Heat vs. Temperature

[SteveJMarist]

This one still kind of alludes me.

What I know:

Temperature = average kinetic energy of a given substance
Heat = total energy of a system? (I'm iffy on this one)

Can anyone give a solid explanation?

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[tkylen]

Temperature is not the average kinetic energy but it is related to (or a measure of) kinetic energy of a substance. Heat, however, is the total kinetic energy of a substance and it depends on type and the amount of that substance. Would you rather have a drop of boiling water on your hand or dip your hand in a bucket of boiling water? The temperature is the same in both cases but the total kinetic energy (heat) in the bucket is greater than that of the water droplet. I hope this answers your question.

This one still kind of alludes me.

What I know:

Temperature = average kinetic energy of a given substance
Heat = total energy of a system? (I'm iffy on this one)

Can anyone give a solid explanation?

 

[phltz]

Temperature is not the average kinetic energy but it is related to (or a measure of) kinetic energy of a substance. Heat, however, is the total kinetic energy of a substance and it depends on type and the amount of that substance. Would you rather have a drop of boiling water on your hand or dip your hand in a bucket of boiling water? The temperature is the same in both cases but the total kinetic energy (heat) in the bucket is greater than that of the water droplet. I hope this answers your question.

That's a decent explanation. Also, we often talk about adding an amount of heat to an object, but rarely talk of adding temperature. We might say that the coils in an electric kettle add 240 Joules of heat to the water inside of it in a second. Heat often comes up in discussions of energy being transferred from one system to another, or converted from one form of energy to another.

 

[Rabolisk]

That's a decent explanation. Also, we often talk about adding an amount of heat to an object, but rarely talk of adding temperature. We might say that the coils in an electric kettle add 240 Joules of heat to the water inside of it in a second. Heat often comes up in discussions of energy being transferred from one system to another, or converted from one form of energy to another.

It is an ok explanation. Heat is not the total of the kinetic energy of any substance, although heat is related to random molecular motion. In fact, it is wrong to state that any substance has heat. Heat is not a property of a system, but a process.

 

[SteveJMarist]

Thanks, guys. This is pretty helpful. I knew most of the things you guys were talking about, but where I got confused is where I came across a question that went something like this: "A substance in a flask goes from the gas to solid phase. Thus A) The temperature in the flask decreases B) The temperature in the flask increases C) The temperature in the flask stays the same D)some bogus answer choice.

I thought the answer was C, since the HEAT would increase in the flask (exothermic reaction from gas --> solid).

The answer was B. It's almost like the book didn't differentiate between temperature and heat. Note that this was in the TPR Science Workbook.

Was the book right?

 

[Rabolisk]

Thanks, guys. This is pretty helpful. I knew most of the things you guys were talking about, but where I got confused is where I came across a question that went something like this: "A substance in a flask goes from the gas to solid phase. Thus A) The temperature in the flask decreases B) The temperature in the flask increases C) The temperature in the flask stays the same D)some bogus answer choice.

I thought the answer was C, since the HEAT would decrease in the flask (endothermic reaction from gas --> solid).

The answer was A. It's almost like the book didn't differentiate between temperature and heat. Note that this was in the TPR Science Workbook.

Was the book right?

I think this is a bad question, at least as you reproduced it here.

I can think of three situations that the question is really asking about. Based on how you interpret, either A, B, or C could be the answer. A would be the case in which you cool a gas down to a solid, and is the easiest to reason out. B is based on the fact that when gas is converted into solid, heat is released (exothermic) and that the temperature "in the flask" increases as a result. C is the case in which a gas turns to a solid at the deposition temperature. However, temperature of the substance does not change while the process is happening, so C makes sense. It all depends on what they mean by "temperature in the flask" and what they mean when a substance goes from gas to solid. Can you post the full question?

As far as some more things about heat and temperature though...

