NEM 10V Lecture Notes - Lecture 5: Hydrogen Bond, Acid Rain, Surface Tension

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6 May 2018

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Nematology

Part 5 Water Part 2

Life could not exist without water. Over 70% of the planet is covered in water and our bodies are

around 70% water.

• The structure of a water molecule and its ability to form hydrogen bonds with

other water molecules, provide it with some very unique characteristics.

-the ability to act as a universal solvent

-the properties of cohesion and adhesion

-high surface tension

-a high heat capacity

-changes in density based on temperature.

• Universal solvent means that many substances dissolve in it.

• Water’s solvency has to do with the polarity and hydrogen bonding ability of

water molecules.

• Cohesion – the hydrogen bonds in water cause water molecules to stick together.

• Adhesion - Polar bonds give water molecules the ability to cling to other polar

surfaces. Water also has a high surface tension. Because water molecules at the

surface are more strongly attracted to each other than to the air above, water

molecules cling tightly to each other. The surface tension of water explains why it

beads up on waxy surfaces, and why water-striding insects are able to walk on the

surface of water. Water has a high capacity for heat, meaning that the hydrogen

bonds that link water molecules let water absorb a large amount of heat without

changing its chemical state. The density of water is based on its temperature.

• Most compounds contract when they freeze, but the configuration of the hydrogen

bonds in water causes it to expand when it freezes. This expansion causes ice to

have a lower density than liquid water, and therefore ice floats on liquid water. If

ice were more dense than liquid water it would sink, and ponds and lakes would

freeze solid. Bodies of water always freeze from the top down.

• Hydrologic cycle- water remains chemically unchanged throughout the cycle.

driven by solar energy and by gravity

• This is different from other nutrient cycles. The major reservoirs of water are the

oceans that cover about ¾ of the earth’s surface and contain 97% of the

available water.

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Document Summary

The ability to act as a universal solvent. Because water molecules at the surface are more strongly attracted to each other than to the air above, water molecules cling tightly to each other. The surface tension of water explains why it beads up on waxy surfaces, and why water-striding insects are able to walk on the surface of water. Water has a high capacity for heat, meaning that the hydrogen bonds that link water molecules let water absorb a large amount of heat without changing its chemical state. The density of water is based on its temperature: most compounds contract when they freeze, but the configuration of the hydrogen bonds in water causes it to expand when it freezes. This expansion causes ice to have a lower density than liquid water, and therefore ice floats on liquid water. If ice were more dense than liquid water it would sink, and ponds and lakes would freeze solid.

Related Questions

Which of the following is not a property of water?

a. It has a low boiling point compared with other molecules.

b. It has a high heat of vaporization.

c. Its molecules resist separation, a property called cohesion.

d. It has the property of adhesion, the ability to stick to charged and polar groups in molecules.

e. It can form hydrogen bonds to molecules below but not above its surface.

champagneleopard228

Chemistry 162 Lab Activity

Introduction Water has an amazing ability to adhere (stick) toitself and to other substances. Hydrogen bonds form when hydrogenatoms covalently bonded to nitrogen (N), oxygen (O), or fluorine(F) in the form of covalent compounds such as ammonia (NH3), water(H2O) and hydrogen fluoride gas (HF). In these molecules, thehydrogen atoms do not pull as strongly on the shared electrons asthe N, O, or F atoms. Therefore, the molecules are polar; thehydrogen atoms become positively charged and are able to formhydrogen bonds to negative ions or negatively charged parts ofother molecules (such as the N, O, and F atoms that becomenegatively charged in these compounds). Hydrogen bonds are not truebonds like covalent bonds or ionic bonds. Hydrogen bonds areattractions of electrostatic force caused by the difference incharge between slightly positive hydrogen ions and other, slightlynegative ions. These attractions are much weaker than true ionic orcovalent bonds, but they are strong enough to result in someinteresting properties.

