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Postby Novelpreet_Boparai1N » Sun Jan 29, 2017 2:26 am

When would you use q=C*deltaT as opposed to q=m*C*deltaT?

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Re: Heat

Postby AnkitaNair1E » Sun Jan 29, 2017 12:16 pm

Hi Novelpreet,

I think the best way to figure out how to use the equations you're confused about is to understand what each equation means.

The first equation q=C*delta T essentially tells us that we don't need know how much substance is in the reaction (i.e.: we don't need to know the mass or the # of moles of stuff involved in the reaction). All we need to know is the heat capacity (how much energy it takes to raise the temperature) and the change in temperature. Usually, this equation is used in calorimetry experiments where we often aren't given the mass of the reactants or products. Instead, we must first the heat capacity of the calorimeter and then use that as our C value in the above equation to solve the problem.

If you want a good example of this application, look at problem 8.25. In this problem you're asked to calculate how much heat was released in a calorimetry experiment where the temperature change was 7.32 degrees celsius. You aren't given a mass value so the only way to solve this equation is to use q=C*deltaT. However first we must figure out what C is, since usually the Cvalue isn't given in these problems. To figure out C, the problems will usually give you information on how the Calorimeter was calibrated. In other words, they will tell you give you a value for Q and a value for (delta T) from another experiment which you can use to find the C value (using the same q= C* delta T equation). You plug in that C value with 7.32 degrees celsius to solve your problem

The other equation: q=mC*(delta T) is used for situations where we know a specific heat capacity (i.e. we know how much energy it takes to raise 1g of a substance to 1 degree) and you have a specific amount mass. You could use this equation for example to figure out how much energy it would take to heat 400 grams of water from 10 degrees celsius to 100 degrees celsius.

Essentially, the first equation depends only on the C value you find/are given and the change in temperature. You don't really need to know mass or moles to solve the problem. This equation is most often used in calorimetry. The second equation depends on both a specific heat capacity and on how much mass of substance you have and is used to solve a different type of problem where you have more specific information.

Hope this helps!

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