Intermolecular forces, electronegativity and bond properties

Bromine is a liquid at room temperature. Weak van der Waals’ forces hold the bromine molecules together. Describe how van der Waals’ forces arise.

Van der Waals’ forces or London dispersion forces refer to the electron charge clouds in a non-polar molecule (or atom) that are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short moment, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ+ end of the dipole in one molecule and the δ– end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. van der Waals’ forces are sometimes called temporary dipole–induced dipole forces.

Water is extensively hydrogen bonded. This gives it anomalous (peculiar) properties.

a. Water is extensively hydrogen bonded. This gives it anomalous (peculiar) properties.

b. State two other anomalous properties of water.

a. Most solids are denser than their liquids. This is because the molecules are more closely packed in the solid state. But this is not true of water. In ice, there is a three-dimensional hydrogen-bonded network of water molecules. This produces a rigid lattice in which each oxygen atom is surrounded by a tetrahedron of hydrogen atoms. This ‘more open’ arrangement, due to the relatively long hydrogen bonds, allows the water molecules to be slightly further apart than in the liquid. So the density of ice is less than that of liquid water.

b. In addition to the non-directional interactions found in simple liquids, the capacity to create up to four hydrogen bonds (H-bonds) results in a number of peculiar characteristics, including high surface tension, decreased viscosity under pressure, increased density upon melting, and density maximum at 4 °C.

Propanone has the structure shown below.

When propanone dissolves in water, it forms a hydrogen bond with water. 

i. What features must water and propanone molecules posses in order to form a hydrogen bond?

ii. Draw a diagram to show a propanone molecule and a water molecule forming a hydrogen bond.

i. both propanone and water must possess highly electronegative atom with al east one lone pair of electron and at least one of the molecules must have a hydrogen atom attached to such an electronegative atom

ii.

There are a number of different types of intermolecular forces possible between molecules. 

Name the three main types and place them in ascending order of strength.

Three main types of intermolecular forces are 

(1) induced dipole-dipole interactions

(2) permanent dipole-dipole interactions

(3) hydrogen bonding

In order of ascending strength: induced dipole-dipole interactions < permanent dipole-dipole interactions < hydrogen bonding

Below are four solids. Which of these contains more than one kind of bonding?

A. Diamond

B. Sodium chloride

C. Iron

D. Ice

The answer is D.

Ice contains covalent bonds between atoms in the water and hydrogen bonds between the molecules

A is incorrect because it has only covalent bond

B is incorrect because it has only ionic bond

C is incorrect because it has only metallic bond

Which quantity gives the best indication of the relative strengths of the hydrogen bonds between water molecules in the liquid state?

A. Enthalpy changes of vaporisation

B. Bond dissociation energies

C. Enthalpy of formation

D. Activation energy

The answer is A.

Vaporisation is the process by which a liquid is converted into a gas. Hydrogen bonds helps water molecules to hold together in liquid state. Thus, vaporisation is obtained by overcoming the hydrogen bonds.

B is incorrect because this energy is related to breaking of a covalent bond

C is incorrect because the enthalpy of formation is defined as the enthalpy change when one mole of a substance is formed from its elements in their standard states

D is incorrect because activation energy is the minimum energy required for a chemical reaction to occur.

Which of the following changes of state shows only dispersion van der Waals’ forces being overcome?

A. `C_3H_6 (l) → C_3H_6 (s)`

B. `CO_2 (l) → CO_2 (g)`

C. `H_2O (s) → H_2O (l)`

D. `C_2H_5OH (l) → C_2H_5OH (g)`

The answer is B.

Van der Waals’ force is the weakest of all intermolecular interactions and the molecular size and shape will affect the strength of this force and the boiling points. Thus, CO₂ will have weak dispersion forces → a very low boiling point.

A is incorrect because dispersion forces is generated

C is incorrect because water has hydrogen bonds

D is incorrect because ethanol has permanent hydrogen bonds

Silver and iodine are both shiny crystalline solids. Which forces exist between neighboring iodine molecules in solid iodine and particles in solid silver? 

A. Metallic bonds - Covalent bonds

B. Ionic bonds - Metallic

C. Covalent bonds - Covalent bonds

D. Van der Waals’ forces - Metallic

The answer is D.

Van der Waals’ force is the weakest of all intermolecular interactions and the molecular size and shape will affect the strength of this force and the boiling points. 

Metallic bonding is formed between metal atoms and share the valence electrons in delocalised electrons.

A is incorrect because iodine is non-metal

B is incorrect because iodine is non-metal

C is incorrect because silver is a metal

Which of the following statements is not correct? 

A. The strongest type of intermolecular force in ammonia is hydrogen bonding

B. The strongest type of intermolecular force in water is hydrogen bonding

C. The strongest type of intermolecular force in hydrogen chloride is permanent dipole-dipole

D. The strongest type of intermolecular forces in ethanol is permanent dipole-dipole

The answer is D.

Ethanol can form hydrogen bonds 

A is correct because NH3 can form hydrogen bonds

B is correct because H2O can form hydrogen bonds

C is correct because HCl can form permanent dipole-dipole forces

Which element has induced dipole–dipole interactions (London forces) in its solid lattice?

A. Boron

B. Magnesium

C. Silicon

D. Sulfur

The answer is D.

Induced dipole-dipole interactions exists in-between molecules of substances. Sulfure molecules form simple molecular structures

A, B and C are incorrect because B, Mg, Si are not molecular substances.