We are taking a closer look at what happens when hot water is cooled down or ice is heating up.
We are taking a closer look at what happens when hot water is cooled down or ice is heating up (0:30).
After a brief review of the terminology of phase changes (1:11) we are taking a closer look at the particles in solid, liquids and gases (1:29). Focusing on the energy component of phase changes, we are describing the five segments of a heating curve (2:22). In segments where the temperature increases with increasing heat added, we are increasing the particle motion and can calculate the heat added using mCΔT (3:26). In segments where the temperature is not increased, the energy added is used to overcome the intermolecular forces. For these segments we use the enthalpies of fusion and vaporization to calculate the heat added (5:54). The episode closes with three important reminders about the heat of vaporization, the magnitude of enthalpies for a cooling curve and a reminder about the units for the calculations (6:26).
Question: Which of the following substances would have the greatest enthalpy of fusion?
A. methane (CH4) B. acetic acid (CH3COOH) C. ethanol (C2H5OH)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Energy of Phase Changes
Lets Zoom out:
Unit 6 - Thermodynamics
Topic - 6.5 Energy of Phase Changes
Big idea - Energy
Introduction:
What do you do when you have leftover hot water? Just put it in the freezer and save it for later! Always good to start an episode with some laughter! But let’s take a closer look at the hot water in the freezer as well when we want to use it again as hot water: The hot water cools down and freezes, releasing energy. In the opposite direction, we have to add energy to raise the temperature from solid to hot water. How can we calculate the energy absorbed and released? What happens on a particle level? And how does this relate to our macroscopic observations?
Let’s zoom in:
Since we are talking about phase changes, let’s briefly review the terminology: solid to liquid is melting, liquid to gas is vaporization, and solid to gas is sublimation. For these phase changes, heat has to be added to the system. These phase changes are endothermic. In reverse order: gas to liquid is condensation, liquid to solid is freezing and gas to solid is deposition. For these phase changes, heat is being released by the system to the surroundings. Therefore, these phase changes are exothermic.
Let’s take a closer look at the particles: in a solid the particles are experiencing strong intermolecular forces - as strong as they can be for the substance we are looking at, the arrangement is very orderly and the particle motion is very limited. When heating the liquid, the particle motion increases, the intermolecular forces are weaker and the particle arrangement is less orderly. The liquid assumes the shape of the container. Gas particles move freely throughout the entire container - and even leave it if you don’t have a lid. They don’t experience intermolecular forces, at least if we assume they behave like ideal gases.
Let’s focus on the energy component of the phase change. Experimentally, we could throw some very cold ice cubes in a beaker and heat the beaker consistently, measuring the temperature frequently. When graphing our data with heat added as the independent variable on the x-axis and temperature as dependent variable on the y-axis, we’ll get a graph that has five areas: in the first segment, the heat added leads to an increase in temperature, the second segment, which is at melting point, shows no increases in temperature, even though we kept adding heat. The third segment, above melting point, again shows an increase in temperature with increasing heat. Then we reach our fourth, the boiling point. Similarly to the melting point, the temperature remains constant for a while. Lastly, beyond boiling point, the temperature rises again with increasing heat.
Summarizing this, we have two different aspects to look at: The areas in which the added heat leads to an increase in temperature and the areas of phase change, in which the temperature remains constant.
Let’s start with the areas in which the temperature increases. These are the areas between the phase changes. In these areas, the heat or energy added increases the kinetic energy of the particles and therefore increases the particle motion. We also know from the gas laws that temperature is proportional to kinetic energy. Therefore, the particles move faster with increasing temperature. We can calculate the energy for these areas using everyone's favorite thermodynamic formula: mCΔT. Quick reminder: m is the mass in grams of the substance we are heating. C is the specific heat capacity, for example for liquid water this is 4.18 J/g ℃ and delta T is the change in temperature we want to calculate the energy for. Here’s an example: Imagine we want to know the amount of energy needed for the 100.g of the hot water that we put in the freezer to get back to boiling. Assuming we are going from 0℃ to 100℃, we calculate: 100.g x 4.18 J/g ℃ x 100℃. Take note: units are friends! grams and ℃ are cancelling out, and we are left with joules, the unit of energy. This one we can even do in our head: We need 4.18 x 104 J of energy.
What about the segments where the temperature doesn’t increase, at melting point and boiling point? The energy added during those segments is used to overcome the intermolecular forces. The length of this segment can give an indication of how strong the intermolecular forces in the substance are. Additionally, the length of the boiling point segment is longer than the one at melting point, because to go from liquid to gas ALL intermolecular forces have to be overcome, whereas from solid to liquid only some do.
How do we do our energy calculations for these? Since we don’t have a change in temperature, mcΔT doesn’t work. For these segments we use the enthalpies of fusion and vaporization. When heating a substance, we can calculate the energy at melting point by using: q, heat, equals the number of moles of substance times the enthalpy of fusion, which has to be given. Important: Notice that you’ll need the number of MOLES because the enthalpy of fusion is in kJ/MOL - don’t forget, units are friends. So if you heat 100.g of water, it will be 5.55 moles of water times the heat of fusion, which is 6.01 kJ/mol. The moles cancel out and you’ll need to add 33.4 kJ to overcome the forces of attraction between water particles to transition from solid to liquid. Three important aspects: 1) as I’ve already mentioned, the heat of vaporization will be greater. In comparison, for water it is 40.67 kJ/mol. 2) if you have a cooling curve instead of a heating curve, the magnitude stays the same, but instead of heat added, the energy is being released so the values are negative. To go from water vapor to liquid water at boiling point you would have 5.55 moles of water times negative 40.67 kJ/mol. The formation of attractive forces between the water molecules therefore releases 225.7 kJ of energy. 3) when you add the values together, be careful with the units: the mCΔT calculations give you J, the calculations with enthalpy kJ.
To recap:
When heating a substance, energy is added to the system and the phase changes are endothermic. When cooling a substance, energy is released by the system to the surroundings and the phase change is exothermic. Particle movement increases from solid to gas and the forces of attraction decrease from solid to gas. When graphing a heating curve, we have three areas in which the increase in heat leads to an increase in temperature. The energy can be calculated using mCΔT. At melting and boiling point the temperature remains constant and added energy is used to overcome intermolecular attraction. The amount of energy needed can be calculated using the enthalpy of fusion and vaporization.
Coming up next on the APsolute RecAP Chemistry Edition: Unit 6 Selected FRQs
Today’s Question of the day is about the enthalpy of fusion.
Which of the following substances would have the greatest enthalpy of fusion?
A. methane (CH4) B. acetic acid (CH3COOH) C. ethanol (C2H5OH)