Introduction to the Heat Curve
Before we take a look at what a heat curve is, we must first understand what can happen when heat is added to a system.
More Information on Each of These Important Equations
q is heat
n is moles
ΔHphase change is the enthalpy value that describes the heat of transition (energy required to complete a transition)
Example: the heat of vapourization (ΔHvap) is describing the amount of energy that is required to convert a liquid → gas (vapourization).
q is heat
m is mass (g)
c is the specific heat capacity (J/goC or J/goK)
ΔT is the change in temperature ΔT=Tf-Ti (oC or oK)
Note: Each phase will have a different "c" value!
Example: We will see that water in liquid form and solid ice form have different c values!
- H2O(l), c=4.18 J/goC
- H2O(s), c=2.06 J/goC
Heat curve for H2O
- Label the melting and boiling points of water on the y axis.
- Label what is happening on the horizontal lines as we move to the right.
- Label the appropriate equation for each column in the diagram (q=mcΔT or q=nΔH).
- What c values and ΔH values would you use in each column?
- What is the slope of horizontal sections? ________
- Are the phase changes occuring at the parts of the graph that have a slope of 0 or a positive slope? ________
Summary
Important Things to Know About the Heat Curve
Do you think the ΔHfusion or ΔHvapourization would be a larger number? Why? (You can look at the heat curve to help you decide)
- ΔHvap because there are more intermolecular forces that need to be broken to go from liquid to gas than there are from solid to liquid.
- This is because gases have no intermolecular forces (or almost none) so to go from a liquid to a gas, it requires more energy since all the intermolecular forces need to be broken.
- This will always be the case!
- The longer the horizontal line in the heat curve, the more/less: ________ the value of the ΔH for that phase change!
- qvap will also be greater than qfusion
Based on the given values of ΔHfusion and ΔHvapourization in the diagram above, what do you think the ΔHcrystallization and ΔHcondensation would be?
Same magnitude but opposite sign so for crystallization it would be -6 kJ and for condensation it would be -40 kJ
Melting/Boiling Points
The melting point is the temperature where (________) ⇌ (________) phase
The boiling point is the temperature where (________) ⇌ (________) phase
Both these points (melting and boiling) occur at a pressure of 1atm. We'll see how this will come in handy for us to know soon!
Example: Heat Curve
What is the amount of heat required to melt 10 g of ice at –5°C and convert it to liquid at 60°C?
cice = 2.06 J/g°C, ΔHfus = 334 J/g, cH2O(l) = 4.18 J/g°C
Hint: For a problem like this we need to think of what we are doing (increasing temperature or changing phase) and when
- We start with ice at –5°C. The ice will need to be heated until it reaches 0°C
- At 0°C we then change ice's phase into liquid by melting it
- Once we melt the ice into a liquid, we then heat the liquid from 0°C to 60°C
Now that we have thought of the different steps, let's write out an equation that describes each step:
qT = mciceΔT + nΔHfusion + mcwaterΔT
= (10 g)(2.06 J/goC)(5oC) + (10 g)(334 J/g) + (10 g)(4.18 J/goC)(60oC)
= 5951 J
If we wanted to answer in kJ, what would be our answer (if our answer only has 2 significant digits):
= 5.951 kJ
= 6.0 kJ