Answer:
Dipole-dipole
Step-by-step explanation:
Intermolecular forces, IMFs, are the attractive forces between molecules in a mixture or substance.
Dipoles
Before we talk about IMFs we need to understand the forces within an individual molecule. Molecules can have something called a dipole moment. Dipole moments are a separation of charges caused by a difference in electronegativity. Electronegativity is the tendency of an atom to pull electrons to itself. So, if one atom in a molecule has a high electronegativity and the other has a low electronegativity, the electrons will be pulled to one side of the molecule. This side will then be more negative, and the other will be more positive. The slightly negative and positive charges cause dipole moments.
Both hydrogen iodide, HI, and hydrogen bromide, HBr, have dipole moments. The hydrogen atom has a slight positive charge in both molecules.
Dipole-Dipole Interactions
To understand dipole-dipole IMFs it's important to remember that opposite charges attract. When the slightly positive end of one molecule is close to the slightly negative end of another, they are attracted to each other. This attraction is known as dipole-dipole interactions. So, since both HI and HBr have dipoles, we can see dipole-dipole interactions between them.
Other IMFs
In a mixture of HI and HBr, dipole-dipole IMFs are the most prominent. However, it should be noted that there is another type of IMF visible- London Dispersion Forces, LDFs. LDFs are caused by the random shifting of electrons in a molecule that causes very slight, very temporary dipole moments. These moments only last for a split second and are weaker than permanent dipoles (the type mentioned above), but they do exist. Since dipole-dipole is stronger, we tend to mention it as the main IMF.