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Tips on When to Use Specific Heat 2,433 views

Tips on when to use specific heat. So sometimes we are asked to find the heat, amount of energy released or absorbed in a particular chemical process or reaction. And a lot of times my students go straight to the specific heat equation which is; q=mc delta t.

A lot of times this is used. And then my students get stuck because they don’t know what to do or how it can be used. So I’m going to first off tell you, that the only time you are ever to use specific heat, the specific heat equation, is if you have a change in temperature.

If you do not have a change in temperature, do not use specific heat. So make sure you have a change in temperature. If you do not, if you do a process that does not have a change in temperature but requires energy, you're probably going to be using a delta H.

But we are not talking about that right, now we talking on when to use specific heat. So you have that change in temperature. So if you are talking about, I'm going to draw a heating curve. I’m sure everybody has seen a heating curve before.

And you are asked to decide how much energy is required to heat something up from here to here. This is let’s say, this is water, 0 degrees, 100 degrees. We know this is a freezing point. We know this is boiling point. So let’s say we are asked to raise the temperature, sure, use q equals mc delta t.

So if you are asked to do anything in this region, anything in this region or anything in this region, between whether it is a solid, liquid or gas, it doesn’t matter. You are going to use q equals mc delta t. If you are asked about something change in phases, the energy it takes, do not use q equals mc delta t. Also be very aware that each state of matter; solid, liquid and gas, know that even if it’s the same substance, H20, it has a different c’s. So I’ll say it's unique for all states of matter. It’s not the same for a solid, liquid and gas, I’ll write that too.

All of these three phases have different c’s. You cannot use the same c, make sure you are very aware of that. That is a very common mistake.

Another problem you are going to use specific heat. Let’s say you are given and this is very another common problem. And we have some water in a bucket or beaker and you have some metal. So this is a rock or anything and you are throwing the rock in the water. The rock is very hot. This is very cool and you are asked how much heat is changed?

So basically the rock is going to transfer heat from the hot rock to the cold water. The temperature of the water is going to rise. The temperature of the rock is going to drop. This is a change in temperature. The amount of heat released from the rock is the same as amount of heat absorbed by the water. So q equals mc delta t, q of the rock equals mc delta t of the water.

Rock is going to loose energy somewhere so it’s a negative. Water is going to gain energy, so I’m going to say it's positive. So -q of the rock is equal to q of the water, if 100 percent of the energy is being transferred and not absorbed in the container that is being held in, or anything like that.

So you might say negative mc delta t equals mc delta t. So this is another place, the only other place you're going to use specific heat. So you are talking about the temperature of something rising and it transferred heat from one hot substance to a cool substance.

Notice, the energy, I put a negative sign here. That does not mean that there is negative amount of energy. It basically means that energy is been dropped. Temperature is lowering, so energy has dropped. So these are the two places, I’m going to say this is the rock, this is some water. These are the only two places you are going to use specific heat. Otherwise there are other equations you can use that will better help your answer. But only when you're raising temperature. You make sure you have a change in temperature before you start plugging in that q equals mc delta t equation. So hopefully that helped.