Chapter 15: Equilibrium Systems

Question 1

What else should you go through?

Answer 1

Learning outcomes, old diplomas, and fix all labs

Question 2

What is the simplest equilibrium?

Answer 2

A static equilibrium where nothing is moving/changing to maintain balance.

Question 3

What is a chemical equilibrium?

Answer 3

Balance between two opposing agents, but in a dynamic system (both are occurring/moving, but at same rate) with no observable/macroscopic changes.

Question 4

What is a closed system?

Answer 4

A chemical system separated from its surroundings by a definite boundary so that no matter can enter or leave, but energy can enter and leave. They are useful in experiments because they give scientists more control over the experiment.

Question 5

What should you do to study more?

Answer 5

Read through textbook.

Question 6

What is a phase equilibrium?

Answer 6

A type of equilibrium involving a chemical substance existing in more than one phase/state IN A CLOSED SYSTEM. (Ice cube in water cup at room temperature not at equilibrium since no water will turn back into ice).

Question 7

What is a solubility equilibrium?

Answer 7

A constant state of a system in which excess solute is in contact with the saturated solution. Rate dissolving= Rate crystallization.

Question 8

What is a chemical reaction equilibrium?

Answer 8

An apparently static state of a reaction system where the reactants and products may be favoured but the rates of forward and reverse reactions are equal.

Question 9

What is a dynamic equilibrium?

Answer 9

When two opposing processes are occurring at the same rate.

Question 10

What should you be asking yourself when memorizing concepts?

Answer 10

What are the implications of this, and how can I apply this to real life? What are some questions I could be asked to test the extent of my comprehension?

Question 11

What is being referred to when talking about the ‘forward’ and ‘reverse’ reaction?

Answer 11

The terms forward and reverse identify which process is referred to and ARE SPECIFIC TO A WRITTEN EQUILIBRIUM EQUATION (!!!). Forward reaction is going right, while reverse is going left.

Question 12

What is the percent yield of a an equilibrium reaction?

Answer 12

The yield (amount) of product measured at equilibrium compared to the maximum possible yield of product, which is found assuming quantitative rxn in reference to L.R . This is useful for communicating the position of an equilibrium. Actual/Maximum x 100%

Question 13

How do you communicate that an equilibrium exists in a reaction?

Answer 13

With a right and left arrow superimposed between products and reactants.

Question 14

Which chemical reactions occur in both forward and reverse directions?

Answer 14

While some do so to a negligible extent, ALL chemical reactions occur in both forward and reverse directions.

Question 15

What are the classifications of chemical reaction equilibria (%)?

Answer 15

<1% yield is a non-spontaneous rxn in the way equation is written (forward rxn negligible), and there is no apparent reaction. <50% implies that the reactants are favoured, while >50% implies that the products are favoured. >99% yield indicates a quantitative reaction occurred, and only one right arrow need be drawn.

Question 16

What should you review?

Answer 16

Look through problems in notebook, and do all hmwk questions.

Question 17

When can stoichiometric equations use concentrations directly rather than chemical amounts?

Answer 17

When all entities in a rxn are in the same state because substances in the same state cover the same volume.

Question 18

What is the Kc?

Answer 18

The equilibrium constant, a constant value for a rxn system despite variable concentrations.

Question 19

What is the equilibrium law?

Answer 19

Mathematical expression relating concentration of reactants and products to equilibrium constant. [products]/[reactants]=Kc, with molar coefficients as exponents.

Question 20

When does the equilibrium law relationship hold true?

Answer 20

When the concentrations of the entities involved are constant, in a closed system, at a given temperature.

Question 21

What should do after answering questions?

Answer 21

Say “I am saying that …” and go over word for word, your thinking process, along with a definition of the concepts involved, and their properties.

Question 22

What happens to the equilibrium law expression and Kc for a reversed equation?

Answer 22

The equilibrium law expression is reversed, same as Kc (1/OG).

Question 23

What does a large Kc value indicate?

Answer 23

As Kc increases, there is a higher tendency for the system to favour the forward direction (products favoured at equilibrium). Opposite for small Kc.

Question 24

How is Kc related to rate of reaction?

Answer 24

While the equilibrium constant gives information on the position of an equilibrium, it DOES NOT provide any information on the rate at which the reaction is happening.

Question 25

Why are condensed states not included in Kc expressions/calculations?

Answer 25

Because solids and liquids have concentrations that are fixed, they will never change the position of an equilirium, which is the purpose of calculating the equilibrium constant.

Question 26

Which substances are always included in Kc calculations/expressions?

