Pressure

• Define vapour pressure.Include: volatility, dynamic equilibrium • Define normal boiling point in terms of vapour

pressure. • Interpolate and extrapolate information from

pressure versus temperature graphs.

Additional KEY TermsManometer

Pressure is defined as force per unit area.

Air is a gas...it must also be exerting pressure.

Called air pressure or atmospheric pressure.

The standard unit of atmospheric pressure (at sea level):

1 atmosphere = 760 mmHg = 101.3 kPa kilopascalsmillimeters of Mercury

Closed system (constant temperature)

As liquid continues to vaporize, a point is reached:

For every molecule that vapourizes, one condenses

When vapourization = condensation we have reached equilibrium.

dynamicequilibrium

The pressure of a gas at equilibrium with the liquid in a closed system is known as the vapour pressure.

characteristic physical property

A manometer measures vapour pressure.

• Compares vapour pressure to air pressure. • Measures difference in height on both sides.

IMFs determine the rate of vapourization.

Weak IMFs: more particles vapourized - higher vapour pressure – more volatile.

Heat increases particles kinetic energy - breaks IMFs.

Increases rate of vapourization - pressure increases.

Boiling Point:When vapour pressure is equal to atmospheric pressure.

Particles must have:• Energy to break IMFs• Energy to push out of the liquid

and through the air.

(1 atmosphere, 101.3 kPa, 760 mmHg)

Any change in air pressure will produce a change in the boiling

point.

Particles with strong IMFs require more energy (higher temperatures) to phase change to a gas with enough pressure to equal atmospheric pressure.

Longer cooking time to prepare foods at higher altitudes.

At lower pressure the water boils at a lower temperature – longer cooking time.

• Standard air pressure is 101.3 kPa.

Boiling Temp

• Boil: when vapour pressure = air pressure

CAN YOU / HAVE YOU?

• Define vapour pressure.Include: volatility, dynamic equilibrium • Define normal boiling point in terms of vapour

pressure. • Interpolate and extrapolate information from

pressure versus temperature graphs.

Additional KEY TermsManometer