Cracking the SAT Chemistry Subject Test

And you’d read text such as this.

We  may consider    free-energy change  in  a   spontaneous reaction    much    as  we  consider    the

potential   energy  change  that    accompanies the rolling of  an  ordinary    ball    down    a   hill.   The ball

is  driven  down    the hill    by  the potential   energy  within  a   gravitational   field.  By  analogy,    the free

energy   within  a   chemical    system  decreases   continuously    over    time...blah,    blah,   blah...

ultimately  reaching    a   minimum.    When    potential   energy  is  at  a   minimum,    the reaction    reaches

its equilibrium.

We  might   best    illustrate  the concept by  reference   to  the formation   of  ammonia from    its

elements     hydrogen    and...blah,     blah,   blah....Imagine     that    a   particular  number  of  moles   of

nitrogen    react   with    three   times   the number  of  hydrogen    atoms.  The formation   of  ammonia will

not be  complete    because...blah, blah,   blah....An  equilibrium will    be  attained    by  the system,

and at  equilibrium the reaction    chamber will    contain a   mixture of...blah,  blah,   blah....At  that

time    there   can be  no  additional  spontaneous formation   of  ammonia because the system  has

reached  a   minimum     state   of  free    energy  that    ...blah,    blah,   blah....Free    energy  is  a   state

function,   and that    is  why...blah, blah,   blah.   ...

The text would go on and on, intimidating and boring you, but offering nothing
that raises your test score. You’d get so sick of it that you’d stop reading.

When we teach you about Gibbs free energy, we tell you exactly what you have
to know to raise your test score. As we do that, we give you opportunities to
practice on realistic chemistry problems, to make sure you’re with us at every
step. The most important thing for you to remember about Gibbs free energy is
that it is symbolized as ∆G, and that if ∆G is negative, the reaction proceeds
spontaneously in the forward direction, but if it’s positive, the reaction proceeds
spontaneously in the reverse direction.

Gibbs   Free    Energy

ΔG  <   0   a   reaction    proceeds    spontaneously   in  the forward direction

ΔG  >   0   a   reaction    proceeds    spontaneously   in  the reverse direction

Now try to answer the following two questions:

Directions:  Each    set     of  lettered    choices     below   refers  to  the     numbered

statements   or  questions   immediately     following   it.     Select  the     one     lettered

choice   that    best    fits    each    statement   or  formula     and     then    fill    in  the

corresponding   oval    on  the answer  sheet.  A   choice  may be  used    once,   more

than    once,   or  not at  all in  each    set.