The reaction from gas to solid is not endothermic, but exothermic. The heat of deposition (or enthalpy*) is negative, and heat is released into the surroundings. It is wrong to say that heat decreases from state 1 to state 2. Certainly you have a negative value for heat, but that defines the direction of heat flow rather than a "decrease" in heat (negative = from system to surrounding). Heat is NOT a state function, and there is no such thing as Δq. Heat is characteristic of the process (chemical reaction, physical expansion, phase transition, etc.). You cannot say that at temperature 1, a substance has a certain heat value q1, and that at temperature 2, its heat value is q2.

What IS a state function is temperature. A functional definition of temperature is that it is an intensive (independent of amount (e.g. density)) physical property that drives heat flow between two systems. That is, if A is at a higher temperature than B, heat flows from A to B. It is related to "thermal energy" or "kinetic energy of random motion". Heat is defined as energy that flows between two systems at different temperatures. That seems like circular reasoning, so another definition for heat is that it is energy that drives random motion, rather than organized motion. Regardless, temperature of a system decreases when heat flows out of a system (q < 0), and increases when heat flows into a system (q > 0), generally. Hope you enjoy the wall of text.

 

[SteveJMarist]

I think this is a bad question, at least as you reproduced it here.

I can think of three situations that the question is really asking about. Based on how you interpret, either A, B, or C could be the answer. A would be the case in which you cool a gas down to a solid, and is the easiest to reason out. B is based on the fact that when gas is converted into solid, heat is released (exothermic) and that the temperature "in the flask" increases as a result. C is the case in which a gas turns to a solid at the deposition temperature. However, temperature of the substance does not change while the process is happening, so C makes sense. It all depends on what they mean by "temperature in the flask" and what they mean when a substance goes from gas to solid. Can you post the full question?

As far as some more things about heat and temperature though...

The reaction from gas to solid is not endothermic, but exothermic. The heat of deposition (or enthalpy*) is negative, and heat is released into the surroundings. It is wrong to say that heat decreases from state 1 to state 2. Certainly you have a negative value for heat, but that defines the direction of heat flow rather than a "decrease" in heat (negative = from system to surrounding). Heat is NOT a state function, and there is no such thing as &#916;q. Heat is characteristic of the process (chemical reaction, physical expansion, phase transition, etc.). You cannot say that at temperature 1, a substance has a certain heat value q1, and that at temperature 2, its heat value is q2.

What IS a state function is temperature. A functional definition of temperature is that it is an intensive (independent of amount (e.g. density)) physical property that drives heat flow between two systems. That is, if A is at a higher temperature than B, heat flows from A to B. It is related to "thermal energy" or "kinetic energy of random motion". Heat is defined as energy that flows between two systems at different temperatures. That seems like circular reasoning, so another definition for heat is that it is energy that drives random motion, rather than organized motion. Regardless, temperature of a system decreases when heat flows out of a system (q < 0), and increases when heat flows into a system (q > 0), generally. Hope you enjoy the wall of text.

First off, I made a critical mistake and typed up the answer wrong. Please read my revised post above. The exothermic/endothermic part was my careless mistake.

Right now I don't have the book with me, but I can post the question when I get home later.

I believe the answer actually stated that the temperature in the flask INCREASED. I remember the explanation being like "since gas -->solid is exothermic, heat is being released, so the temperature of the flask increases". A key part of the question was that it was asking the temperature of the FLASK, not the substance in the flask (i.e. the surroundings vs. the substance). Does this make more sense?

 

[Rabolisk]

The more I read it, the more I thought that B was the best interpretation. Your reasoning is right. If you take the substance as the system and the rest of the flask as the surroundings, then energy or heat flow is from the system to the surroundings, implying an increase in the temperature of the surroundings.

 

[SteveJMarist]

The more I read it, the more I thought that B was the best interpretation. Your reasoning is right. If you take the substance as the system and the rest of the flask as the surroundings, then energy or heat flow is from the system to the surroundings, implying an increase in the temperature of the surroundings.

Yep, that's exactly right. I confirmed with both my teacher and some others. Thanks for all of your help.