5.3 - Ice Box Problems - chem30
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Transcript 5.3 - Ice Box Problems - chem30
5.3 – ICE BOX
PROBLEMS
Unit 5: Equilibrium
As we already know, a chemical equation tells us
several pieces of information.
H2(g) + I2(g) ↔ 2HI (g)
This equation tells us how the number of
molecules or moles of each substance relate to
each other by looking at the balancing
coefficients.
The balancing coefficients also tell us how the
concentration of each substance changes as a
reaction goes on.
STUFF WE SHOULD KNOW
ALREADY…
H2(g) + I2(g) ↔ 2HI (g)
For example, if [H2] changes by “x” mols/L, then:
[I2] changes by “x” mols/L, and
[HI] changes by “2x” mols/L.
However, [H2] and [I2] are decreasing
while [HI] is increasing.
Thus, we say that Δ[H2] = Δ[I2] = “
x”, and the Δ[HI] = “2x”.
Note that the negatives mean decrease (or losing) and a positive value
means gaining.
This knowledge of changing concentrations is key for an ICE box
problem.
NEW-ISH STUFF…
The ICE is an acronym for:
I = Initial concentration
C = Change in concentration
E = Equilibrium concentration
An ICE box problem looks at the ENTIRE equilibrium chemical
reaction:
The concentration of initial reactant (s) before they interact with each
other
The change in concentrations for each substance (both increasing
and decreasing)
The final concentrations of each substance once equilibrium has been
established
Therefore you will be working with three (3) sets of values in each
ICE box question – so make sure to be thorough and show all your
work; it’s easy to make simple mistakes!
ICE BOX PROBLEMS
The following process can be used to solve an ICE box problem
for a general reaction: A + B ↔ C
1. Balance the equation.
2. Set up the ICE box:
[A]
[B]
[C]
[Initial]
[Change]
[Equilibrium]
3. Use information from the question to plug values into the ICE
box.
ICE BOX PROBLEMS
ICE BOX PROBLEMS
Some hints you may want to consider:
i. Initially, you should have no product.
Think about it, if the substances haven’t even touched or interacted with each
other, how can any product be formed at the start of the reaction?
Therefore, the initial product box (es) can have a value of zero
ii. All of the values in the ICE box represent concentrations (molarity,
moles/L). If the information given is not expressed in molarity, make sure you
convert it to molarity first!
iii. Read the information given in the question very carefully – it will often
give away many of the values for you to place in the ice box
iv. You can manipulate, use stoichiometry, or assign variables for the
CHANGE values ONLY.
You can NOT do this for your initial or equilibrium values
v. Information given in the question will always be for the INITIAL or
EQUILIBRIUM concentrations.
The CHANGE values will never be explicitly given to you, rather, you will have to
problem-solve to figure them out
Type 1: Solving for Keq when initial and equilibrium conditions are given:
Example 1: Ammonia is created by the following process:
N2(g) + 3H2(g) ↔ 2 NH3(g)
If the initially [N2] = 0.96 M and [H2] = 0.72 M, and at equilibrium [NH3] =
0.24 M what is the equilibrium constant?
Use the information given to problem-solve for a CHANGE value
Use stoichiometry to solve for the remaining CHANGE values
Go where the questions asks
ICE BOX PROBLEMS – TYPES OF
QUESTIONS
Type 2: Solving for equilibrium concentrations when Keq is given
Example 2: Initially, for the reaction below, [H2] = [I2] = 0.200 M.
H2(g) + I2(g) ↔ 2HI(g)
Calculate all three equilibrium concentrations if Keq = 64.0.
Since no equilibrium concentrations are given, you need to assign variables
for your CHANGE values
Let the balancing coefficients assist you in assigning variables
i.e.: if there is a balancing coefficient of “1”, assign that variable “x”
i.e.: if there is a balancing coefficient of “2”, assign that variable “2x”
ICE BOX PROBLEMS – TYPES OF
QUESTIONS
ICE BOX PROBLEMS – TYPES OF
QUESTIONS
Type 3: Solving for equilibrium concentrations when Keq is given BUT you
have to use the quadratic formula
When solving for x, if you are left with the following pattern:
ax2 + bx + c
You will need to use the quadratic formula to solve for x
This will give you two answers; always neglect the negative answer for “x”
Remember, “x” represents concentration, so you can’t have a negative
concentration
Example 3: Given this equation,
PCl5(g) ↔ PCl3(g) + Cl2(g)
Calculate all three equilibrium concentrations if initially [PCl 5] = 1.00 M and
Keq = 16.0.