Recall
Is the following a physical or chemical change? A piece of iron rusts after being exposed to oxygen and moisture.
Chemical Change
A substance has particles that are closely packed but can move past one another. It has a definite volume but no definite shape. What state of matter is being described?
Liquid
Everyday Chemistry: Why does ice float on liquid water?
Solid water has a less dense structure than liquid water.
According to the kinetic molecular theory, particles of matter are constantly ________.
moving
Identification: What type of force acts between temporary dipoles caused by fluctuations in electron distribution?
London dispersion force
How many significant figures are in 0.0040500 g?
5SF
A substance changes directly from a solid into a gas without becoming a liquid. What is this process called?
Sublimation
Why does perfume sprayed on your skin eventually spread through the surrounding air?
The particles evaporate and move through the air due to their kinetic motion.
hen the temperature of a substance increases, what generally happens to the average kinetic energy of its particles?
Increase
Which molecule can form hydrogen bonds with other molecules of itself?
A. CH₄ B. H₂O C. CO₂ D. Cl₂
B. H₂O
A student obtains measurements of 25.1 g, 25.2 g, and 25.1 g. The accepted value is 30.0 g. Are the measurements precise, accurate, both, or neither? Explain.
Precise but not accurate. The values are close to one another but far from the accepted value.
TRUE OR FALSE:
If the statement is FALSE, replace the bolded word or phrase to make the statement correct.
During melting, the temperature of a pure substance continuously increases as the solid changes into a liquid.
remains constant
Why does sweating help cool your body?
Evaporation of water removes higher-energy molecules from the surface, reducing the average kinetic energy of the remaining liquid.
True or False: Gas particles are assumed to experience significant attractive forces between one another according to the ideal KMT model.
False
What is the dominant intermolecular force between two HCl molecules?
Dipole-dipole
Balancing Chemical Equations
Balance the following chemical equation:
__Al+__O2→__Al2O3
4Al+3O2→2Al2O3
Substance X and Substance Y are at the same temperature.
A student concludes:
"The particles in X have less kinetic energy because solids have less energy than gases."
Is this necessarily correct? Why or Why not?
No.
At the same temperature, particles have the same average kinetic energy, regardless of state. The major difference is in their arrangement, motion constraints, and intermolecular interactions, not simply that solid particles have "less kinetic energy."
You place equal amounts of water and rubbing alcohol in separate containers under the same conditions. The alcohol evaporates more quickly. Give a molecular-level explanation involving intermolecular forces.
The intermolecular attractions in water are stronger, particularly because of hydrogen bonding. Alcohol generally has weaker overall intermolecular attractions, so its molecules can escape into the gas phase more readily.
Two samples of the same gas are at the same temperature. Sample A has particles moving at an average speed greater than Sample B. Is this possible under the same conditions? Explain.
Not if they are the same gas at the same temperature; their average kinetic energy is determined by temperature.
What type of intermolecular attraction occurs between Na⁺ and H₂O molecules? Which part of the water molecule is attracted to Na⁺?
Ion-dipole; the partially negative oxygen side of H₂O is attracted to Na⁺
Given the balanced equation:
2H2+O2→2H2O
If 4.0 mol of H₂ reacts completely with excess O₂, how many moles of H₂O can be produced?
4.0 mol H2O
Because the mole ratio is: 2 mol H₂ : 2 mol H₂O
Therefore: 4.0 mol H₂ → 4.0 mol H₂O
A student places an ice cube on a table. After several minutes, the ice cube becomes liquid water. The student says:
"The water molecules melted because the molecules themselves became less solid."
Is the student's explanation scientifically correct? Explain what happens to the particles during melting.
No. The molecules themselves do not become "less solid." During melting, the particles gain energy, allowing them to overcome enough intermolecular attractions to move more freely while remaining chemically the same substance, H₂O.
A pressure cooker allows food to cook at a temperature higher than the normal boiling point of water. Why?
A. Increasing pressure increases the kinetic energy of water molecules automatically.
B. Increasing pressure makes it more difficult for water molecules to escape into the gas phase, raising the boiling point.
C. Pressure decreases the intermolecular attractions between water molecules.
D. Water becomes a different substance under pressure.
B.
Two identical balloons are filled with the same amount of gas. One balloon is placed in a refrigerator, while the other remains at room temperature.
After some time, the refrigerated balloon becomes smaller.
Using the Kinetic Molecular Theory, explain why the balloon's volume decreases.
Lower temperature means the gas particles have lower average kinetic energy. They move more slowly and collide less energetically with the walls of the balloon. As a result, the flexible balloon contracts until the internal and external pressures are balanced.
Four substances have approximately similar molecular sizes. Substance A is nonpolar, B is polar, C can form hydrogen bonds, and D is an ionic compound dissolved in water. Identify the dominant interaction for each.
A – London dispersion; B – dipole-dipole; C – hydrogen bonding; D – ion-dipole.