Chemistry – April 19, 2017

P3 Challenge –

Consider: 2 C2H2 + 5 O2  4 CO2 + 2 H2O

What is the equilibrium constant if the concentrations at
equilibrium are [C2H2] = 0.035 M, [O2] = 0.58 M, [CO2] = 3.8 M, and
[H2O] = 1.7 M ?
Get out Equilibrium Constant
worksheet for HMW check
Chemistry – April 19, 2017

Today’s Objective –


LeChatelier’s Principle
Assignment:

LeChatelier’s Worksheet

Agenda

Homework Review

LeChatelier’s Principle

Concentration

Pressure

Volume

Temperature
Rest of Year Overview

April 17 – Equilibrium Constants

May 5 – Review Packet

April 19 – LeChatlier’s Principle

May 8 – Review Units 1, 2 and 3

April 21 – Acids and Bases, pH

May 10 – Review Unit 4 and 5

April 24 – Kinematics

May 15 – Review Unit 6 and 7

April 26 – Review WS

May 17 – Retakes

May 1 – Review

May 19 – Last questions

May 3 – Equilibrium Test

May 22 – 25 Finals – If you pass the final
with a raw score 70% you pass the course.
LeChatlier’s Principle

A system at equilibrium will oppose any change to the system in a way
that helps eliminate the change.

The types of changes that a chemical equilibrium may experience include
changes in….

Concentrations of reactants or products

Pressure

Volume

Temperature

An equilibrium responds by shifting left or shifting right.
Concentration changes

To determine how an equilibrium responds imagine a set of marbles
balanced on a seesaw.

If you increase a reactant concentration, that’s like pushing up on the left
side. The marbles and the equilibrium move right.

If you decrease a reactant concentration, that’s like pulling down on the
left side. The marbles and the equilibrium move left.

If you increase a product concentration, that’s like pushing up on the right
side. The marbles and the equilibrium move left.

If you decrease a product concentration, that’s like pulling down on the
right side. The marbles and the equilibrium move right.
Practice:

Consider: 2 C2H2 + 5 O2  4 CO2 + 2 H2O

Predict if the equilibrium will shift right or left when the equilibrium is
disturbed in the following ways:

Increase in [O2]

Decrease in [CO2]

Decrease in [H2O]

Increase in [CO2]
Pressure and volume changes

Pressure and volume changes are all about the gasses that are present in
a reaction.

Count up the number of moles of gas on each side of the reaction. If they
are equal, changing pressure and/or volume will have no effect.

If they are different, both pressure and volume will effect an equilibrium in
a similar way.

An increase in pressure (or decrease in volume) will shift the equilibrium to
the side with fewer moles of gas.

A decrease in pressure (or increase in volume) will shift the equilibrium to
the side with more moles of gas.
Practice:

Consider: 2 C2H2 + 5 O2  4 CO2 + 2 H2O

Predict if the equilibrium will shift right or left when the equilibrium is
disturbed in the following ways:

Increase pressure

Decrease pressure

Decrease volume

Increase volume
Temperature changes

Temperature changes effect an equilibrium in a different way.

All other changes keep the same equilibrium constant value, which in our
model is the location of the fulcrum.

Changing temperature changes the value of the equilibrium constant

How a reaction responds depends on whether it is endothermic, requiring
a heat source, or exothermic, producing heat.

Treat heat as a reactant (endothermic) or as a product (exothermic) and
then use the same reasoning as concentration changes.
Practice:

Consider: 2 C2H2 + 5 O2  4 CO2 + 2 H2O
(exothermic)

Predict if the equilibrium will shift right or left when the equilibrium is
disturbed in the following ways:

Increase temperature

Decrease temperature
Exit Slip - Homework

Exit Slip: What direction does the equilibrium shift if the
pressure is increased?

2 H2O (g)  2 H2 (g) + 1 O2 (g)

What’s Due? (Pending assignments to complete.)


LeChatelier’s Principle worksheet.
What’s Next? (How to prepare for the next day)

Read p496 - 520