Explain : Temperature vs Time graph
Answers
Answer:
We are going to heat a container that has 72.0 grams of ice (no liquid water yet!) in it. To make the illustration simple, please consider that 100% of the heat applied goes into the water. There is no loss of heat into heating the container and no heat is lost to the air.
Let us suppose the ice starts at −10.0 °C and that the pressure is always one atmosphere. We will end the example with steam at 120.0 °C.
There are five major steps to discuss in turn before this problem is completely solved. Here they are:
1) the ice rises in temperature from −10.0 to 0.00 °C.
2) the ice melts at 0.00 °C.
3) the liquid water then rises in temperature from zero to 100.0 °C.
4) the liquid water then boils at 100.0 °C.
5) the steam then rises in temperature from 100.0 to 120.0 °C
Each one of these steps will have a calculation associated with it. WARNING: many homework and test questions can be written which use less than the five steps. For example, suppose the water in the problem above started at 10.0 °C. Then, only steps 3, 4, and 5 would be required for solution.
To the right is the type of graph which is typically used to show this process over time.
The ChemTeam hopes that you can figure out that the five numbered sections on the graph relate to the five numbered parts of the list just above the graph.
Also, note that numbers 2 and 4 are phases changes: solid to liquid in #2 and liquid to gas in #4.
Here are some symbols that will be used, A LOT!!
1) Δt = the change in temperature from start to finish in degrees Celsius (°C)
2) m = mass of substance in grams
3) Cp = the specific heat. Its unit is Joules per gram-degree Celsius (J / g °C is one way to write the unit; J g¯1 °C¯1 is another)
4) q = the amount of heat involved, measured in Joules or kilojoules (symbols = J and kJ)
5) mol = moles of substance.
6) ΔHfus is the symbol for the molar heat of fusion and ΔHvap is the symbol for the molar heat of vaporization.
We will also require the molar mass of the substance. In this example it is water, so the molar mass is 18.0 g/mol.
By the way, the p means the specific heat is measured at constant pressure; there is a related specific heat we will not discuss (yet) which is measured at constant volume. Not too suprisingly (I hope), it has the symbol Cv.
Step One: solid ice rises in temperature
As we apply heat, the ice will rise in temperature until it arrives at its normal melting point of zero Celsius.
Once it arrives at zero, the Δt equals 10.0 °C.
Here is an important point: THE ICE HAS NOT MELTED YET.
At the end of this step we have SOLID ice at zero degrees. It has not melted yet. That's an important point.
Each gram of water requires a constant amount of energy to go up each degree Celsius. This amount of energy is called specific heat and has the symbol Cp.
Follow this link to calculate the energy used in this step.