States & Particle Behavior
A gas inside a sealed container is cooled significantly.
A student claims the mass of the gas particles decreases because the pressure becomes lower.
Explain why this reasoning is incorrect.
The mass of gas particles does not change. Lower pressure occurs because particles move slower and collide less forcefully with the container walls.
Determine the oxidation number of manganese in:
KMnO4
+7
FeCl3
Iron(III) chloride
Aluminum sulfate
Al2(SO4)3
A reaction has:
ΔH=−450 kJ
What does the negative value indicate about the energy flow?
Energy is released to the surroundings.
A sealed syringe containing gas is compressed to half its original volume. A student claims the particles themselves became smaller.
Why is this explanation incorrect?
Gas particles do not shrink; the spaces between particles decrease.
In the reaction below, identify the element that is oxidized:
Mg+2HCl→MgCl2+H2
Magnesium
Mg goes from 0 to 2, so it is oxidized.
Fe₂O₃
Iron(III) oxide
Iron(III) sulfate
Fe₂(SO₄)₃
A student says combustion reactions can sometimes be endothermic.
Evaluate the statement.
Incorrect; combustion reactions are always exothermic because they release heat.
Two liquids are heated equally. Liquid A evaporates much faster than Liquid B.
What can be inferred about the intermolecular forces of Liquid A?
Liquid A has weaker intermolecular forces.
Determine the oxidation number of chlorine in:
Ca(OCl)2
+1
Calcium is +2, Oxygen is -2. The hypochlorite ion OCl- gives Chlorine an oxidation state of +1.
Na₂CO₃
Sodium carbonate
Ammonium carbonate
(NH₄)₂CO₃
A reaction absorbs 250 kJ of energy while bond formation releases only 180 kJ.
Determine whether the reaction is exothermic or endothermic.
ΔH=Bonds Broken−Bonds Formed
Endothermic because more energy is absorbed than released.
A sealed flask containing gas is heated.
One student says pressure increases because particles expand.
Another says pressure increases because collisions become more frequent and forceful.
Which explanation is scientifically correct?
The second explanation is correct; particles gain kinetic energy, causing more frequent and forceful collisions.
I in Mg(IO3)2
+5
Pb(NO₃)₂
Lead(II) nitrate
Potassium nitrate(III)
KNO2
Given:
H2+Cl2→2HCl
Breaking bonds requires 436 kJ and 242 kJ respectively, while forming H–Cl bonds releases 431 kJ.
Determine whether the reaction is exothermic or endothermic.
ΔH=Bonds Broken−Bonds Formed
(436+242)−862=−184 kJ
exothermic
A substance changes from liquid to gas at constant temperature.
Even though the temperature remains constant, energy is continuously absorbed.
Explain where the absorbed energy goes.
The absorbed energy is used to overcome intermolecular forces between particles rather than increasing kinetic energy. Since kinetic energy does not increase, the temperature remains constant.
A student claims nitrogen undergoes reduction in the following reaction:
2NO2→2NO+O2
Using oxidation numbers, determine whether the claim is correct.
Correct. Nitrogen changes from +4 in NO₂ to +2 in NO, so it is reduced.
(NH₄)₃PO₄
Ammonium phosphate
A student writes the formula of magnesium nitrate as MgNO₃.
Explain the error and provide the correct formula.
Magnesium forms Mg²⁺ while nitrate is NO₃⁻, so two nitrates are needed to balance the charge.
Correct formula: Mg(NO₃)₂
A chemical reaction is found to be exothermic overall, even though a large amount of energy is needed at the beginning of the reaction.
Explain how this is possible using bond breaking and bond making concepts.
Breaking bonds absorbs energy, while forming bonds releases energy. The reaction is exothermic because the energy released during bond formation is greater than the energy absorbed during bond breaking.