Problem involving specific heat and graph

In summary: C.In summary, a researcher studied the heat transference between an iron block and an unknown substance using an adiabatic calorimeter. The quotient of the mass of the iron block and the mass of the unknown substance was found to be 0.8. By considering the specific heat of the iron, the specific heat of the unknown substance in its solid state was determined to be 0.28 cal/g-°C based on measurements of equilibrium temperatures.
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Homework Statement


A researcher studies the heat transference between an iron block and an unknown substance, inside an adiabatic calorimeter. In successive experiences he puts inside the calorimeter the unknown substance, always in its solid state and with temperature θs = 20ºC, and the iron block, in each case with a different initial temperature "θi". In each case he measures the equilibrium temperature "θe". The results are shown in the graphic.

The quotient of the mass of the iron block and the mass of the unknown substance, "mi/ms", is equal to 0,8.
Consider the specific heat of the iron = 0.1 cal/g ºC

Image:
https://imgur.com/a/QLu3NQI

Question: Find the specific heat, "cs", of the unknown substance in its solid state

Homework Equations



Q = m c Δθ

3. The Attempt at a Solution

Let mi = mass of the iron block; ms = mass of the substance; cs = specific heat of the substance; ci = specific heat of the iron; θe = the equilibrium temperature; θi = the initial temperature of the iron block; θs = the initial temperature of the substance

The process is thermally isolated, so Q received = Q given ⇒ [(ms)(cs)(θe - θs)] = [(mi)(ci)(θe-θi)] ⇒

mi/ms is constant = 0,8; θs = 20 and ci = 0,1 ⇒ (cs)(θe - θs) = 0,08(θe-θi) ⇔ cs = 0,08(θe-θi)/(θe - 20)So, here's the problem. In the graph I initially picked the (150;50) point because it gives me the easy 100, so the cs = 4/15 cal/g ºC, but upon checking online the only answer I found (albeit not in a truly trustworthy source) is 0,28 cal/g ºC. Getting back to the exercise, this is the answer that we get when we input (200;60). (150;50) = 4/15 cal/g ºC; (100;40) = 0,24 cal/g ºC; (50;30) = 0,16 cal/g ºC.

My forced conclusion here is that the specific heat of the substance in its solid state is not constant (because the question did say the iron one was constant). So how should I know that the correct one was the (200;60)? Is my conclusion that cs is not a constant correct?

PS: Just to be sure, in the graph of the image, the Y axis are the equilibrium temperatures, the X - axis are the different temperatures of the iron block.

PS2: This is from an acceptance exam from 1997 and there are no definitive solutions of this online, so, again, I'm not entirely sure about the 0,28 cal/ g ºC

PS3: I wasn't able to host the image for some reason, in the preview it never showed up.

PS4: I'm sorry if I broke any rule, it's my first time posting.
 
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  • #2
You are not reading your graph carefully. It is small and unclear, I admit, but it is perfectly clear that the last point you mention is not (50,30) but about (50,26); i.e Te doesn't increase by 10° for every 50° increase in Ti, but a bit more.
 
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  • #3
Damn, I can't believe I spent so much time on this because I simply wasn't careful reading the graph :cry::cry:. Upon further inspection in my printed graph, the only point which is exact from those I cited is (200;60), even (150;50) and (100;40) are slightly off.

Thank you so much for pointing this out, mjc123.
 
  • #4
frkCarl said:

Homework Statement


A researcher studies the heat transference between an iron block and an unknown substance, inside an adiabatic calorimeter. In successive experiences he puts inside the calorimeter the unknown substance, always in its solid state and with temperature θs = 20ºC, and the iron block, in each case with a different initial temperature "θi". In each case he measures the equilibrium temperature "θe". The results are shown in the graphic.

The quotient of the mass of the iron block and the mass of the unknown substance, "mi/ms", is equal to 0,8.
Consider the specific heat of the iron = 0.1 cal/g ºC

Image:
https://imgur.com/a/QLu3NQI

Question: Find the specific heat, "cs", of the unknown substance in its solid state

Homework Equations



Q = m c Δθ

3. The Attempt at a Solution

Let mi = mass of the iron block; ms = mass of the substance; cs = specific heat of the substance; ci = specific heat of the iron; θe = the equilibrium temperature; θi = the initial temperature of the iron block; θs = the initial temperature of the substance

The process is thermally isolated, so Q received = Q given ⇒ [(ms)(cs)(θe - θs)] = [(mi)(ci)(θe-θi)] ⇒

The above equation is incorrect. It should read:

[(ms)(cs)(θe - θs)] + [(mi)(ci)(θe-θi)] =0

or, equivalently,

Heat lost by iron = Heat gained by solid

where

heat lost by iron ##=m_ic_i(\theta_i-\theta_e)##

and

heat gained by solid ##=m_sc_s(\theta_e-\theta_s)##

So, the solution to the correct equation is $$\theta _e=\frac{m_sc_s}{(m_sc_s+m_ic_i)}\theta_s+\frac{m_ic_i}{(m_sc_s+m_ic_i)}\theta_i$$
So the slope is ##\frac{m_ic_i}{(m_sc_s+m_ic_i)}## and the intercept is ##\frac{m_sc_s}{(m_sc_s+m_ic_i)}\theta_s##

This leads to a value of cs of 0.28 cal/gm.C
 
Last edited:

1. What is specific heat and why is it important in problem solving?

Specific heat is the amount of heat energy required to raise the temperature of a substance by 1 degree Celsius. It is important in problem solving because it helps us understand how much heat is needed to change the temperature of a substance and how different substances respond to heat.

2. How do you calculate specific heat?

To calculate specific heat, you need to know the mass of the substance, the change in temperature, and the amount of heat energy used. The formula for specific heat is q = mcΔT, where q is heat energy, m is mass, c is specific heat, and ΔT is the change in temperature.

3. What does the graph of specific heat look like?

The graph of specific heat is a straight line with a positive slope. This means that as the temperature of the substance increases, the amount of heat energy needed also increases.

4. How can specific heat be used to identify a substance?

Specific heat is a characteristic property of a substance, meaning that it is unique for each substance. By measuring the specific heat of a substance, we can compare it to known values and identify the substance.

5. How does specific heat affect the temperature of a substance?

The specific heat of a substance determines how much heat energy is needed to change its temperature. A substance with a high specific heat will require more heat energy to raise its temperature compared to a substance with a lower specific heat. This is why substances like water can maintain a relatively constant temperature, as it has a high specific heat compared to other substances.

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