Gases Mr. Chan Northwestern University To insert your company logo on this slide From the Insert Menu Select “Picture” Locate your logo file Click OK To resize the logo Click anywhere inside the logo. The boxes that appear outside the logo are known as “resize handles.” Use these to resize the object. If you hold down the shift key before using the resize handles, you will maintain the proportions of the object you wish to resize.

Describing Properties of Gases Recall Kinetic Theory for Gases – Recall Gas simulator – Large relative distance between molecules – No attractive or repulsive forces exist – Constant random motion, straight line path – Average kinetic energy of particles directly proportional to Kelvin temperature of gas

Factors that affect gas pressure Amount of gas – Increase in gas, increase in gas pressure – Direct proportion Explain with KMT – Increasing number of gas particles, number of collisions, pressure increases – Sealed container example

Factors that affect gas pressure Volume – Decrease in volume, Increase in pressure – Inverse proportion Explain with KMT – Decrease in volume, less room for particles to move, more collisions with sides, increase pressure

Factors that affect gas pressure Temperature – Increase temperature, increase pressure – Direct proportion Explain with KMT – Increase temp, particles move faster, more collisions, increase pressure

Gas Laws – Boyle’s Law Pressure-Volume Relationship – Fixed temperature and amount of gas Mathematical Expression – P1 X V1 = P2 X V2 – Inverse proportion Animation Lee Marek – 55 gallon drum Practice 10-11

Gas Laws – Charles’s Law Temperature-Volume Relationship – Fixed pressure and amount of gas Mathematical Expression – V1 / T1 = V2 / T2 – Direct proportion Animation Practice 12-13

Gas Laws – Gay-Lussac’s Law Temperature-Pressure Relationship – Fixed volume and amount of gas Mathematical Expression – P1 / T1 = P2 / T2 – Direct proportion Animation Practice 14-15

Combined Gas Law Put all three gas laws together – Try to write equation (P1 X V1)/T1 = (P2 X V2)/T2 Also, taking each case separately + thinking about gas law Practice 16-17

Using the Ideal Gas Law Determining any of the variables associated with gases Derived from combined gas law PV/Tn = constant (R) – R = 8.31 L kPA / mol K – R = L atm / mol K P x V = n x R x T – PivNert! Practice 22-25

Differences between Ideal and Real Gases No gas behaves ideally at all temperatures and pressures Where are the exceptions? Gases vs. Liquids vs. Solids – Attractive forces exist in liquids/solids (intermolecular forces) – Volume As pressure increases, volume decreases Particles have real volume

Deviations from Ideal Gases

Avogadro’s Hypothesis Equal volumes of gases at the same temp and pressure contain equal numbers of particles. – Easier to imagine since gas particles are so far apart Calculating volumes of gases at STP

Dalton’s Law of Partial Pressures Partial Pressure – Each gas makes a contribution to the total pressure of a mixture Dalton’s Law – Total pressure equals sum of the partial pressures Practice 37-38

Graham’s Law of Effusion Diffusion – Molecules move toward areas of lower concentration until concentration is uniform throughout Effusion – Gas escapes through tiny hole in container – Gases with lower molar mass effuse faster than those with higher molar mass

Graham’s Law RateA/RateB = MolarMassB/MolarMassA Practice Elements Song

Lab – Changes of Physical State Objectives – Observe behavior of substance during melting and freezing – Graph heating and cooling curve Techniques/Notes – Freezing point determination Use hot and room temp water – Melting point determination Same Use clock or watch to estimate time Hypothesis – What happens to temperature at melting point? – If energy is still being added when going from solid to liquid, where does the energy go? Confirmation of melting points using MelTemps?