Topic 3: Energy

Write the equation from the reference tables with units on every value, and say where the energy goes, into or out of the system.

A lesson for this topic

Quick checks: multiple choice

Question 1

An instant cold pack holds solid ammonium nitrate, NH4NO3, and a pouch of water. When the pouch breaks, the solid dissolves and the pack becomes cold enough to soothe a sprained ankle. Which statement describes the energy change?

Question 2

A student warms 250.0 grams of water on a hot plate from 18.0 °C to 42.0 °C. The specific heat capacity of water is 4.18 J/(g·°C). How much heat did the water absorb? Use the reference tables.

Question 3

Specific heat capacities of four metals
Metal	Specific heat capacity (J/(g·°C))
aluminum	0.897
iron	0.449
copper	0.385
lead	0.129

To identify a metal, a student heats a 60.0 gram sample to 99.0 °C in boiling water and drops it into 120.0 grams of water at 21.0 °C in an insulated foam cup. The metal and the water both end at 24.4 °C. Assume all the heat lost by the metal is gained by the water. The specific heat capacity of water is 4.18 J/(g·°C). Using the table, which metal is the sample most likely to be?

Question 4

A 25.0 gram ice cube at 0.0 °C is dropped into a glass of lemonade. How much heat must the ice absorb to melt completely and then warm, as liquid water, to 10.0 °C? The heat of fusion of water is 334 J/g, and the specific heat capacity of liquid water is 4.18 J/(g·°C). Use the reference tables.

Question 5

A hot plate supplies energy at a steady rate to a beaker of crushed ice that starts at −20 °C. The graph shows the temperature of the sample. Between minutes 1 and 9 the temperature stays at 0 °C. What happens to the energy the hot plate supplies during those minutes?

Question 6

An electric kettle that uses 1500 watts runs for 120 seconds and heats 500.0 grams of water from 20.0 °C to 95.0 °C. The electrical energy used equals power times time, and 1 watt is 1 joule per second. The specific heat capacity of water is 4.18 J/(g·°C). What percentage of the electrical energy ends up in the water?

Question 7

In a Buffalo high school lab, 100.0 grams of water at 80.0 °C is poured into 300.0 grams of water at 20.0 °C in an insulated container. No energy enters or leaves the container. Which final temperature and reason are correct?

Question 8

A disposable hand warmer contains iron powder that reacts slowly with oxygen from the air: 4Fe + 3O2 -> 2Fe2O3. The packet keeps a skier's hands warm for several hours at a resort in the Catskills. Which statement describes the energy in this system?

Question 9

A student drops a hot brass cylinder into water in a glass beaker. From her measurements she calculates that the brass lost 2150 J of heat and the water gained 1980 J. Which explanation of the 170 J difference fits the law of conservation of energy?

Question 10

In a computer model, two oppositely charged ions are held 0.40 nm apart. The model then moves them to 0.80 nm apart without changing either charge. Compared with the first position, the electrostatic force between them is now

Question 11

Magnesium oxide, MgO, melts at about 2800 °C, while sodium chloride, NaCl, melts at about 800 °C. Both are ionic solids. Which explanation is best supported by a model of charged particles?

Question 12

Removing the outermost electron from a potassium atom takes less energy than removing the outermost electron from a sodium atom. Which explanation fits a model of the electron attracted to the nucleus?

Question 13

A sodium ion, Na+, and a chloride ion, Cl-, start far apart in a gas and move toward each other until they form an ion pair. Which statement describes the energy of the two-ion system as the distance between them shrinks?

Question 14

Ion pairs in a student's computer model
Ion pair	Charges	Distance in the model (nm)
Li+ and F-	1+ and 1-	0.2
Na+ and Cl-	1+ and 1-	0.28
Mg2+ and O2-	2+ and 2-	0.4
Ca2+ and S2-	2+ and 2-	0.5

A student builds a computer model of four ion pairs. The model uses the charges shown and distances she chose herself (they are not the real distances in these compounds). In her model, which two pairs attract each other with the same force?

Question 15

A cylinder of carbon dioxide is cooled to a low temperature at a moderately high pressure. Its measured volume is smaller than the combined gas law predicts. Which model of the molecules explains the difference?

Heating water with a candle

Students in Syracuse burn a paraffin candle under a steel can that holds 200.0 grams of water. The water warms from 19.0 °C to 34.0 °C, and the candle's mass falls by 1.25 grams. They assume that all the energy released by the burning wax goes into the water. The specific heat capacity of water is 4.18 J/(g·°C).

Question 16

Using the students' data and assumption, how much energy is released per gram of wax burned? Use the reference tables.

Question 17

A data book gives a much larger value for the energy released per gram of paraffin wax than the students found. Which change to their procedure would most likely bring their result closer to the data book's value?

Question clusters

A reusable hand warmerQuestion cluster · 5 credits

A company in Rochester makes a reusable hand warmer: a sealed pouch of sodium acetate solution with a small metal disc inside. Clicking the disc starts crystals forming, and the pouch warms. Boiling a used pouch in water for ten minutes turns the crystals back into a solution, ready to use again. To test one pouch, students clicked it and at once sealed it in an insulated box with 300.0 grams of water (table). The specific heat capacity of water is 4.18 J/(g·°C).

Temperature of 300.0 g of water in an insulated box with a clicked pouch
Time (minutes)	Water temperature (°C)
0	20
5	26.2
10	29.8
15	31.5
20	31.5

(a) Calculate the energy, in kilojoules, that the water absorbed from the pouch. [1]

(b) Which statement describes the energy flow while the crystals were forming? [1]

(d) Explain, in terms of energy, why a used pouch must be boiled before it can warm hands again. [1]

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(e) Identify one reason why the energy the water absorbed could be less than the energy the crystallizing pouch released. [1]

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Charges, distances and melting pointsQuestion cluster · 4 credits

Students compared four ionic solids. In each solid, every ion is surrounded by ions of the opposite charge. The table gives the charges of the ions, the distance between the centers of neighboring ions, and the approximate melting point of each solid. The force between two ions is proportional to q1q2/d², where q1 and q2 are the charges and d is the distance between them.

Four ionic solids
Solid	Ions	Distance between ion centers (nm)	Approximate melting point (°C)
NaCl	Na+ and Cl-	0.28	801
KCl	K+ and Cl-	0.31	771
MgO	Mg2+ and O2-	0.21	2825
CaO	Ca2+ and O2-	0.24	2613

(a) Which two solids best show how the distance between ions affects the melting point when the charges stay the same? [1]

(b) Use the relationship F ∝ q1q2/d² and the table to calculate how many times larger the force between neighboring Mg2+ and O2- ions is than the force between neighboring Na+ and Cl- ions. [1]

times

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(d) The ions in KCl carry the same charges as the ions in NaCl. Explain why KCl melts at a lower temperature. [1]

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