HL CHEMISTRY

HL1-8.ppt

Gas Laws

Review – Avogadro’s Law

Avogadro’s Law: Equal volumes of gases at the same T & P contain the same number of molecules.

(which means that coefficients in a balanced equation can be ‘read’ as volumes of gases)

Review – Molar Volume1 mole of any gas takes up 22.4 dm3 of space at STP

Standard Temperature (273K or 0oC)& Pressure (101 kPa or 1 atm)

VSTP = 22.4 dm3 / mol

Which can be rearranged…

n = VSTP / 22.4 dm3 mol

Review – Molar Volume1 mole of any gas takes up 22.4 dm3 of space at STP

Standard Temperature (273K or 0oC)& Pressure (101 kPa or 1 atm)

VSTP = 22.4 dm3 / mol

Which can be rearranged…

n = VSTP / 22.4 dm3 mol

Other applications of Avagadro’s law &/or Molar Volume:1. Determine number of moles (or g) of a given

volume of gas at STP. (1.4.2 # 3, part of 5, part of 7)

2. Determine volume of a known quantity (mol or g) of a gas at STP (1.4.2 # 4, part of # 10)

3. Calculate molar mass given information about moles (or g) and volume… and knowing molar volume (1.4.2 # 8)

4. Determine density of a gas at STP (1.4.2 # 9)

The Ideal Gas Law Q – Can all gases be considered ‘ideal’? Most real gases act ideally under normal

conditions. They tend to act less ideally when:

They have high molar masses Why?

They are at very high pressures Why?

They are at very low temperatures Why?

Which of these gases would act the least ideally?

The Ideal Gas Law

PV=nRT whereP = pressure in kPaV = volume in dm3 (Litres)n = moles of gasR = universal gas constant

○ (8.314 kPa dm3 mol-1 K-1)○ (0.0821 if you’re using atm, which you won’t in

IB…)T = temperature in K

Pressure -

You want pressure in kPa…What do you do if it is given in mm Hg?

What do you do if it is given in atm?

The Ideal Gas Law As long as you can remember PV=nRT,

you can derive all of the other gas laws. If the number of moles of gas is not

changing, we can just solve for n.n=PV/RT

Since the moles aren’t changing, we can set the right side of the equation equal to itself.P1V1/RT1= P2V2/RT2 and because R is the

same…P1V1/T1= P2V2/T2 (combined gas law)

If you have a problem where one of these variables is held constant, you can simplify the combined gas law by eliminating that variable:

P1 V1 / T1 = P2 V2 / T2

P1 V1 = P2 V2

V1 / T1 = V2 / T2

P1 / T1 = P2 / T2

We now have this collection of gas laws:

P1V1/T1=P2V2/T2 (Combined Gas Law)

P1V1=P2V2 (Boyle’s Law)

V1/T1=V2/T2 (Charles’ Law)

P1/T1=P2/T2 (Gay Lussac’s Law)

Gas Laws TIP #1: Always use K for temperatures because it is an “absolute” scale (no negatives)T

Gas Laws TIP #2: If P or V are on both sides, the units don’t matter as long as they are the same on both sides.

Gas Laws TIP #3: You must be VERY careful with your units when using the Ideal Gas Law!

Your Assignment:

Read pp. 21-26 Do Ex 1.4.3

on pp. 25-26# 1-10 Identify which gas law you are using to solve

the problem… then solve the problem . Show your work…. (at minimum include the set up using some form of a gas law)

Charles Law

Charles's law is an experimental gas law which describes how gases tend to expand when heated. It was first published by French natural philosopher Joseph Louis Gay-Lussac in 1802, although he credits the discovery to unpublished work from the 1780s by Jacques Charles.

Charles Law

T V The following is a more useful way of

expressing this law.

Create a hypothesis for what happened here.

Here’s what really happened.1. Workers used hot pressurized water to

clean the tanker car.

2. Tank was drained.

3. Door was closed and tanker was left overnight.

4. Temperature inside the car plummeted overnight.

5. V/T=V/T (Temp goes down, Volume must go down also)