Surface tension and intermolecular forces relationship

Intermolecular Forces

surface tension and intermolecular forces relationship

Surface tension of water is caused by an intermolecular force called hydrogen the water molecules thereby decreasing the surface tension. Surface tension is an effect within the surface layer of a liquid that causes the Surface tension is caused by the effects of intermolecular forces at the interface. Stronger intermolecular forces will produce greater surface tension. The intermolecular forces present in a sample of water are hydrogen.

The bond lengths and angles are oscillating around the predicted values. The amount of heat required to raise the temperature of the ice is determined by the heat capacity of ice, the heat required to change the temperature of 1 gram of ice by 1oC.

The heat capacity of each phase of each substance is unique, and depends on the chemical nature of the substance. When the temperature reaches 0oC, the melting point of ice, further addition of heat does not change the temperature.

At this phase transition temperature, the added energy goes to changing the Potential Energy of the system. It is coulombic in nature, arising from the attraction of charged species. In the case of H2O, it is the attraction between the partial positive charges on the H and the partial negative charges on the O.

As we discussed earlier in the semester, these are hydrogen bonds, holding the water molecules in the crystalline structure of ice. At the phase transition temperature, 0oC, all of the ice will be converted to liquid water. The increase in temperature is, again, an increase in the KE of the system.

The movement of the water molecules will increase in the liquid phase. There is still some degree of hydrogen bonding between molecules, but they are no longer in fixed positions in a crystal lattice. There is a second phase transition at oC. At this temperature, the water, at oC, is converted to steam at oC.

The remaining hydrogen bonds are broken, and all of the water molecules are now moving independently of each other, with no remaining hydrogen bonding. The liquid water is converted to steam.

Tension Superficial

As soon as this happens, addition of heat raises the temperature of the steam and increases the average kinetic energy of the gas molecules, as predicted by the Molecular Kinetic Theory.

Strength of IMF The heat of fusion heat required to melt a solid and heat of vaporization heat required to vaporize a liquid are determined by the strength of the Intermolecular Forces. Substances with high IMF will have higher melting and boiling points.

It will require more energy to break the IMF. Most IMF are weaker than chemical bonds. To break the IMF in ice heat of fusion requires 6. All IMF are electrostatic in nature, the interaction of positive and negative charges. The strength of the IMF will, then, depend on the magnitude of these charges.

Ionic bonding The strongest IMF is ionic bonding. These are the bonds between metals and non-metals, involving ions.

Coulomb's law states that the potential energy, E is proportional to the amplitude of the charges, Q1 and Q2, divided by the distance squared, d2. In salts, there are full positive charges on the cations, which have lost electrons, and full negative charges on the anions, which have gained electrons.

surface tension and intermolecular forces relationship

One of the defining features of salts is their extremely high melting points. A large amount of energy is required to separate the positive and negative ions from their positions in the crystalline lattice. The next strongest IMF is ion-dipole. The partial charge on the polar compound is smaller than a full positive or negative charge on ions, so the interaction will not be as strong.

Water has both strong adhesion to glass, which contains polar SiOH groups, and strong intermolecular cohesion. When a glass capillary is put into water, the surface tension due to cohesive forces constricts the surface area of water within the tube, while adhesion between the water and the glass creates an upward force that maximizes the amount of glass surface in contact with the water. If the adhesive forces are stronger than the cohesive forces, as is the case for water, then the liquid in the capillary rises to the level where the downward force of gravity exactly balances this upward force.

The upper surface of a liquid in a tube is called the meniscus, and the shape of the meniscus depends on the relative strengths of the cohesive and adhesive forces. Capillary action of water compared to mercury, in each case with respect to a polar surface such as glass. Differences in the relative strengths of cohesive and adhesive forces result in different meniscus shapes for mercury left and water right in glass tubes.

11.4: Intermolecular Forces in Action: Surface Tension, Viscosity, and Capillary Action

Mark Ott Polar substances are drawn up a glass capillary and generally have a concave meniscus. Fluids and nutrients are transported up the stems of plants or the trunks of trees by capillary action. Plants contain tiny rigid tubes composed of cellulose, to which water has strong adhesion.

Because of the strong adhesive forces, nutrients can be transported from the roots to the tops of trees that are more than 50 m tall. The moisture is absorbed by the entire fabric, not just the layer in contact with your body. Some liquids, such as gasoline, ethanol, and water, flow very readily and hence have a low viscosity. Others, such as motor oil, molasses, and maple syrup, flow very slowly and have a high viscosity.

surface tension and intermolecular forces relationship

The two most common methods for evaluating the viscosity of a liquid are 1 to measure the time it takes for a quantity of liquid to flow through a narrow vertical tube and 2 to measure the time it takes steel balls to fall through a given volume of the liquid. The higher the viscosity, the slower the liquid flows through the tube and the steel balls fall. The viscosities of some representative liquids are listed in Table 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.

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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.

surface tension and intermolecular forces relationship

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. 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?