Specific heat of Isoprene 1

[RonHRob]

Can somepone help me out with a value (or range for the specific heat of isoprene or isobutylene rubbers?

Thanks in advance for your help.

Ron.

 

[MintJulep]

For polyisoprene my CRC handbook lists 0.40.

For Isobutylene-isoprene 0.45.

 

[RonHRob]

Mint Julep:

Thanks for the reply, but can you tell me the units that go with the values that you provided.

 

[MintJulep]

As will all "specific" quantities, they are dimensionless, being the ratio some property of the material of interest over the same property of some reference material (typically water).

In the case of specific heat, the property of interest is the amount of heat required to raise a unit mass of the material 1 degree. The units of mass or temperature are irrelavent.

 

[Zoobie]

MintJulep,

I respectfully disagree with your last post. The units do matter. If the numbers you quote are simply the specific heat ratioed to the heat for water than fine but you need to state that.

You define the property as the amount of heat required to a raise a unit of mass by 1 degree. For a given substance: 1 BTU of heat added to 1 lb of material raising it by 1 F is not the same as 1 KJ of heat added to 1 kg of material raising it by 1 C. Not only do the system of units matter, mass vs molar basis matters. Different references tabulate them differently.

You are also incorrect when stating that all specific quantities are dimensionless. Specific gravity is but most 'specific' terms are actually ratios to mass....specific heat (kJ/kgC), specific volume (m3/kg), etc.

 

[RonHRob]

Thanks for the inputs guys, I appreciate your comments.

Ron.

 

[MintJulep]

Zoobie,

You are correct that the amount of heat needed to raise 1 pound of water 1 degree F is not the same amount of heat required to raise 1 kg of the same material 1 degree C.

However, we are dealing with a ratio. For any arbirary set of units X amount of heat is required to increase the temperature of a unit mass of material by one degree.

That same X amount of heat will raise a unit mass of a material with a different heat capacity a different number of degrees.

The ratio of the delta T is not dependant on the units (provided the same unit system is used to measure both materials)

Encyclopedia.com does a nice job of explaining the common confusion between specific heat and heat capacity, and is consistent with my Mark's STandard Handbook.

http://www.encyclopedia.com/html/s1/specheat.asp

 

[Zoobie]

Well then, we certainly have a problem. I have never used 'specific' heat capacities as you identify them. Not only that, I have several engineering references that tabulate specific heat in units of kJ/kg-K. Also, my process simulator provides 'specific heat' with units of heat/mass-temp.

I looked up specific heats in my CRC handbook....64th edition pgs F-12&13. Specific heat is given for air in units of cal/g-K. If in fact these were 'specific' heats the table would be filled with values close to 1.

Also, take a look at some steam tables. Specific volume is definitely not a dimensionless quantity.

I would appreciate it if someone else could lend a hand...I have always thought that 'specific' terms were properties that were mass dependent (ie more mass...more heat, more mass...more volume).

 

[Zoobie]

Cp is often referred to as specific heat (even in CRC). If in fact we treated this is a dimensionless quantity than you would need an extra term in the equation Q=m x Cp x delta T. I have never seen it expressed differently.

Also, I take back my some of my comments about air. The tabulation is as I stated but if we follow the definition provided of specific heat than you could generate ratios to water using appropriate units.

 

[MintJulep]

The confusion would appear to arise from the defintions of units of heat.

1 BTU is the heat required to raise one pound of water one degree F.

1 calorie is the heat required to raise one gram of water one degree C.

Thus by inverse of the definitions of the units of heat the heat capacity of water is:

1 BTU/pound F our 1 cal/g K

This in turn forces the specific heat - that is the ratio of temperature rise per unit mass per unit heat input of a substance over that of water - to be numerically equal to its heat capacity in either BTU/pound F or cal/g K.

So, apparent widespread colloquial misuse notwithstanding, "Specific heat" is properly dimensionless. "Heat capacity" requires units of heat/mass x temperature.

Joules don't work out so nicely.

It is my opinion that Joules, being properly a unit of work, should not be used for "heat capacity" (although if the units are clearly stated this does not appear to create a problem).

 

[MintJulep]

I wish I'd found this before typing out my last post.

http://www.encyclopedia.com/html/section/heat_measuresofheat.asp

 

[Zoobie]

Two comments (and then I think this issue has been beaten to death).

The area of engineering is no less susceptible to colloquial misuse (leading to accepted use). If we look at languages (english in particular) colloquialisms are the major evolutionary force. I would argue that the term Cp (as in the equation Q=mCpDT) has become widely referred to as 'specific heat' (at least in my part of the world). Just like any equation, the units used can be just about anything as long as they are consistent.

The encyclopedia articles that you reference seem to be having the same difficulty that we are. They give contradicting definitions. The first article refers to the ratio of heat capacities while the second defines specific heat the same way it does heat capacity in the first article.

 

[25362]

If I'm allowed to resuscitate the subject I'd say there should be no confusion as long as we adopt the following definitions.

The heat [Δ]Q transferred to an object and the resulting change [Δ]T in temperature are proportional:

[Δ]Q = C[Δ]T​

where C, the proportionality constant, is called the heat capacity of the object. Thus the units of heat capacity are J/K. C applies to a specific object and depends both on its mass and on the substance from which it's made.

One usually characterizes different substances in terms of c, specific heat, or heat capacity per unit mass. The heat capacity of an object is then the product of its mass m and specific heat, and we write:

Q = mc[Δ]T​

The SI units of specific heat are J/(kg.K)

The "specificity" in this case refers to the unit of mass. So would be with specific volume, specific (electrical) charge, and specific humidity.

 

[Zoobie]

Thank you 25362, I fully agree with you.

 

[RvL04]

Can i suggest you getting your isoprene tested? Specific heat values differ so much from one batch to the next and depend on so many factors, that (in my view) it is always worth getting the 'real' numbers that apply in your case. Check out

http://www.axelproducts.com/downloads/ThermalConductivity.pdf

on a way to do this.

Ron