hydrogen bonding

hydrogen bonds and changes of state

Changes of state from solid to liquid and from liquid to gas occur when matter gains energy. The energy allows individual molecules to separate and move apart from one another. It takes more energy to bring about these changes of state for polar molecules. Although hydrogen bonds are weak, they add to the energy needed for molecules to move apart from one another, so it takes higher temperatures for these changes of state to occur in polar compounds. This explains why polar compounds have relatively high melting and boiling points. The Table 1.1 compares melting and boiling points for some polar and nonpolar covalent compounds. Name of Compound (Chemical Formula) Methane (CH4 ) Ethylene (C2 H2 ) Ammonia (NH3 ) Water (H2 O) Polar or Nonpolar? Melting Point( C) Boiling Point ( C) nonpolar nonpolar polar polar -182 -169 -78 0 -162 -104 -33 100 Q: Which compound in the Table 1.1 do you think is more polar, ammonia or water?

hydrogen bonding

Because of waters polarity, individual water molecules are attracted to one another. You can see this in the Figure of a nearby water molecule. This force of attraction is called a hydrogen bond. Hydrogen bonds are intermolecular (between-molecule) bonds, rather than intramolecular (within-molecule) bonds. They occur not only in water but in other polar molecules in which positive hydrogen atoms are attracted to negative atoms in nearby molecules. Hydrogen bonds are relatively weak as chemical bonds go. For example, they are much weaker than the bonds holding atoms together within molecules of covalent compounds. Click image to the left or use the URL below. URL:


what are polar compounds

Polar compounds, such as water, are compounds that have a partial negative charge on one side of each molecule and a partial positive charge on the other side. All polar compounds contain polar bonds (although not all compounds that contain polar bonds are polar.) In a polar bond, two atoms share electrons unequally. One atom attracts the shared electrons more strongly, so it has a partial negative charge. The other atom attracts the shared electrons less strongly, so it is has a partial positive charge. In a water molecule, the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do. This explains why the oxygen side of the water molecule has a partial negative charge and the hydrogen side of the molecule has a partial positive charge. Q: If a molecule is polar, how might this affect its interactions with nearby molecules of the same compound? A: Opposite charges on different molecules of the same compound might cause the molecules to be attracted to each other.

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water is a polar compound.

-->  a. true

b. false

hydrogen bonds are very strong bonds.

a. true

-->  b. false

in water, hydrogen bonds hold together

a) hydrogen and oxygen atoms in the same molecule.

-->  b) hydrogen and oxygen atoms in different molecules.

c) hydrogen atoms in different molecules.

d) hydrogen atoms in the same molecule.

polar molecules tend to have lower boiling points than nonpolar molecules.

a. true

-->  b. false

which of the following compounds has the highest melting point?

a) methane

b) ethylene

c) ammonia

-->  d) water

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