Test #7 Review - Heat and Gas Laws

Object A at 40ºC and object B at 80ºC are placed in contact with each other. Which statement describes the heat flow between the objects?

• Heat flows from object A to object B

• Heat flows from object B to object A

• Heat flows in both directions between the objects

• No heat flow occurs between the objects

Answer explanation

Recall that heat energy (thermal energy) always flows from high to low temperature.

Object A = 40^oC and Object B = 80^oC

Heat flows from HOT to COLD

Therefore, heat will flow from Object B to Object A.

What is the amount of heat absorbed when the temperature of 75 grams of water increases from 20^oC to 35^oC?

• 1,100 J
• 4,700 J
• 6,300 J
• 11,000 J

Answer explanation

Look at the heat equations on the back of the reference table. Since this problem gives us 2 different temperatures, we need to use the equation q=mcΔT.

q=mcΔT

m= 75 g

c= 4.18 (Reference Table B)

ΔT = 35-20 = 15

q =75 x 4.18 x 15

q = 4,700 J

What is the total amount of heat required to completely melt 347 grams of ice at its melting point?

• 334 J
• 1450 J
• 116,000 J
• 784,000 J

Answer explanation

Look at the heat equations on the back of the reference table. Since this problem tell us that a substance is melting, we need to use the heat of fusion equation q = mH_f

q=mH_f

m = 347 g

H_f = 334 (Reference Table B)

q= 347 x 334

q = 116,000 J

Refer to the image of a heating curve. Which statement describes the energy of the particles in this sample during interval DE?

• Both potential energy and average kinetic energy increase

• Both potential energy and average kinetic energy decrease

• Potential energy increases and average kinetic energy remains the same

• Potential energy remains the same and average kinetic energy increases

Answer explanation

Recall how kinetic and potential energy changes throughout a heating curve.

Kinetic energy increases as temperature increases. (When the lines are going up)

Potential energy increases during phase changes in which particles spread apart such as Solid --> Liquid and Liquid --> Gas (Flat Lines)

Interval DE is a flat line which means temperature and kinetic energy remains constant.

Interval DE represents boiling in which liquid changes into a gas. Therefore, the potential energy increases.

Refer to the image of a heating curve. Identify the amount of time that the substance exists in the liquid phase, only.

• 10 min

• 20 min

• 30 min

• 60 min

Answer explanation

Recall the parts of a heating curve.

Line 1 (0-10 min) = Solid

Line 2 (10-30 min) = Melting (S-->L)

Line 3 = (30-60 min) = Liquid

Line 4 = (60-120 min) = Boiling (L-->G)

Line 5= 120 min + = Gas

In this curve, the substance is a liquid from 30-60 min which is a period of 30 minutes.

Under which conditions of temperature and pressure does a real gas behave most like an ideal gas?

• low temperature and low pressure
• low temperature and high pressure
• high temperautre and low pressure
• high temperautre and high pressure

Answer explanation

Recall that an ideal gas is a gas that follows kinetic molecular theory perfectly. This occurs when the gas particles are moving really fast and are far apart from each other.

A gas behaves most ideal under conditions of high temperature and low pressure.

HoT LiPs (High Temp and Low Pressure)

A sample of gas occupies a volume of 50.0 milliliters in a cylinder with a movable piston. The pressure of the sample is 0.90 atmosphere and the temperature is 298 K. What is the volume of the sample at STP?

• 41 mL

• 49 mL

• 51 mL

• 55 mL

Answer explanation

Use the combined gas law equation on the back of the reference table.

P₁V₁ = P₂V₂

T₁ T₂

P₁ = 0.9 atm P₂ = 1.0 atm (STP Table A)

V₁ = 50 mL V₂ = ?

T₁ = 298 K T₂ = 273 K (STP Table A)

Solve for V₂

(0.9 x 50) = (1.0 x V₂)

298 273

45 = V₂ 298V₂ = 12,285

298 273 V₂ = 41 mL

A sample of gas is in a rigid cylinder with a movable piston. The pressure of the gas is kept constant. If the Kelvin temperature of the gas is doubled, the volume of the gas is

• halved
• doubled
• tripled
• unchanged

Answer explanation

Recall the 3 gas law relationships:

As Temp increases, volume increases (Direct)

As Pressure increases, volume decreases (Inverse)

As Temp increases, pressure increases (Direct)

Therefore, if the temperature is doubled, the volume will also double (Direct).

A rigid cylinder with a moveable piston contains 50.0 L of gas at 303 K and constant pressure. What is the volume of the gas in the cylinder at STP?

• 5.49 L
• 55.5 L
• 45.0 L
• 455 L

Answer explanation

Use the combined gas law equation on the back of the reference table.

P₁V₁/T₁ = P₂V₂/T₂

Since the pressure is constant, we can remove pressure from the equation.

The equation becomes:

V₁/T₁ = V₂/T₂

V₁ = 50 L T₁ = 303 K

V₂ = ? T₂ = 273 K (STP Table A)

Solve for V₂

50/300 = V₂/273

300V₂ = 13,650

V₂ = 45 L

The temperature 30 K expressed in degree Celsius is

• 243^oC

• -243^oC

• 303^oC

• -303^oC

Answer explanation

Use the temperature equation on the back of the reference table.

K = ^oC + 273

30 K = ^oC + 273

^oC = 30 K - 273

^oC = -243