Unit Two: Energy

Question 1

To break a chemical bond, energy is ______________

Answer 1

To break a chemical bond, energy is used.

Question 2

To form a chemical bond, energy is ______________

Answer 2

To form a chemical bond, energy is released.

Question 3

Potential energy

Answer 3

Energy that is stored in chemical bonds (Chemistry definition).

Question 4

Kinetic energy

Answer 4

energy that a body possesses by virtue of being in motion.

Question 5

Which of the following has the highest average kinetic energy: A. 20 g of CO2 gas molecules at 100K B. 10 g of CO2 gas molecules at 200K C. 5 g of CO2 gas molecules at 100 deg C D. 2.5 g of CO2 gas molecules at 200 deg C

Answer 5

Which of the following has the highest average kinetic energy: D. 2.5 g of CO2 gas molecules at 200 deg C

Question 6

Distinguish between temperature and heat.

Answer 6

Temperature is the average kinetic energy of the molecules of a substance while heat is the total kinetic energy of all its molecules.

The hotter an object is, the faster the motion of the molecules inside it. Thus, the heat of an object is the total energy of all the molecular motion inside that object. Temperature, on the other hand, is a measure of the average heat or thermal energy of the molecules in a substance. From Caltech.

Question 7

Define vapor pressure

Answer 7

The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container.

Question 8

What happens to the vapor pressure of a liquid when the temperature increases and why?

Answer 8

When temperature increases, vapor pressure increases.

Increasing the temperature increases the average energy of the particles present. That means that more of them are likely to have enough energy to escape from the surface of the liquid. That will increase the vapour pressure.

Question 9

Define boiling point using vapor pressure.

Answer 9

The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

(Water will boil at a lower temperature on Mars or on Everest).

Question 10

What is the normal boiling point?

Answer 10

Normal boiling point: the temperature at which a liquid’s vapor pressure equals one atmosphere.

Question 11

What are the two formulae for calculating heat changes of a substance?

Answer 11

Q=mcΔT

Q=mH (For phase change)

Question 12

What is a calorimeter?

Answer 12

an apparatus for measuring the amount of heat involved in a chemical reaction or other process.

Question 13

What physical law is relevant to the calorimeter?

Answer 13

The law of Conservation of Energy

Question 14

List the six phase transitions; state the states of matter involved and indicate if the process is exothermic or endothermic.

Answer 14

1. Liquid -> Gas: Vaporization, Endothermic
2. Gas -> Liquid: Condensation, Exothermic
3. Liquid -> Solid: Freezing/solidification, Exothermic
4. Solid -> Liquid: Melting/fusion; Endothermic
5. Solid -> Gas: Sublimation, Endothermic
6. Gas –> Solid: Deposition, Exothermic

Question 15

Latent heat

Answer 15

The energy that is used to change the phases of a substance.

Question 16

Why does it take longer for the water ocean to warm up relative to the air, in summer? And why does it take longer for it to cool down in the fall?

Answer 16

The specific heat of water in the ocean is much higher than air; also the mass of the ocean water is likely much more.

Question 17

Calculate the final temperature of 34.2 g of mercury after it 0.184 Cal of energy was released(?). Initial temperature: 54 degrees C. c_p mercury = 0.45 J/g/deg C.

Answer 17

0. 184C = 4.8*1000*0.184 J = 883.2 J
1. 2 = 34.2*0.45*(delta_t)

delta_t =

Question 18

Enthalpy

Answer 18

Enthalpy is a defined thermodynamic potential, designated by the letter “H”, that consists of the internal energy of the system (U) plus the product of pressure (p) and volume (V) of the system.

The total enthalpy, H, of a system cannot be measured directly. The same situation exists in classical mechanics: only a change or difference in energy carries physical meaning. Enthalpy itself is a thermodynamic potential, so in order to measure the enthalpy of a system, we must refer to a defined reference point; therefore what we measure is the change in enthalpy, ΔH. The change ΔH is positive inendothermic reactions, and negative in heat-releasing exothermic processes.

Question 19

What is the algorithm fo calculating the total heat absorbed or relased from heating/cooling curves?

Answer 19

1. Divide the graph into stages Q₁.. Q₂.. Q_nwhere each Q corresponds to a part of the graph.
2. For a full phase change graph there are 5 Q_5.
3. Answer is ΣQ_n
4. Sanity check at each step.

Question 20

Given a phase diagram, how do you find out the normal melting point, triple point and critical point?

Answer 20

1. Find the y value corresponding to 1 atmosphere.
2. Draw a horizontal line from that y value to the melting point line: that is the melting point.
3. The point at which all three phase lines interesect is the triple point.
4. There will be a certain value of x for which y increases very sharply.

Question 21

What do you need to watch out for when doing sig. fig. calculations?

Answer 21

1. Check that the addition rules have been applied to the additions.
2. Check that the multiplication rules have been added to the multiplications.
3. Sanity check.