In the case of water, hydrogen bonds form between neighboringhydrogen and oxygen atoms of adjacent water molecules. Theattraction between individual water molecules creates a bond knownas a hydrogen bond (Fig. 1).
Fig. 1: Hydrogen bonds shown as the dotted lines between watermolecules.
Cohesion Molecules of pure substances are attracted to themselves.This sticking together of like substances is called cohesion.Depending on how attracted molecules of the same substance are toone another, the substance will be more or less cohesive. Hydrogenbonds cause water to be exceptionally attracted to each other.Therefore, water is very cohesive.
Adhesion Adhesion is similar to cohesion, but it involves unlike(i.e. different) substances sticking together. Water is veryadhesive; it sticks well to a variety of different substances.Water sticks to other things for the same reason it sticks toitself â because it is polar so it is attracted to substances thathave charges. In this lab activity, you will be asked to use thematerials and procedures provided to explore the properties ofwater that involve both cohesion and adhesion. Be sure to writedown your observations and results as you progress. sults as youprogress.

Fig. 1

Hydrogen bonds shown as the dotted

lines between water molecules.

Part 1:

Exploring Water

Procedure

How many drops of water can you fit on the surface of a pennywithout the water spilling over?

Predict how many drops of water will fit on a penny. Record yourprediction.

Use the plastic pipette to drop water on the surface of a penny.Drop car

efully and slowly to fit as

many drops as possible on the penny. Count each drop until thewater spills over the side of the

penny and record your results.

Compare your results to those of your classmates. Record therange and calculate the average

number

of drops that fit on a penny.

Add more drops to the penny, without letting the water spillover. Carefully observe and draw the

water on the penny.

Record

a hypothesis for the behavior of water that you observed, usingthe language of cohesion

and adhesion

in relation to the water and the penny.

Record your results and

thinking in your notebook.

Can you

disturb

the water on the penny without spilling it?

dd drops of water to the penny, until the water is almost readyto spill over.

Predict what will

happen when you insert a toothpick into the surface of the waterpiled on the

penny. Record your prediction.

Insert the

toothpick

and carefully observe the surface of the water.

Try inserting the

toothpick

in various ways. Observe the surface of the water.

Pay special attention to

the water surface where the skewer enters the water.

Record your observations and draw what you see.

Repeat

these

procedures using a paper clip and the metal clip part of abinder clip. Try to indent the

pile of water as much as p

ossible without spilling it.

Record your observations and dra

w what you see, paying careful

attention to the surface of the

water.

Record

a hypothesis for the behavior of water that you observed usingthe language of cohesion

and adhesion in relation to the

toothpick

, the paperclip, the binder clip, and the water.

Record your results and

thinking in your notebook.would you please answer the questionfor me . Thanks

Question about mixed solvents for recrystallization.

I am having a hard time conceptualizing how a mixed solvent would work in a scenario where P and O nitronitroacetanilide are recrystallized with a small amount of hot Ethanol followed by a larger volume of water.

From the structure of the molecules I assumed that the Para form of nitroacetanilide is more polar than the Ortho form. Second, I know that water>ethanol for polarity. Finally, I know that water and ethanol can hydrogen bond with one-another and with itself. Also I know that water and ethanol will hydrogen bond together to form a more densely packed liquid. I imagine that this would look somewhat like this: [ Eth-OH] --[O-H2]---[OH-Eth]

My understanding of dissolving is that the solute gets surrounded by solvent molecules and separates from other solute molecules. This happens to all the solute molecules until no more solvent molecules are left to dissolve the solute.

Having said that, does the following make sense? If not, please correct me

1) the O/P mixture of crude nitroacetanilide dissolves in hot ethanol because ethanol hydrogen bonds with the nitroacetanilide

2) when water is added, ethanol and water hydrogen bond with one another creating a denser solvent because the ethanol hydrogens are attracted to the oxygen in water and vice versa

3) because there's less hydrogens "available" to form new hydrogen bonds after the water is added, the solvent can't make new hydrogen bonds as easily

4) because the P-nitroacetanilide needs more hydrogens to dissolve due to it being more polar, it is now less soluble in the ethanol/water mixture. this causes it to precipitate out while leaving the O-nitroacetanilide that's less polar still in solution

Is this correct? if not please explain how the mixed solvent helps precipitate the p-nitroacetanilide but maintain the o-nitroacetanilide in solution.

NEM 10V Lecture Notes - Lecture 5: Hydrogen Bond, Acid Rain, Surface Tension

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