Answer 26

Substances in a gaseous or dissolved (aqueous) state have variable concentrations and must always be shown in an equilibrium law expression.

Question 27

What must be given along with the Kc of any given reaction?

Answer 27

The temperature it’s occurring at.

Question 28

How are entities represented in equilibrium law expressions?

Answer 28

Gases as they are seen in the equation, but ions are represented as INDIVIDUAL ENTITIES. Kc expressions are ALWAYS WRITTEN FROM NET IONIC FORM OF REACTION EQUATIONS. This implies that spectator ions are omitted from equilibrium law expressions.

Question 29

What is the equilibrium constant (Kc) value dependent on?

Answer 29

Temperature.

Question 30

What is the equilibrium constant (Kc) value independent of?

Answer 30

Of reagent concentrations (entities involved in reactions), catalysts, time taken to reach equilibrium, units.

Question 31

What is LeChatelier’s principle?

Answer 31

When a chemical system at equilibrium is disturbed by a change in a property of the system, the system always appears to shift/react in the direction that opposes the change, until a new equilibrium is established.

Question 32

What should you do?

Answer 32

STUDY.

Question 33

What is the relationship between lechatelier’s principle and concentration changes?

Answer 33

If the addition/removal of a reactant/product affects the concentration of said substance, then the substance will undergo an equilibrium shift to counteract the change (decrease in [C] causes equilibrium shift to reproduce substance, and increase causes shift to consume it). In a chemical reaction, an imposed concentration change is partially counteracted, and the final equilibrium state [C] is different from OG value.

Question 34

How does the collision-reaction theory support (and further) lechatelier’s principle on its theory about the impact of concentration changes on a system at equilibrium?

Answer 34

If reactant is added for example, its concentration increases and more forward reactions happen since the # of collisions increases. Eventually as more forward reactions occur, there will be more products as added reactants are consumed, and the rate of reverse reaction will increase until 2 RATES are equal again (not necessarily [C]. Both rates at the new equilibrium are faster than those at the original equilibrium, since the system contains more particles in its reaction medium.

Question 35

How does addition/removal of reagent present in a condensed state affect the concentration of that substance?

Answer 35

Addition/removal of reagent present in liquid/solid state DOES NOT change the concentration of that substance. The reaction of condensed states take place only on an exposed surface, and if the surface area is changed, it is always the same change in available area for both forward and reverse reactions. If they change at all, forward and reverse rates do so by the same amount and there is no equilibrium shift.

Question 36

What should you look for in notebook?

Answer 36

All sidebar notes, and find any remaining questions.

Question 37

How does lechatelier’s principle approach temperature changes?

Answer 37

Energy in chemical equilibrium equation is treated as though it were a reactant or product. Reactant + E» Products = endothermic (H=+),While Reactant» Products + E = exothermic (H=negative). Heating/Cooling a system adds/removes E from it, and equilibrium shifts to undo change as if concentration of an entity had been affected. THINK OF ENERGY AS REACTANT: If it increases (heated), system shifts to consume it.

Question 38

In an exothermic reaction (reactants <> products + E), what happens as temperature changes?

Answer 38

The percent yield (forward rxn, which is causing release in E) increases at lower temperatures, and decreases at higher temperatures to consume E.

Question 39

How does Lechatelier’s principle approach gas volume changes (try to search vids on concepts you can’t fully grasp)?

Answer 39

According to Boyle’s law (p1v1=p2v2), pressure of gas in container inversely proportional to volume of container (example:decrease in volume by half doubles gas [C] and pressure). If the volume of a container is decreased, the equilibrium shifts towards side with least gas molecules as pressure increases. If the volume of a container is increased, system shifts towards side with larger amount of gaseous entities as pressure decreases.

Question 40

Which types of systems are not affected by changes in pressure/volume?

Answer 40

Systems with equal number of gaseous molecules on each side of equation, systems involving only liquid/solids are not affected by changes in pressure (aqueous affected?). Also adding a gas not involved in equilibrium, such as an inert/noble gas to the container will increase the overall pressure, but will not cause an equilibrium shift??????

Question 41

How does the collision-reaction theory support/further lechatelier’s principle approach to pressure changes?

Answer 41

Example rxn: 2SO2(g)+O2(g)><2SO3(g)+198KJ (exo):
Decreasing volume of this system causes both forward and reverse reactions to become faster since [C] of products and reactants both increase. However, the forward reaction increases more than the reverse reaction because there are more particles involved in the forward rxn. This shift causes concentration changes that produce more SO3(g) and gradually increase the reverse reaction and decreases the forward rate until they become equal again.

