Effects of Intermolecular Forces - ChemistNATE | Lessons
If you are asked to rank molecules in order of melting point, boiling point, viscosity, surface tension or vapour pressure what they are actually asking is for you. Boiling point is highly dependent on the intermolecular forces of a compound. with the highest vapor pressure will have the weakest intermolecular forces. In this lesson we will review what intermolecular forces are and how they will affect Lesson; Quiz & Worksheet - Intermolecular Forces & Physical Properties Quiz; Course such as boiling point, freezing point, melting point, and vapor pressure. The higher the difference in electronegativity, the strong the dipole- dipole.
Because a liquid can flow only if the molecules can move past one another with minimal resistance, strong intermolecular attractive forces make it more difficult for molecules to move with respect to one another.
This effect is due to the increased number of hydrogen bonds that can form between hydroxyl groups in adjacent molecules, resulting in dramatically stronger intermolecular attractive forces. There is also a correlation between viscosity and molecular shape. Liquids consisting of long, flexible molecules tend to have higher viscosities than those composed of more spherical or shorter-chain molecules.
London dispersion forces also increase with chain length. Due to a combination of these two effects, long-chain hydrocarbons such as motor oils are highly viscous. Viscosity increases as intermolecular interactions or molecular size increases.
Motor Oils Motor oils and other lubricants demonstrate the practical importance of controlling viscosity. Viscosity decreases rapidly with increasing temperatures because the kinetic energy of the molecules increases, and higher kinetic energy enables the molecules to overcome the attractive forces that prevent the liquid from flowing.
So-called single-grade oils can cause major problems.
Properties of Liquids | Chemistry | Visionlearning
If they are viscous enough to work at high operating temperatures SAE 50, for examplethen at low temperatures, they can be so viscous that a car is difficult to start or an engine is not properly lubricated. These properties are achieved by a careful blend of additives that modulate the intermolecular interactions in the oil, thereby controlling the temperature dependence of the viscosity. Will the oil be pulled up into the tube by capillary action or pushed down below the surface of the liquid in the beaker?
What will be the shape of the meniscus convex or concave? Identify the cohesive forces in the motor oil. Determine whether the forces interact with the surface of glass. From the strength of this interaction, predict the behavior of the oil and the shape of the meniscus. Solution A Motor oil is a nonpolar liquid consisting largely of hydrocarbon chains.
The cohesive forces responsible for its high boiling point are almost solely London dispersion forces between the hydrocarbon chains. This is one of the reasons that oil has high viscosity.
The molecules consist of long chains of 15 or more carbon atoms. A great deal of research has gone into designing motor oils that maintain a uniform viscosity over a broad range of temperatures.
Surface tension is the resistance of the surface of a liquid to being broken. It is caused by the fact that the molecules on the surface of a liquid are not bonded to as many neighboring molecules as those beneath the surface.
As with viscosity, the greater the attraction between molecules, the higher the surface tension. Any action that breaks the surface of a liquid must first distort the surface and therefore increase the surface area. Because molecules on the surface of a liquid are bonded to fewer neighboring molecules there are none above themincreasing the surface area necessarily involves breaking some intermolecular bonds.
An object will break the surface of a liquid only if it exerts a force due to gravity, momentum, etc. Volatility and vapor pressure really measure the same phenomenon. Volatility is the tendency of a liquid to evaporate and, as you might expect, the stronger the intermolecular bonds, the less the tendency to evaporate and the lower the volatility. Volatility and vapor pressure are two properties that increase in value as the strength of intermolecular bonds decreases. Most physical properties boiling point, melting point, viscosity, and surface tension, for example correlate directly to the strength of intermolecular bonds — increasing in value as the bond strength increases.
It might be a good idea to commit that distinction to memory. If a liquid is placed in a closed container, it will tend to evaporate. The more volatile it is, the more rapidly it will evaporate.
It is also the case that raising the temperature of the liquid will cause it to evaporate more rapidly.
At any given temperature the molecules in a liquid have a broad range of kinetic energies — some are very high, some are very low, but most are close to the average. To escape from the liquid, a surface molecule must have sufficient kinetic energy to overcome the intermolecular bonds holding it to its neighbors.
The higher the temperature, the greater the fraction of surface molecules that will have this amount of energy and the greater the rate of evaporation. Further, at any given temperature, in a liquid whose molecules are held together by weak intermolecular bonds, a relatively larger fraction of the surface molecules will have sufficient kinetic energy to overcome those bonds than in a liquid whose molecules are held together by strong intermolecular bonds. As the liquid evaporates, gas begins to accumulate in the empty volume above the liquid, and this gas exerts a pressure against the walls of the container.
As more liquid evaporates, the pressure increases. Recall from the ideal gas law that the pressure is determined by the volume, temperature, and number of moles of gas present. These weak forces lead to low cohesion.
On the other end of the cohesion spectrumconsider a dewdrop on a leaf in the early morning Figure 6. How can such a thing exist if, as explained earlier, liquids flow and take the shape of the container holding them? As described above and in the Water module, water molecules are held together by strong hydrogen bonds. These strong forces lead to high cohesion: The water molecules interact with each other more strongly than they interact with the air or the leaf itself.
Dew drops on a leaf. Surface tension results from the strong cohesive forces of some liquids. These forces are strong enough to be maintained even when they experience external forces like the gravity of an insect walking across its surface.
The water strider Gerris remigisa common water-walking insect. Remember that, in contrast, cohesion is the tendency of a compound to interact with itself.Vapor pressure - States of matter and intermolecular forces - Chemistry - Khan Academy
Adhesion helps explain how liquids interact with their containers and with other liquids. One example of an interaction with high adhesion is that between water and glass. Both water and glass are held together by polar bonds. Therefore, the two materials can also form favorable polar interactions with each other, leading to high adhesion. You may have even seen these attractive adhesive forces in action in lab. When water is in a glass graduated cylinder, for example, the water creeps up the sides of the glass, creating a concave curve at the top called a meniscus, as shown in the figure below.
Water in graduated cylinders made out of some types of non-polar plastic, on the other hand, forms a flat meniscus because there are neither attractive nor repellant cohesive forces between the water and the plastic.
See Figure 8 for a comparison of polar and non-polar graduated cylinders.
Lesson 2: Intermolecular Bonds
In graduated cylinder A, made of glass, the meniscus is concave; in cylinder B, made of plastic, the meniscus is flat. Viscosity At the beginning of the module, we said that one of the defining features of liquids is their ability to flow. But among liquids there is a huge range in how easily this happens. Consider the ease with which you can pour yourself a glass of water, as compared to the relative challenge of pouring thick, slow-moving motor oil into an engine.
The difference is their viscosityor resistance to flow. Motor oil is quite viscous ; water, not so much. Remember, water molecules form strong hydrogen bonds with each other. Pentane, on the other hand, made up of just hydrogen and carbon atoms, is nonpolar, so the only types of intermolecular forces it can form are the relatively weak London dispersion forces.
The weaker intermolecular forces mean that the molecules can more easily move past each other, or flow — hence, lower viscosity.