Calculating Energy Required to Heat and Change State of Ice in an Oven

In summary, to change ice into water, you need Q=mL. To change water into steam, you need Q=mc(change in T). To heat ice to its melting point, you need Q=mc(change in T). To change liquid water into steam, you need Q=mL. To change steam into 134 degrees Celsius, you need Q=mc(change in T).
  • #1
srose9625
21
0
A student takes 0.45kg of ice. The ice is initally at -23'C. She heats the sample in an oven until the temp is 134'C.
a). What is the Q needed to heat the ice to its melting point?
b). What is the Q needed to change the solid ice into liquid water?
c). What is the Q needed to heat the liquid water to its boiling point?
d). What is the Q needed to change the liquid water to steam?
e). What is the Q needed to heat the steam to 134'C?
f). Draw a graph with the temp of the sample in Kelvin on y axis, and NRG absorbed in Joules by the sample on the x axis.


Equations of use:
Q=mc(change in T)
Q=mL

a) (.45kg) (4.2x10^3 J/kg) (23'C)=4.3x10^6 J
b) (.45kg) (3.3x10^5 J/kg) = 1.5x10^5 J
c). (.45kg) (3.3x10^5 J/kg) (100'C) = 1.2x10^7 J
I am guessing that water boils at 100'C. Would that be the right answer?
d) (.45kg) (2.3x10^6 J/kg) = 1.0x10^6 J
e). (.45kg) (3.3x10^5 J/kg) (134'C) = 1.2x10^7 J
OR
(.45kg) (2.3x10^6 J/kg) (134'C) = 1.4x10^8 J
f) I have ice drawn in a straight/horizontal line, then water drawn in a linear line followed by a straight/horizontal line, then steam in a linear line, then boiling in a straight/horizontal line.
 
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  • #2
I think for part a) you need to find the heat req'd to raise the temp from -23'C to 0'C
 
  • #3
I think that what I did, not sure though.
 
  • #4
Be careful with what values you use for your heat capacity "c". It is different for ice, liquid water, and steam. I think you have some mixed up.
In part (c) you have used the latent heat of fusion "L" as your heat capacity. That is wrong since you are now increasing the temperature of liquid water, not undergoing a phase change. Look up the proper values in your book.
 
  • #5
It also helps to draw a little diagram with arrows between each step and work out if the step involves taking in/giving out heat, rather than just relying on getting all the signs correct in your head.
 
  • #6
so for c) it would be (.45kg) (2.0x10^3 J/kg) (100'C) = 9.0x10^4 J
this is using the specific heat of steam.
Would that be right?
 
  • #7
srose9625 said:
so for c) it would be (.45kg) (2.0x10^3 J/kg) (100'C) = 9.0x10^4 J
this is using the specific heat of steam.
Would that be right?

No, you need the specific heat for water. The water is being brought up to 100 degrees, but is not yet steam so you can't use the specific heat for steam.
 
  • #8
c) (.45kg) (4186 J/kg) (100'C) = 1884x10^2 J


When will I use latent heat vs. specific heat?

How did I do for e , f ?
 
Last edited:
  • #9
For (e) you've got the wrong "c" value again. You need the one for steam and again you have used latent heat of fusion.

Actually, you are using the wrong one in part (a) as well. You need the one for ice.

For (f), you've sort of got the right idea, but not quite there. Think about what is happening to the ice. It is initially at -23 and is being heated to 0. So in that time, you are adding energy and it is causing the temperature to increase. So what will that look like on your graph?
The next step is the phase change from ice at 0 degrees to water at 0 degrees. What do you think this should look like? If you understand these two you can draw the rest of the graph.
 
  • #10
a) (.45kg) (2090 J/kg) (23'C) = 2163x10 J
e) (.45kg) (2010 J/kg) (134'C) = 1212x10^2 J
f) I have a linear line for -23'C then I have ice drawn in a straight/horizontal line, then water drawn in a linear line followed by a straight/horizontal line, then steam in a linear line, then boiling in a straight/horizontal line.
Is that right?
 
  • #11
srose9625 said:
a) (.45kg) (2090 J/kg) (23'C) = 2163x10 J
e) (.45kg) (2010 J/kg) (134'C) = 1212x10^2 J
f) I have a linear line for -23'C then I have ice drawn in a straight/horizontal line, then water drawn in a linear line followed by a straight/horizontal line, then steam in a linear line, then boiling in a straight/horizontal line.
Is that right?

(a) looks good.

For (e), it should look like (0.45 kg)(2010 J/kg)(134-100 degrees C)
Remember, it's the change in temperature. The temperature is going from 100 to 134, so a temperature change of 34 degrees. The way you had it was how much energy would be needed to raise the temperature of steam by 134 degrees, not to 134 degrees. I should have caught that earlier.

For your graph, you should have 5 sections. You are OK up until the end. After you heat the steam to 134 that's it. There is no boiling after that. I'm not sure what you're thinking there.
 
  • #12
Thank you for all your help and FAST, easy to understand responses! I will chat with you again sometime:)
 
  • #13
You're welcome. :smile:
 

1. What is latent heat in an oven?

Latent heat in an oven refers to the heat energy required to cause a substance to undergo a phase change, such as from solid to liquid or liquid to gas, without changing its temperature.

2. How does latent heat affect cooking in an oven?

The release of latent heat during cooking helps to evenly distribute heat and maintain a steady temperature in the oven, resulting in more efficient and consistent cooking of food.

3. What is the difference between latent heat and sensible heat in an oven?

Sensible heat in an oven refers to the heat that can be felt or measured by a thermometer, while latent heat is the energy required for a phase change to occur without changing the temperature. In other words, sensible heat affects temperature, while latent heat affects the physical state of a substance.

4. Can latent heat cause food to overcook in an oven?

Yes, if too much latent heat is released during cooking, it can cause food to overcook or even burn. This is why it is important to monitor the cooking process and adjust the temperature or cooking time as needed.

5. How can latent heat be utilized in cooking other than in an oven?

Latent heat can also be utilized in other cooking methods such as boiling, steaming, and deep frying. These methods all involve the transfer of latent heat to the food being cooked, resulting in changes in its physical state and texture.

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