3.3: Electrical Properties of Pure Water
- Page ID
- 294282
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Electrical Properties of Pure Water
The motion of water’s dipoles guide almost everything that happens in the liquid. Two important contributions:
- Permanent dipole moment of molecule lies along symmetry axis.
- Induced dipole moments (polarization) along the hydrogen bonds. Strengthening hydrogen bond increases \(r_{OH}\) and decreases \(R_{OO}\), which increases the dipole moment. The dipole moment per molecule changes from 1.7 to 3.0 D going from gas phase to liquid.
Water Dielectric Response
Pure water is a strong dielectric medium, meaning that long-range electrostatic forces acting between two charges in water are dramatically reduced. The static dielectric constant is \(\varepsilon = 80\), also known as the relative permittivity \(\varepsilon_r = \varepsilon /\varepsilon_0\). The dielectric response is strongly frequency and temperature dependent. Motion of water charges encoded in complex dielectric constant (\(\varepsilon\)) or index of refraction (\(\tilde{n}\)).
Water Autoionization and pH
- Protons and hydroxide govern acid base chemistry.
- Any water molecule in the bulk lives about 10 hours before dissociating.
- In a liter, a water molecule dissociates every 30 microseconds.
Protons in Water
- Structure of \(H^+\) in water and the extent to which the excess charge is delocalized is still unresolved. It is associated strongly enough to describe as covalently interacting, but its time evolution is so rapid (<1 ps) that it is difficult to define a structure.
- Much higher mobility than expected by diffusion of a cation of similar size.
- Explained by Grotthus mechanism for transfer of proton to neighboring water molecules.
- OH– is also very mobile and acts as a proton acceptor from water.
Water Physical Properties
