Hydrogen Bonds

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What 3 things do molecules require to form H bonds?
A large dipole between an H atom and a strongly electronegative atom such as N, O or F / A 'small H atom that is able to get in close proximity with other molecules / An electron pair
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What properties do H bonds give substances and why?
High melting and boiling points. This is because H bonds require a great deal of energy to break.
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How do H bonds affect the viscosity of liquids?
The more H bonds are present, the more viscous the liquid. Low viscosities rely on intermolecular bonds readily breaking and reforming, which cannot occur so easily due to the strength of H bonds.
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Give an example of a viscous liquid containing H bonds.
Glycerol contains 3 hydroxyl group and therefore readily forms H bonds (CH2OH x 3)
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How do H bonds affect a substance's solubility in water?
As substances such as glycerol and ethanol contain hydroxyl groups, hydrogen bonds form readily between them and water molecules. This facilitates the dissolving process.
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Describe the nature of the H bonds within water.
As molecules of H2O have two electron pairs on their O atoms as well as 2 positively charged H atoms, the H bonds formed are particularly strong.
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How does this affect the properties of water?
As a result, the boiling point of water is unusually high for a substance with such a low molecular mass. Also, its specific heat capacity is greater than almost any other liquid.
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How do H bonds affect ice formation?
Water is unique in so far as, like other liquids it contracts at low temperatures of around 278K, however below this point it begins to expand again until it freezes. This means it actually decreases in density upon freezing.
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Why does the density of water decrease when it is frozen?
The 4 H atoms around a water molecule's O atom are arranged in a tetrahedral shape due to electron repulsion. As a result, large open spaces develop between molecules upon freezing, thus lowering the density of the substance.
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How is this different from water in liquid form?
When the ice melts, the large open spaces between the tetrahedral molecules collapse in on themselves and are filled by other water molecules. As a result, the density of the liquid is greater than that of the solid.
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Other cards in this set

Card 2

Front

What properties do H bonds give substances and why?

Back

High melting and boiling points. This is because H bonds require a great deal of energy to break.

Card 3

Front

How do H bonds affect the viscosity of liquids?

Back

Preview of the front of card 3

Card 4

Front

Give an example of a viscous liquid containing H bonds.

Back

Preview of the front of card 4

Card 5

Front

How do H bonds affect a substance's solubility in water?

Back

Preview of the front of card 5
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