Is hydrogen bonding not factored in with specific heat, or am I confusing myself?

[totaldorkist]

Just before I delve into my question I'm just gonna say, I think my misunderstanding has to do with 1) the definition of a calorie and 2) how energy and temperature work together.

 

So, my textbook says that a calorie is how much energy one would need to raise the temperature of 1g of water 1°C (also 4.184 joules), is this not true?

 

BUT, it is also tells me a page later, in the context of heat of vaporization, that it takes 580 calories to evaporate 1g of water that is 25°C. From the definition of a calorie, wouldn't it be 75 calories to evaporate it? Also, the reason why they say this is to show water's high heat of vaporization due to hydrogen bonding, does it have to do with that?

Edited July 7, 2013 by totaldorkist

[swansont]

Evaporation does not raise the temperature to 100 ºC, it's a phase change from liquid to gas.

 

The hydrogen bonds are between the molecules, and you have to break these to get a molecule free when it evaporates.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html

[studiot]

You should certainly talk this subject over with your teachers as you have got yourself a bit muddled up.

 

Yes 1 calorie is the amount of energy required to raise one gram of liquid water 1 degree centigrade.

 

Note the emphasis on liquid. It applies from 0C to 99C. If you raise some water at 100C to 101C it is no longer liquid.

 

So to raise 56 grams of water from 25C to 35C would require 56 times (35-25) = 560 calories.

 

But water can also exist as a solid or gas (ice or steam).

 

When water changes from solid to liquid or liquid to gas or back again, there is no change in temperature until the whole sample has changed.

 

This emphasised bit is very important since it means that there is no heat energy involved due to temperature change.

 

However it still takes energy to change the state of the water. And this energy is much larger per gram, than the amount needed to change the liquid by 1 degree.

 

This energy is called latent heat.

 

For the change from solid to liquid at it is called the latent heat of fusion. For ice at 0C to water at 0C this is 80cals/gm/C

 

For the change from liquid to gas it is called the latent heat of evaporation. For water at 100C to water vapour at 100C it is 541cals/gm/C

 

You can see that it take more energy to boil than to melt a substance.

 

As to the second part of you question,

 

Yes your have started correctly, it takes 75 calories to raise 1gram of liquid water from 25C to 1 grma of liquid water at 100C, it then takes an additional 541 calories to turn this into gas.

 

So the total is 75 + 541 = 616 calories.

 

This is slightly higher than your book figure.

 

Finally about the hydrogen bonding.

 

Water has unusual properties due to hydrogen bonding. The melting and boiling points are higher than for the next larger molecule in the periodic table. Normally it is the other way round since it atakes more energy to move a large molecule around. Water is hydrogen oxide and is a liquid at normal temperatures. Hydrogen sulphide is a gas at normal temperatures, which means it has a lower boiling point.

 

Does this help?

[The young professor vik.]

1 calorie is approximately 4.184joules.

Energy and temperature are related thermodynamically. Read the topic of thermodynamics in your textbook for explanation.

 

Water has a latent(hidden) heat of vaporization of 2260joules. Since 1 calorie = 4.184joules; from simple proportion 2260 would be 2260/4.184 = 565 calories. This value is close to what is in your textbook. The value in your textbook might have been gotten experimentally. An as you know error cannot be avoided in experiments. Hence an error (probably impurities in water or cheer carelessness) in the experiment might be responsible for the increase in the value (580cals). This increase has nothing at all to do with hydrogen bonding.

1 calorie is approximately 4.184joules.

Energy and temperature are related thermodynamically. Read the topic of thermodynamics in your textbook for explanation.

 

Water has a latent(hidden) heat of vaporization of 2260joules. Since 1 calorie = 4.184joules; from simple proportion 2260 would be 2260/4.184 = 565 calories. This value is close to what is in your textbook. The value in your textbook might have been gotten experimentally. An as you know error cannot be avoided in experiments. Hence an error (probably impurities in water or cheer carelessness) in the experiment might be responsible for the increase in the value (580cals). This increase has nothing at all to do with hydrogen bonding. Reply if this helps or for further questions!

[swansont]

1 calorie is approximately 4.184joules.

Energy and temperature are related thermodynamically. Read the topic of thermodynamics in your textbook for explanation.

 

Water has a latent(hidden) heat of vaporization of 2260joules. Since 1 calorie = 4.184joules; from simple proportion 2260 would be 2260/4.184 = 565 calories. This value is close to what is in your textbook. The value in your textbook might have been gotten experimentally. An as you know error cannot be avoided in experiments. Hence an error (probably impurities in water or cheer carelessness) in the experiment might be responsible for the increase in the value (580cals). This increase has nothing at all to do with hydrogen bonding.

1 calorie is approximately 4.184joules.

Energy and temperature are related thermodynamically. Read the topic of thermodynamics in your textbook for explanation.

15 cal would be a huge error.

 

The difference between the value given and the latent heat value is that the latent heat is defined for the boiling point, rather than 25 ºC. At the lower temperature the molecules have less energy, on average, so you need to add that extra energy to remove the molecule.

[The young professor vik.]

Thanks a lot. Didn't think of that!