Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermochemistry Question

  1. Feb 2, 2014 #1

    I am currently studying intro to thermochemistry. I noticed in some problem solutions the equation q=mc(Tf-Ti) but in other the equation is q=c(Tf-Ti). How come sometimes the mass is not used in the equation? When do you know which one to use?

  2. jcsd
  3. Feb 2, 2014 #2
    In one case you are using the Heat Capacity which has units of J/K, in the other case they are using the Specific Heat Capacity which has units of J/(gK) or J/(molK). The latter is an intensive property of a system, meaning you don't have to worry about how much material you have, whereas the former can change depending on how much material you are working with. To intuitively understand this a little better just consider that 1g of water will have a much greater change in temperature when absorbing 1 J of energy whereas 1000g of water may hardly change temperature at all.

    The relation between them is nCs = C, where I will define n to be the relevant mass unit (either moles or grams), Cs = specific heat capacity, and C = heat capacity.
  4. Feb 3, 2014 #3
    The second one is "per unit mass", where the "unit mass" has to be in units consistent with the rest of the equation.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook