A Level Chemistry OCR Practice Exam

What is a permanent dipole-dipole force?

1. An attractive force between similar non-polar molecules

2. An attractive force between permanent dipoles of neighbouring polar molecules

3. A repulsive force between charged ions

4. A force that exists only in gaseous states

The correct answer is: An attractive force between permanent dipoles of neighbouring polar molecules

A permanent dipole-dipole force is defined as an attractive interaction that occurs between the permanent dipoles of neighboring polar molecules. This phenomenon arises due to the unequal distribution of electron density in polar molecules, leading to regions of partial positive and negative charges. When these polar molecules approach one another, the positive end of one molecule is attracted to the negative end of another, forming a stable interaction. This type of intermolecular force is significant because it contributes to the physical properties of substances, such as boiling and melting points. The strength of dipole-dipole interactions can often affect the solubility and reactivity of polar substances in various environments. The other choices do not accurately describe permanent dipole-dipole forces. Non-polar molecules do not have permanent dipoles, so the first option is not applicable. The third option refers to the forces between charged ions, which are classified as ionic or electrostatic forces rather than dipole-dipole interactions. Lastly, the fourth option suggests that these forces only exist in gaseous states, which is incorrect as dipole-dipole interactions can occur in both liquid and solid states where polar molecules are present.