Laws versus Theories - PDF Free Download

Announcements Text HW due tomorrow (Friday) Online HW #3 (Type 1) due Monday, September 17 by 7:00 p.m. Online HW #3 (Type 2) due Wednesday, September 19 by 7:00 p.m. Lab write-up for Gases Lab due Wednesday, September 19 at the start of lab

Laws versus Theories Laws: Tell us what s true Observations Facts Theories: Tells us why (gives us an explanation) a MODEL (not reality) We like our theories to be simple

Gas Laws Think about what you know is true about gases Let s make some observations of gases and their behavior

Gas Laws First observation: gases take up the volume of its container (has no definite shape or volume) Fill a balloon with gas. What is the shape of the gas? Fill a soda bottle with gas. What is the shape of the gas?

Gas Laws Second observation: gases mix well (diffusion) Constant random motion and collisions Water vapor mixes with air

Gas Laws Third observation: gases exert pressure Collisions of gas particles with walls of its container Why does a balloon stay inflated? Because the gases inside are pushing on the walls of the balloon. A barometer works because of atmospheric pressure

Gas Laws So what does this tell us? For gases, we need to look at: Pressure (P) Volume (V) Temperature (T) Moles (n) All of these factors matter with gases; these aren t as important when talking about liquids and solids

V versus n Volume versus moles of gas (V vs. n) Take two balloons Inflate them to different sizes I blew more CO 2 into the larger balloon, so the volume got bigger So as the number of moles increase, the volume increases Temperature is constant; but what about pressure?

Clicker #1 Is the pressure in the larger balloon greater than, less than, or equal to the smaller balloon? A) greater than the smaller balloon B) less than the smaller balloon C) equal to the smaller balloon

V versus n The pressure is equal because it has to be about the same as atmospheric pressure, otherwise it would expand more or deflate n, V (at constant T and P)

P versus n Pressure versus moles of gas (P vs. n) Carbon dioxide can sublimate: CO 2 (s) CO 2 (g) Place solid CO 2 in bag What happens as solid CO 2 becomes gaseous CO 2? The bag inflates The number of moles of gaseous CO 2 increases But what about the pressure?

Clicker #2 As the number of moles of carbon dioxide increase, the pressure in the bag will A) increase B) decrease C) stay the same

P versus n So since the T and V (of the bag) are constant, as the number of moles of CO 2 increase, the pressure in bag will increase n, P (at constant T and V)

P versus V Pressure versus volume (P vs. V) Syringe If I push the plunger on a syringe, the volume inside decreases But what if I block the tip of the syringe and push? Is it easier or harder to push the plunger down? What does this mean about the pressure?

Clicker #3 If the volume of gas inside the syringe decreases, the pressure will A) increase B) decrease C) stay the same

P versus V So as the volume of a container decreases, the pressure inside the container increases V, P (at constant T and n)

V versus T Volume versus temperature (V vs. T) Liquid nitrogen boils at -196 C Air in balloon is at room temperature (25 C) What will happen to the temperature of the gas inside the balloon if it is submerged in liquid nitrogen?

Clicker #4 If the temperature of the gas inside the balloon decreases, the volume will A) increase B) decrease C) stay the same

V versus T As the temperature of a gas decreases, the volume also decreases T, V (constant n and P)

P versus T Pressure versus temperature (P vs. T) Aerosol can filled with gas Can is sealed so volume is constant What would happen if I heated the sealed aerosol can?

Clicker #5 If the temperature of the gas inside the can increases, the pressure will A) increase B) decrease C) stay the same

P versus T As the temperature of a gas increases, the pressure also increases T, P (constant V and n)

P, V, n, T relationships We can also quantify these relationships (think about these, don t memorize!)

P, V, n, T relationships How are V and n related? Directly related

P, V, n, T relationships How are P and n related? Directly related

P, V, n, T relationships How are P and V related? Indirect and inverse relationship Note: cannot go to zero because gas will always have some volume and pressure!

P, V, n, T relationships How are P and T related? Directly related

P, V, n, T relationships How are V and T related? Directly related

V versus T But if T=0 C or 0 F no temperature? No! T can be lower than this! If you lower the temperature enough, gas will become a liquid so you have to extrapolate Shift to Kelvin scale: K = C + 273 Absolute scale

P, V, n and T To summarize: V is directly proportional to n V is directly proportional to T P is directly proportional to n P is directly proportional to T P is inversely proportional to V P is proportional to 1/V V is proportional to 1/P

Ideal Gas Law PV is directly proportional to nt Turns out that PV/nT = R and for any gas that is the same PV = nrt If P is in atm, V in L, n in mol, T in K, then R = 0.08206 L atm/mol K All of the common sense things you know about gases is in this law!

V versus T and Energy Temperature is related to the kinetic energy of the gas molecules (measures motion of the gas molecules) At 0 Kelvin (absolute zero) there would be no energy in the system

V versus T and Energy If I increase the temperature of a gas from 20 C to 40 C, have I doubled the kinetic energy? No! But why not? Celsius is not an absolute scale (0 C is not the lowest possible temperature)

V, T, and KE The kinetic energy does not double because the temperature has not really doubled! The difference between -273 C and 20 C is not the same as the difference between 20 C and 40 C

V, T, and KE But what if the temperature changed from 100K to 200K? Does the kinetic energy double?

V, T, and KE What if you compare the kinetic energy of the change between 0 C to 20 C and 0 C to 40 C?