Help with entropy/reaction rates homework?

Okay, but you'll have to read through this, and you cannot copy what I say. You'll have to put it into your own words.

#1)# Explain the effects of reactant concentration and particle size on the rate of a reaction.

Given that a general rate law is

#r(t) = k[A]^m[B]^n#,

higher concentrations lead to higher rates; more collisions occur when there are more particles flying around per unit volume.

Smaller particles travel faster, and thus collide more strongly (at a higher average kinetic energy), making it easier to overcome the activation energy.

#2)# Characterize spontaneous and non spontaneous reactions. Give one example for each.

A reaction is:

• spontaneous when the change in Gibbs' free energy is negative.
• nonspontaneous when the change in Gibbs' free energy is positive.

Essentially all we want for a spontaneous reaction is that, IF it is fast enough (if the kinetics are observable in a human life span), it will occur.

If it's nonspontaneous, it will never be energetically favorable BUT can be made possible with a catalyst and heat.

• Spontaneous example:

#color(blue)("AgCl"(s) + 2"NH"_3(aq) -> "Ag"("NH"_3)_2^(+)(aq) + "Cl"^(-)(aq))#

Aqueous ammonia can be added to silver chloride precipitate to force it to dissolve, because ammonia easily binds to #"Ag"^(+)#, with a formation constant #K_f# larger than #10^7#.

• Nonspontaneous example:

#color(blue)(6"CO"_2(g) + 6"H"_2"O"(l) -> "C"_6"H"_12"O"_6(s) + 6"O"_2(g))#

Photosynthesis is known to be nonspontaneous. It doesn't occur at night but it occurs in the day. What is the catalyst?

#3)# What is entropy? Give 2 examples.

Entropy is the amount of energy dispersal at a certain temperature. The more motion a system has, the more entropy it has.

I assume by "examples" it means where entropy is important or where entropy is demonstrated.

Examples

• Mixing #"NaCl"# into water is accompanied by a positive entropy change that is the driving factor for the process.

#color(blue)("NaCl"(s) stackrel("H"_2"O"(l)" ")(->) "Na"^(+)(aq) + "Cl"^(-)(aq))#

The enthalpy of solution #DeltaH_(sol n)# is around #"3.9 kJ/mol"#, so it requires a large enough, positive entropy of solution #DeltaS_(sol n)# to overcome the positive enthalpy, which it has.

• Freezing of water into ice decreases the entropy of the water.

#color(blue)("H"_2"O"(l) stackrel("freeze ")(->) "H"_2"O"(s))#

By freezing water, you release kinetic energy (in the form of heat) that was present from the rotation of water molecules and form more ordered hydrogen-bonding interactions.

Through the release of energy, motion decreases, forming a more rigid structure in ice. Entropy thus has decreased.

#4)# Record the signs in the chart as positive or negative.

#ul("Balanced Eqn.")#
#"Mg"(s) + 2"HCl"(aq) -> "Mg"^(2+)(aq) + 2"Cl"^(-)(aq) + "H"_2(g) + "energy"#

Notice how energy is "on the products' side". That implies it is released, which is exothermic, meaning #DeltaH < 0#. A gas is produced, from a solid plus a liquid, increasing entropy and thus making #DeltaS > 0#.

We know that #DeltaG = DeltaH - TDeltaS#, so

#color(green)(DeltaG) = (-) - (+)(+) = color(green)((-)" always")#

#color(blue)(ul(DeltaH" "" "" "DeltaS" "" "" "DeltaG))#
#color(blue)((-)" "" "(+)" "" "" "(-))#

#ul("Balanced Eqn.")#
#2"H"_2(g) + "O"_2(g) + "energy" -> 2"H"_2"O"(g)#

Energy is "on the reactants' side", so the process absorbs energy (is endothermic). Thus, #DeltaH > 0#.

We had three mols of gas, and ended up with two. Less gas = less motion = less entropy, so #DeltaS < 0#.

We know that #DeltaG = DeltaH - TDeltaS#, so

#color(green)(DeltaG) = (+) - (+)(-) = color(green)((+)" always")#

#color(blue)(ul(DeltaH" "" "" "DeltaS" "" "" "DeltaG))#
#color(blue)((+)" "" "(-)" "" "" "(+))#

#5)# Fill in the blank portions of the following table. Use the terms "positive", "negative", "yes", and "no".

This is evidently made to reflect what you saw in question 4. Just regurgitate (and analyze...).

#ul("Reaction"" "" "" "DeltaH" "" "" "DeltaS" "" "" ""Spontaneous?")#
#"Exothermic"" "" ""Negative"" ""Positive"" ""Yes of course"#
#"Endothermic"" "color(white)(/)"Positive"" ""Negative"" ""No sirree"#

[As we said before, #DeltaG < 0# means spontaneous. Read #(2)#.]