Download Name ………………………………………………… Unit 7: States of

Name …………………………………………………
Period …….
Unit 7: States of Matter,
Thermochemistry,
& Gas Laws
KEY - More Practice with Regents Questions
(–)
1. Which equation represents an exothermic
reaction at 298 K?
TABLE I !
(1) N2(g) + O2(g)  2NO(g) (+)
(2) C(s) + O2(g)  CO2(g)(–)
(3) KNO3(s)  K+(aq) + (NO3)– (aq) (+)
(4) NH4Cl(s)  (NH4)+(aq) + Cl–(aq) (+)
2. Which 10-milliliter sample of water has the
greatest degree of disorder? Disorder = ENTROPY
ENTROPY increases as we go from s  l  g
(1) H2O(g) at 120°C
(2) H2O(l) at 80°C
(3) H2O(l) at 20°C
(4) H2O(s) at 0°C
The graph below represents the uniform heating
of a substance, starting with the substance as
a solid below its melting point.
4. The average kinetic energy of water
molecules is greatest in which of these
samples? Which one has the greatest temperature?
(1) 10 g of water at 35°C
(2) 10 g of water at 55°C
(3) 100 g of water at 25°C
(4) 100 g of water at 45°C
5. Using your knowledge of chemistry and the
information in Reference Table H, which
statement concerning propanone and water
at 50°C is true?
(1) Propanone has a higher vapor pressure
and stronger intermolecular forces than
water.
(2) Propanone has a higher vapor pressure
and weaker intermolecular forces than
water.
(3) Propanone has a lower vapor pressure
and stronger intermolecular forces than
water.
(4) Propanone has a lower vapor pressure
and weaker intermolecular forces than
water.
3. Which line segment represents an increase in
potential energy and no change in average
kinetic energy? No change in temperature…must be a PLATEAU
Naphthalene (aka “moth balls”), a nonpolar substance that sublimes at room temperature, can be used to
protect wool clothing from being eaten by moths.
Solid  Gas
6. Explain, in terms of intermolecular forces, why naphthalene sublimes. [1]
Napthalene has weak intermolecular forces, which allows it to go directly from a solid to a gas.
7. The empirical formula for naphthalene is C5H4 and the molecular mass of naphthalene is 128
grams/mole. What is the molecular formula for naphthalene? [1]
(1) EFM:
(2) MM
EFM
C 5 x 12 g = 60 g
H 4x 1g= 4g
64 g
= 128 g = 2
64 g
(3) (C5H4) x 2
= C10H8
The table below lists physical and chemical properties of six elements at standard pressure that
correspond to known elements on the Periodic Table. The elements are identified by the code letters, D,
E, G, J, L, and Q.
8. What is the total number of elements in the “Properties of Six Elements at Standard Pressure” table
that are solids at STP? [1]
Standard Temperature (on 1st page of Reference Tables) is 0°C or 273K
Anything that is a SOLID at 0°C will have a melting point (go from solid to liquid) ABOVE 0°C
The total number of elements in the table above that have melting points above 0°C is 4
Letter Z corresponds to an element on the Periodic Table other than the six listed elements. Elements
G, Q, L, and Z are in the same group on the Periodic Table, as shown in the diagram below.
9. Based on the trend in the melting points for elements G, Q, and L listed in the “Properties of Six
Elements at Standard Pressure” table, estimate the melting point of element Z, in degrees Celsius. [1]
According to the table above, elements G, Q, and L have melting points of 181°C, 98°C, and 64°C.
The trend is one of DECREASING TEMPERATURE.
Element Z must have a melting point below 64°C.
Crude oil is a mixture of many hydrocarbons that have different numbers of carbon atoms. The use of a
fractionating tower allows the separation of this mixture based on the boiling points of the hydrocarbons.
To begin the separation process, the crude oil is heated to about 400°C in a furnace, causing many of the
hydrocarbons of the crude oil to vaporize. The vaporized mixture is pumped into a fractionating tower
that is usually more than 30 meters tall. The temperature of the tower is highest at the bottom. As
vaporized samples of hydrocarbons travel up the tower, they cool and condense. The liquid hydrocarbons
are collected on trays and removed from the tower. The diagram below illustrates the fractional
distillation of the crude oil and the temperature ranges in which the different hydrocarbons condense.
10. State the trend between the boiling point of the hydrocarbons contained in the crude oil and the
number of carbon atoms in these molecules. [1]

OR

As the number of carbon atoms increases, the boiling point increases. (Bigger molecules will have higher
boiling points)
As the number of carbon atoms decreases, the boiling point decreases. (Smaller molecules will have lower
boiling points)
11. Describe
 the relationship between the strength of the intermolecular forces and the number of carbon
atoms in the different hydrocarbon molecules. [1]

OR

As the number of carbon atoms increases, (the boiling point increases, therefore) the strength of
intermolecular forces increases.
As the number of carbon atoms decreases, (the boiling point decreases, therefore) the strength of
intermolecular forces decreases.
12. A student is holding a test tube containing 5.0 milliliters of water. When a sample of NH4Cl(s) is
placed in the test tube, the test tube feels colder to the student’s hand. Describe the direction of heat
flow between the test tube and the hand. [1]
Heat flows from objects at higher temperatures to objects at lower temperatures.
Heat will flow from the hand to the test tube.
means “evaporates easily”
Ethanol, C2H5OH, is a volatile and flammable liquid with a distinct odor at room temperature. Ethanol is
soluble in water. The boiling point of ethanol is 78.2°C at 1 atmosphere. Ethanol can be used as a fuel to
produce heat energy, as shown by the balanced equation below.
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l) + 1367 kJ
13. At 1 atmosphere, compare the boiling point of pure ethanol to the boiling point of water. [1]
You can look on the 1st page of the Reference Tables if you don’t remember that the boiling point
of water is 100°C.
The boiling point of ethanol is lower. OR The boiling point of water is higher.
14. Is the above reaction endothermic or exothermic? [1] Energy is released – exothermic
15. Identify one physical property of ethanol, stated in the passage above, that can be explained in terms
of chemical bonds and intermolecular forces. [1]
boiling point
A substance is a solid at 15°C. A student heated a sample of the solid substance and recorded the
temperature at one-minute intervals in the data table below.
Plateau, phase change (melting)
16. Based on the data table, what is the melting point of this substance? [1]
53 °C
17. What is the evidence that the average kinetic energy of the particles of this substance is increasing
during the first three minutes? [1]
The temperature is increasing.
18. The heat of fusion for this substance is 122 joules per gram. How many joules of heat are needed to
melt 7.50 grams of this substance at its melting point? [1]
No temperature change, so use q = mHf
q=?
m = 7.50 g
Hf = 122 J/g
q = mHf
= 7.50 g 122 J
g
= 915 J
More practice!
During interval AB, what is the total amount of heat absorbed by a 5.00 g sample of the substance? The
specific heat capacity is 3.23 J/ (gK).
There is a temperature change, so use q = mC∆T
q=?
q = mC∆T
m = 5.00 g
C = 3.23 J/ (gK)
= 5.00 g 3.23 J
∆T = 250 K – 200 K = 50
gK
= 808 J
50 K