Chemistry Notebook Salazar

The Time-Temperature Graph
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.
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.
Q=mcΔT
where ΔT is (Tf – Ti)
Here are some symbols that will be used, A LOT!!
Δt = the change in temperature from start to finish in degrees Celsius (°C)
m = mass of substance in grams
c = the specific heat. Its unit is Joules per gram X degree Celsius (J / g °C is one way to write
the unit; J g¯1 °C¯1 is another)
q = the amount of heat involved, measured in Joules or kilojoules (symbols = J and kJ)
mol = moles of substance.
ΔH is the symbol for the molar heat of fusion and ΔH is the symbol for the molar heat of
vaporization.
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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.
Specific heat (commonly written as c) is very important in being able to calculate the energy used
Notes: in an experiment, particularly water in this case. The specific heat is normally used when referring
to a temperature change, not a change in state (solid-liquid, liquid-gas). The time-temperature
graph is a visual representation of what occurs during all of the steps that are analyzed below.
The graph really is a visual aid, as the chemist can always know where the object is on the graph
based on temperatures, and the energy used by the object. There are several equations that
could be used (4 to be exact) in different events, depending on what is happening. If the
temperature is changing, use the Q=mcDeltaT equation, as this is useful for that situation. When
going from solid to liquid, using the Q=mHf is useful, as it shows how much energy is used
between the plateaus. Q=mHv is useful to describe the liquid-gas change.