Question 42

What is the effect of catalysts on equilibrium systems?

Answer 42

While a catalyst decreases the time required to reach an equilibrium position, it does not affect the final position of the equilibrium (increases forward/reverse reactions by same amount).

Question 43

Describe endothermic and exothermic processes in terms of phase changes:

Answer 43

Forming bonds is exothermic, while breaking them is endothermic.

Question 44

How is the equilibrium constant (Kc) affected by an increase in temperature in an exothermic reaction?

Answer 44

Heat/Energy is a product, so an increase in temperature will cause an equilibrium shift left to consume the added energy.

Question 45

How is the equilibrium constant (Kc) affected by an increase in temperature in an endothermic reaction?

Answer 45

Heat/Energy is a reactant in endothermic reaction, so an increase in temperature will cause an equilibrium shift right to consume the added energy.

Question 46

What is a good generalization to know if Kc changes?

Answer 46

UNLESS TEMPERATURE CHANGES, Kc is always the same. Property changes only affect the position of an equilibrium.

Question 47

Which rules should you remember/look into when answering questions?

Answer 47

Be careful with solubility rules, because adding solid or liquid entities DOES NOT affect concentration (If it tells you solid is added, check if soluble from table).

Question 48

What else can you learn from?

Answer 48

LABS, STUDY ALL.

Question 49

What NEVER belongs on concentration versus time graphs?

Answer 49

Condensed states (Liquids and solids).

Question 50

Quick guidelines for concentration versus time graphs?

Answer 50

Have ticked line to denote instantaneous effect of change in property, and show equilibrium shift affect afterwards. Make sure changes are logical when considering molar coefficients, and even if in opposite directions, entities with same molar coefficients should have same slope.

Question 51

What does it mean for a side to be ‘favoured’?

Answer 51

A side is ‘favoured’, when the equilibrium is shifted towards it.

Question 52

What should you remember to do when graphing?

Answer 52

Properly label them, including with title and axes. Assume entities with larger molar coefficients start at higher concentrations (2:1 should have twice [C]). Also assume WA/WB start at much higher concentrations than the products in their reactions (<50%), but change by same amount. REMEMBER THAT CONCENTRATION CHANGES ARE ONLY PARTIALLY COUNTERACTED, so if concentration decreases it shouldn’t go all the way back up to initial value, if it increases it shouldn’t go all the way back down to initial value. This applies unless a sole entity is involved in equilibrium law expression, as it would need to go all the way back to its initial value for Kc to stay constant.

Question 53

What effect do initial concentrations have on Kc?

Answer 53

Kc is calculated from equilibrium concentrations, and initial concentrations have no effect on its value.

Question 54

What should you watch for when doing calculations involving Kc?

Answer 54

If they tell you the Kc for the forward reaction, but the equation you’ve been given/told of is the reverse reaction, remember to do 1/K to find new Kc.

Question 55

What is the first step of ALL equilibrium questions (EVEN MULTIPLE CHOICE)?

Answer 55

Writing the equilibrium law expression with states of entities in their net ionic equation form.

Question 56

What should you remember to do when writing equations?

Answer 56

Balance atoms, then charge ALWAYS.

Question 57

What is a ‘quantitative’ reaction?

Answer 57

One in which the forward reaction is nearly absolute, draw with 1 right arrow.

Question 58

How do you calculate the percent reaction?

Answer 58

Amount of reactant consumed (change)/amount at beginning x 100%.

Question 59

Why does adding an inert gas, or any gaseous substance not involved in a reaction not cause an equilibrium shift?

Answer 59

Because adding an entity not involved in the reaction has no effect on the concentrations of the reactants and products from equilibrium law expression, there are still just as many particles in the same amount of total space.

Question 60

What to do when the equilibrium constant is smaller than 10^-3?

Answer 60

Get rid of x in denominator, as it is insignificant.

Question 61

How to check Kc questions answers?

Answer 61

Put final values into Kc expression and make sure it’s the same as given Kc.

Question 62

What happens if you decrease the temperature of an exothermic reaction?

Answer 62

You are removing energy, a product in the reaction, which will cause an equilibrium shift right to reproduce the lost energy (product concentration increases too).

Question 63

What to ask yourself after answering calculation questions?

Answer 63

“Have I worked with the proper units? Is my answer in the correct units?”

Question 64

How to predict equilibrium shift when adding a compound?

Answer 64

Check solubility table, if it’s not soluble it doesn’t affect equilibrium.