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Notes on Heat Calculations
Steps to Calculating Heat (Thermodynamic) Problems:
Before Step 1. Define your system (the substance(s) that are changing).
1. Write down unknown information (i.e. what you are looking for).
2. Write down what you know. Remember if you are given heat value (in cal or J) you need to determine the sign of this value:
- What to do with heat term
- If system is losing heat (giving to surrounding), the heat sign is negative
- If system is gaining heat (absorbing from surrounding), the heat sign is positive
- If you are calculating the heat value, you don't determine the sign your calculations will
- What to do with temperature and temperature change
- Show You must show Tinitial and Tfinal
- ,Show calculation, ΔT = Tfinal - Tinitial HERE including sign
- If your unknown is either Tinitial or Tfinal, see special case below
3a. Setting up answer: By equation (Following steps from Set-up for Equations Notes)
- Equation for Phase change (called Latent heat) heat equation (equ): In class
- Equation for Non-phase change (calledSensible heat) heat equ: ΔH = ( m ) ( ΔT ) ( c )
- where ΔH is heat change, ΔT is temperature change and c is specific heat capacity (J/(g-°C) or cal/(g-°C))
3b. Setting up answer: By Dimensional Analysis (Following steps from Dimensional Analysis Notes
- Important information is that c (specific heat capacity) is conversion: unit for c = J/(g-°C) or cal/(g-°C) )
4. Do math. (make sure to have units on all numbers and keep appropriate sig fig and scientific notation).
5. Check answer including correct number of sig figs, scientific notation, and proper units! Remember that the last calculation step determines the method of determining sig figs.
Example of Work
Special Case When Looking for Tinitial and Tfinal
For these problems, the actual equation/math you are trying to solve is not the heat equation ΔH = (m)(ΔT)(c) but ΔT = Tfinal - Tinitial
So, you will need to change the above equation to either:
- Tfinal = ΔT + Tinitial or
- Tinitial = ΔT - Tfinal
You first need to determine the deltaH. This can be done by either Dimensional Analysis or Equation. Once you have calculated ΔH, you now all information to calculate the difference above and find answer.
- Note: since last math step is add/subtract, you will need to do columns/decimal places not number of sig fig to get answer in correct sig figs.
Example of Work