شرح كيمياء thermochemistry

[عاطف خليفة]

Time-Temperature Calculation #1:
Solid Water Changing in Temperature


72.0 grams of ice (no liquid water yet!) has changed 10.0 °C. We need to calculate the energy needed to do this.
This summarizes the information needed:

Dt = 10 °C
The mass = 72.0 g
Cp = 2.06 Joules per gram-degree Celsius

The calculation needed, using words & symbols is:
q = (mass) (Dt) (Cp)
Why is this equation the way it is?

Think about one gram going one degree. The ice needs 2.06 J for that. Now go the second degree. Another 2.06 J. Go the third degree and use another 2.06 J. So one gram going 10 degress needs 2.06 x 10 = 20.6 J. Now we have 72 grams, so gram #2 also needs 20.6, gram #3 needs 20.6 and so on until 72 grams.

I hope that helped.
With the numbers in place, we have:
q = (72.0 g) (10 °C) (2.06 J/g °C)
So we calculate and get 1483.2 J. We won't bother to round off right now since there are four more calculations to go. Maybe you can see that we will have to do five calculations and then sum them all up.
One warning before going on: three of the calculations will yield J as the unit on the answer and two will give kJ. When you add the five values together, you MUST have them all be the same unit. In the context of this problem, kJ is the preferred unit. You might want to think about what 1483.2 J is in kJ.