Intermolecular Forces

Transcription

1 Intermolecular Forces

2 Intermolecular Forces When molecules are attracted to other molecules by a weaker force (when compared to an ionic bond or a covalent bond holding a molecule together) Intermolecular forces refers to an attraction between molecules, intramolecular forces are the forces within the molecule themselves when a substance evaporates, melts, or sublimates (change of state), the molecules gain kinetic energy from an outside source, this allows them to overcome the intermolecular forces holding them close together

3 there are no intermolecular forces in ionic compounds because ionic compounds do not contain molecules! Ionic compounds are solids at room temperature In an ionic crystal ionic bonds hold all the ions together, there is no difference between the bonds holding the compounds together and those holding the crystal together Ionic bonds are generally very strong and accounts for the high melting point Ionic crystal for NaCl

4 Many molecular substances are gases at room temperature, others are liquids with low boiling points or solids that melt easily Forces between the molecules in molecular solids and liquids are relatively weak, addition of thermal energy can easily overcome these intermolecular forces The strength of intermolecular forces determines the physical properties of molecular compounds: state, melting point, boiling point, surface tension, hardness, texture and solubility As the intermolecular forces between molecules increases, the compound s m.p., b.p. and surface tension also increase

5 Types of Intermolecular Forces Dipole-dipole Forces Intermolecular force between oppositely charged ends of a polar molecule Ex: H-Cl These are generally strong intermolecular forces The more polar a substances is (refer to electronegativity and shape), the stronger the dipole-dipole force of attraction

6 London Dispersion Forces All gases will condense to from liquids (even noble gases) when cooled enough which implies that there is some type of attraction between the entities when they are cooled These attractions are caused by the temporary shifts in the electron cloud in an atom or molecule This shift creates a temporary dipole for a fraction of a second that will also cause a temporary dipole in an adjacent atom or molecule Very short lived as the attraction between them continually forms and breaks, as a result these London forces are very weak The larger the molecule, the more electrons and protons there are attracting one another, the stronger the forces, the higher the melting point London dispersion forces exist between all molecules even polar molecules In polar compounds the London forces are insignificant compared to the much stronger dipole forces dipole-dipole forces and London forces are called Van der Waals forces in honour of the scientist who proposed their existence

7 Hydrogen Bonds special category of very strong dipole-dipole intermolecular forces a hydrogen bond is a particularly strong dipole-dipole force that occurs between two molecules each consists of a hydrogen atom covalently bonded to a highly electronegative atom of nitrogen, oxygen or fluorine ex: H-F two reasons for hydrogen bonding are the large difference in electronegativity and the small size of the hydrogen atom (positive pole is highly concentrated and attracts the negative pole of a nearby molecule)

8 ...in biochemistry hydrogen bonds play a significant role in determining the shape and function of large, biologically important molecules (proteins such as enzymes, hormones, antibodies, DNA) DNA Protein

9 Hydrogen Bonding and Water one of most abundant compounds on Earth s surface approx. 22% of Earth s fresh water is in Canada in nature water contains many dissolved substances necessary for life the water cycle, plants and marshes recycle and purify water but cannot take out some pollutants which are dangerous to our health and the health of the planet water is becoming increasingly contaminated by caffeine, cosmetics, hormones, food additive, antibiotics, despite our desperate need for clean water when compared to other similar compounds, water is different it has some very strange properties

10 a) unusually high melting and boiling points water is the only substance that commonly exists in three states the mp and bp are much higher for water than other similarly structured compounds implying the forces holding water together are very strong bent shape, polar bonds and two lone pairs of electrons on the central oxygen atom make water one of the most polar molecules on Earth

11 b) low density of ice solid ice floats on water, the solid is less dense ice also acts as a thermal blanket holding in the heat of the water below it (as in a frozen lake) at 4 o C water is most dense and as it gets closer to freezing it moves to the top of the lake and changes state as it become less dense (leaving warmer water below) because as water freezes the molecules lose kinetic energy (slow down) and hydrogen bonds start to arrange the molecules into a crystalline structure (think snowflakes). This arrangement contains more spaces between the molecules than just a random arrangement of liquid water molecules so the solid expands and is less dense.

12 c) Unusually high surface tension Attraction between molecules at the surface of liquid water is very high Surface tension due to hydrogen bonds causes beads of water to form on smooth surfaces, raindrops to form in free fall and explains why insects can walk on water Hydrogen bonds allow the water molecules to hold on strongly to each other, pulling together and resisting being broken apart so it shapes to the smallest surface area (a droplet)

13 d) Unusually high Specific Heat Capacity Water holds thermal energy very well without changing its temperature Takes a lot of energy to increase the temperature of water by 1 o C This property allows large lakes and oceans to influence climate and weather patterns (think El Nino, El Nina)