Heat capacity salt water?

[grapepopsicle]

TBR Gen Chem Ch 8 passage IX says that the addition of salt to water increases the solution's heat capacity. Literally everyone else, either on the internet or textbooks I've used before, says the opposite. Does anyone know what the deal is?

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[Hexon]

i believe it has to do with colligative properties and boiling point elevation; adding more solutes (in this case, the salt) to water will increase the heat capacity of the water.

 

[N1st]

i believe it has to do with colligative properties and boiling point elevation; adding more solutes (in this case, the salt) to water will increase the heat capacity of the water.

Boiling point elevation is caused by solutes reducing the vapor pressure.

Heat capacity has nothing to do with phase transitions, it is how many Joules are needed to raise the sample one Kelvin. I have never heard of a colligative property for heat capacity.

 

[int3l]

Hmm...
I was under the impression that the addition of salt (NaCl) would disrupt/interfere with water's hydrogen bonding ability, which in turn lowers water's heat capacity. I can't see TBR's reasoning in this case scenario.

And ya, colligative properties usually deal with boiling point elevation and freezing point depression.

 

[Jamil]

Hmm...
I was under the impression that the addition of salt (NaCl) would disrupt/interfere with water's hydrogen bonding ability, which in turn lowers water's heat capacity. I can't see TBR's reasoning in this case scenario.

And ya, colligative properties usually deal with boiling point elevation and freezing point depression.

From my understanding the heat capacity (C), will not change, the reason the additional heat (or removal of heat) is required to either freeze or boil the solution is due to the increase in strength of the bonds due to the ions (Na+ and Cl-) and water. These bonds are stronger than a typical hydrogen bond and thus require more energy to put the water molecules in a state of kinetic energy to evaporate.

 

[gettheleadout]

TBR Gen Chem Ch 8 passage IX says that the addition of salt to water increases the solution's heat capacity. Literally everyone else, either on the internet or textbooks I've used before, says the opposite. Does anyone know what the deal is?

Think about this in a really basic way. You're adding some mass to a beaker of water, right? Adding mass means that to raise the temperature of this solution we now have to add more energy in (via heat flow) than we would if we had less mass. The pure water has less mass than the pure water + salt, so the heat capacity of the solution is greater than that of the solvent alone. For the next point bear in mind that the salt solution also has greater volume than the solvent alone.

The confusion here is that when comparing equal volumes of solutions, say exactly 1.00L of pure water and 1.00L of 20% salt solution, the salt solution has a lower heat capacity. In its dissociated state, salt has a lower heat capacity than water, so by replacing a portion of the mass of the pure water solution with an equal amount of something with a lower heat capacity, we end up with a lower heat capacity of the 20% salt solution in comparison to the pure water.

 

[Intellex]

The total heat capacity rises, while the specific heat capacity decreases

 

[StarlingForce1]

The total heat capacity rises, while the specific heat capacity decreases

What is 'specific heat capacity'? I have only ever heard of heat capacity and specific heat...

 

[Intellex]

What is 'specific heat capacity'? I have only ever heard of heat capacity and specific heat...

Specific heat and specific heat capacity are the same

 

[grapepopsicle]

Think about this in a really basic way. You're adding some mass to a beaker of water, right? Adding mass means that to raise the temperature of this solution we now have to add more energy in (via heat flow) than we would if we had less mass. The pure water has less mass than the pure water + salt, so the heat capacity of the solution is greater than that of the solvent alone. For the next point bear in mind that the salt solution also has greater volume than the solvent alone.

The confusion here is that when comparing equal volumes of solutions, say exactly 1.00L of pure water and 1.00L of 20% salt solution, the salt solution has a lower heat capacity. In its dissociated state, salt has a lower heat capacity than water, so by replacing a portion of the mass of the pure water solution with an equal amount of something with a lower heat capacity, we end up with a lower heat capacity of the 20% salt solution in comparison to the pure water.

The total heat capacity rises, while the specific heat capacity decreases

Thanks! That makes a lot of sense! 👍

 

[Perpetually]

The above post looks good.

If we dissolve a strong electrolyte into water, we will introduce solute and induce some colligative effects.

However, specific heat capacity is a separate concept. From how I understand it, the intermolecular H-bonding in water permits more energy to be absorbed before the molecules begin to increase in their average kinetic energy/ temperature. When we introduce the ionic electrolyte, I think the cations and anions from the salt will disrupt the orderliness of pure water's H-bonding network and make it more easily to increase the temperature of water when a given amount of heat is added. Thus it will decrease the heat capacity.

But again, I think the previous posters better address the OP's question.