Heat Energy and Phase Changes

The heat of fusion for water is  $\Delta H_{fus}=335\ \frac{J}{g}$ . Which of the following explains what this value is? 

• It takes the addition of 335J of energy to boil a gram of water.

• It takes the addition of 335J of energy to melt a gram of water.

• It takes the removal of 335J of energy to boil a gram of water.

• It takes the removal of 335J of energy to melt a gram of water.

A 1 mol sample of solid Li is heated at constant pressure until the pressure starts to melt.

Which of the following is an accurate description of the sample after melting has started?

• The temperature of the sample remains constant as the sample absorbs heat energy.

• The energy of the sample remains constant as the temperature of the sample increases.

• The temperature of the sample increases as the energy of the sample increases.

• Both temperature and energy of the sample remain constant.

Tags

NGSS.HS-PS3-4

The enthalpy of vaporization of Ca is 153.6 kJ/mol under 1 atm of pressure.

Which of the following is an accurate description of 1 mol of liquid Ca if 153.6 kJ of energy is absorbed?

• As energy is absorbed, the Ca atoms are ionized into gaseous ions.

• The average size of the Ca atoms increases as the particles vaporize.

• The temperature of the system increases as energy is absorbed by the system.

• The energy of the system increases as the average distance between Ca atoms increases. The temperature remains constant.

Tags

NGSS.HS-PS3-2

In general, the molar heat of fusion for a substance is a smaller quality than the molar heat of vaporization for the same substance.

The BEST explanation for this observation is which of the following?

• Vaporization involves a greater change in molecular potential energy than does fusion.

• Vaporization involves a greater change in molecular kinetic energy than does fusion.

• Vaporization takes place at a higher temperature than fusion.

• The intermolecular forces (IMF) in liquids are greater than the IMF in solids.

A gas condenses to a liquid at its boiling point at constant pressure.

Which of the following BEST describes this process?

• The process is exothermic; work is done on the system.

• The process is endothermic; work is done on the system.

• The process is exothermic; work is done by the system.

• Work is done on the system, but the process takes place at a constant temperature, so it is neither exothermic nor endothermic.

The molar enthalpy needed to change liquid water to water vapor is 40.7 kJ/mol at 100oC. How much heat is gained/lost by the system when 6.00g of water vapor is condensed?

• 13.6 kJ

• -13.6 kJ

• -40.7 kJ

• 40.7 kJ

Tags

NGSS.HS-PS3-4

The internal energy of a substance will show the largest increase by which of the following?

• Transferring energy from the surroundings of the system, while the system does work on the surroundings.

• Transferring heat from the system to the surroundings, while the system does work on the surroundings.

• Transferring energy from the surroundings to the system and doing work on the system.

• Transferring heat from the system to the surroundings and doing work on the system.

Room temperature Al & H2O are left out in the hot sun. What is the expected change in Temp. of the two samples after 5 hours?
Note that:  $c_{Al}=897\ \frac{J}{kg\ K}$   $c_{H2O}=4181\ \frac{J}{kg\ K}$  

• Both will increase in temp. until they reach the same temp.

• The Al will reach a higher temp.

• The H2O will reach a higher temp.

• Both their temp. will decrease.

Tags

NGSS.HS-PS3-4

In the lab, you touch a metal faucet & a lab bench top made of polymeric composite. The metal faucet feels cooler to your hand than the bench top.

Choose the correct statement.

• Heat flows from you hand to each surface; the faucet has a higher heat capacity than the bench top.

• Heat flows to your hand from each surface; the faucet has a higher heat capacity than the bench top.

• Heat flows from your hand to each surface; the faucet has a lower heat capacity than the bench top.

• Heat flows to your hand from each surface; the faucet has a lower heat capacity than the bench top.

Tags

NGSS.HS-PS3-4

The reaction between  $CH_4$  and  $O_2$  gas to form  $CO_2$  and  $H_2O$  is found to be an exothermic process. Which of the following is TRUE?

• The products contain less stored chemical energy than the reactants.

• The products contain more stored chemical energy than the reactants.

• The products contain the same amount of stored chemical energy as the reactants.

• This cannot be determined based on the classification as endo or exothermic.

Tags

NGSS.HS-PS1-4