10-2----5. 10-2

Ksp = 4 (2.5 10-2)3 = 6.25 10-5 that is different from the Ksp0°C = 8 10-6

Why does this happen?

And answering your question. Mg2+ would precipitate as it it less soluble than Ca2+. But what does this have to do with the first question?
 
  • #9
Solubility of Ca(OH)2 is higher than that predicted by Ksp alone, as Ca2+ gets complexed by OH-.

jaumzaum said:
And answering your question. Mg2+ would precipitate as it it less soluble than Ca2+. But what does this have to do with the first question?

And you still don't see that it means the reaction proceeds to the right?
 
  • #10
Borek said:
Solubility of Ca(OH)2 is higher than that predicted by Ksp alone, as Ca2+ gets complexed by OH-.



And you still don't see that it means the reaction proceeds to the right?

Not yet, haha :(
Could you explain it to me? (I'm not very good in chemistry...)
 
  • #11
To be honest, I have no idea how to help you further. You have came to the right conclusion that when there are two weakly soluble salts competing for an anion (or cation, doesn't matter), reaction will proceed in such a way that the less soluble salt will be the main product. That's exactly what is happening here.
 
  • #12
Borek said:
To be honest, I have no idea how to help you further. You have came to the right conclusion that when there are two weakly soluble salts competing for an anion (or cation, doesn't matter), reaction will proceed in such a way that the less soluble salt will be the main product. That's exactly what is happening here.

But sea water isn't saturated of calcium, it is saturated of magnsium. And we are not adding Mg(OH)2, we are adding CaOH2

I don't see the equivalence
 
  • #13
Will writing the reaction as

Mg2+ + 2OH- -> Mg(OH)2

help?
 
  • #14
Perhaps it is helpful to think of things in terms of energy as Woopydalan suggested. Based on the Ksp values, you require 28 kJ/mol of free energy to break Ca(OH)2 into Ca2+ and two OH-, but forming Mg(OH)2 from Mg2+ and two OH- releases 57 kJ/mol of free energy (under standard conditions). Therefore, breaking apart calcium hydroxide and using the hydroxide ions to form magnesium hydroxide gives an overall decrease in the free energy of the system, and therefore, the reaction is thermodynamically favorable.
 

1. Why does this reaction occur?

The occurrence of a reaction is dependent on several factors such as the reactants, temperature, pressure, and catalyst. These factors affect the energy of the reactants and their ability to overcome the activation energy barrier, which is necessary for a reaction to take place.

2. What determines the direction of a reaction?

The direction of a reaction is determined by the relative energy levels of the reactants and products. In an exothermic reaction, the products have lower energy than the reactants, and the reaction proceeds in the forward direction. In an endothermic reaction, the products have higher energy than the reactants, and the reaction proceeds in the reverse direction.

3. How does the concentration of reactants affect the rate of a reaction?

The concentration of reactants affects the rate of a reaction by influencing the frequency of collisions between reactant molecules. As the concentration increases, the number of collisions increases, leading to a higher chance of successful collisions and a faster rate of reaction.

4. Why does a reaction slow down over time?

Reactions slow down over time due to the depletion of reactants and the accumulation of products. As the concentration of reactants decreases, the frequency of collisions and the rate of reaction also decrease. Additionally, the accumulation of products can create a reverse reaction, which slows down the overall reaction rate.

5. Can the rate of a reaction be increased?

Yes, the rate of a reaction can be increased by increasing the temperature, pressure, or concentration of reactants, or by adding a catalyst. These factors increase the energy of the reactants and/or decrease the activation energy barrier, allowing the reaction to occur more quickly.

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