Rates of Reaction & Equillibrium

Rate of reaction

change in concentration of a reactant or product per unit time

yield (aka extent of reaction)

AMOUNT of products produced → meaning the MOLES (n), not concentration

• go by the DIRECTION BEING FAVOURED, not the actual conc.

For a reaction to occur…

• collision must occur so existing bonds within reactants are broken

• successful collisions require energy >= activation energy

  • + correct orientation

Why is xL of H2 produced?

How can volume of CO2 produced change when mass/volume/conc of a reactant increased?

MUST SAY “IF “X REACTANT” IS LR THEN, “

OR “IF “X REACTANT” IS NOT LR THEN,”

When volume of solution changes in rates reaction but conc doesn’t - effect on ROR?

NONE - bc conc. remains constant even though more particles bc in greater space/vol

How to prove that a temp change is what’s happening

Calculate Kc at equilibrium 1 (before change) and then after change using concs. from graph

When it asks for consequences of increasing temp of equilibrium solution

Describe the changes in conc.

What catalyst being added on equilibrium graph looks like

reaches equilibrium faster - NOTE: wasn’t at equilibrium initially

Transition State

occurs at the maximum potential energy stage (EA)

activation energy

minimum amount of energy required for a reaction to take place

Increasing ROR

increase frequency of successful collisions

increase proportion of particles with energy higher than EA

Increasing ROR Methods

• increasing surface area of solid

• increasing reactant conc. or pressure

increasing surface area of solid

• only surface particles participate in the reaction

• the more exposed solid reactant particles there are, the more particles available to react

• freq. of collisions increases

• freq. of successful collisions increases

increasing reactant conc. or pressure

conc = no of particles in given volume

• when sol conc. increases, more reactant particles moving in a given v(solution).

• freq. of collisions increases

• freq. successful collisions also increases.

• ROR increases

• MUST SAY: PROPORTION OF REACTANTS WITH ENERGY > EA DOES NOT CHANGE

Increase proportion of particles with energy higher than EA methods

increasing temp (Kinetic E)

catalyst (lowers EA)

increasing temp (Kinetic E)

• as temp increases, average kinetic energy of particles increases.

• Greater proportion of particles have E >= EA, proportion of successful collisions increases.

• Therefore, ROR increases

• The increase in kinetic energy also leads to frequency of collisions increasing so freq. of successful collisions also increases

• Therefore, ROR increases

catalyst (lowers EA)

• lowers the activation energy by providing an alternate pathway with lower energy

• because EA is lowered, the proportion of particles with energy >= EA increases

• so proportion of successful collisions increases

• ROR increases

Explaining parts of increase conc. graph

1. At start, many reactant particles so the frequency of collisions increases, the number of successful collisions increases so the rate of reaction increases

2. The rate of reaction slows because as the reaction proceeds, the number of reactant particles decreases. The frequency of collisions decreases and the number of successful collisions decreases

3. The reaction stops because the limiting reactant/ reagent has been used up

When asked to calculate volume of gas

1. Find moles of reactants

2. Find limiting reactant (by dividing moles by coeff)

3. Then solve using pv = rnt or n = v/vm

When graph given and temp increased, sketch expected graph

at the start the rate is just faster so reaches maximum faster but the actual volume of gas produced stays same since no change to moles of reaction

When graph given and conc. increased, sketch expected graph

CHECK IF NEW LIMITING REACTANT based on new conc.

• if in excess, does not affect the volume produced, if not in excess does effect the volume produced

  • re-do calcs to find new LR and new volume

Rate faster so steeper at start

If mass of powder/ some kind of mass change or conc/volume changes the output/volume

Then calculate new final volume and draw the flat line as per that and still steeper at the start.

Finding limiting reactant

• Find moles of each

• Divide each moles by coeff.

Dynamic Equilibrium

when rate of forward reaction = rate of backwards reaction

Open Systems

matter and energy can be exchanged with the surrounding

Closed systems

only energy is exchanged with the surroundings

dynamic equilibrium conditions

1. amounts and concs. of chemical substances remain constant

2. total gas pressure is constant (if gases involved)

3. temperature remains constant

4. reaction is incomplete (all substances are present in reaction mixture)

5. HAS NOT STOPPED REACTING

6. CONCS. CONSTANT BUT DON’T NEED TO BE SAME

what does dynamic equilibrium mean?

point where the rate of the forward reaction and the rate of the reverse reaction are equal (only closed systems and reversible reactions)

extent of reaction vs rate of reaction

The extent of reaction indicates how much product is formed at equilibrium, whereas the rate of reaction is a measure of the change in concentration of the reactants and products over time.

Adjustment period

the reaction is moving towards equilibrium but not at equilibrium

• anything before the lines become flat

Reaction Quotient (Qc)

reaction quotient is used to compare the concentration of the products and reactants at any point in a reaction

• units follow same pattern (M)

Qc Formula

Equilibrium constant (KC)

the equilibrium constant is expression when reaction is at equilibrium

Kc and extent of reaction

Relationship bw Kc and Qc at equilibrium

Qc = Kc

What does equilibrium constant mean?

tells us the extent of the reaction

• eg. high Kc = high yield, low Kc = low yield

large KC value means

[products] incr./ [reactants] decr.

means mostly products present

small KC value means

[products] decr./ [reactants] incr.

mostly reactants present

• reactant favoured so less proportion of reactant has become product

  extent of reaction (proportion of reactant becoming products small) (more reactants at equilibrium compared to products)

Qc < Kc

system needs to form more products to reach equilibrium

• forward reaction favoured

Qc > Kc

system needs to form more reactants to reach equilibrium

• backward reaction favoured

Important thing about stating Kc value

Must state temp too!

• eg. 1.00M at 25 degrees celsius

Equilibrium constant changes

• ONLY affected by temp

• not affected by actions such as adding reactants or products, changes in pressure, use of catalysts

Kc when reaction reversed

Kc becomes reciprocal of original Kc

• eg. Kc2 = 1/Kc1

Kc when coefficients doubled

original Kc value squared

Kc value when coefficient halved

Square root original Kc value

How do you know if ICE table question

The moles of both products and reactants given, if initial moles of something not given, then assume 0

Finding Kc using ICE Tables

Initial moles

Change in moles

Moles at equilibrium

Concentration at equilibrium

Le Chatelier’s principle

If an equilibrium system is subject to change, the system will adjust itself to partially oppose the effect of the change

How equilibrium position can be changed

equilibrium position (relative amounts of reactants + products) of a reaction can be changed by

• adding or removing a reactant or product

• changing pressure by changing volume (gases)

• dilution (for solution)

• changing temperature (only thing that effects Kc)

When asked to find Kp

Kp = P(NH3)/ P(H2) * P(N2)

• P and round brackets

When describing/ graphing what to consider

C - what is the change you are doing to the system?

O - partially oppose by ______

D - direction favoured (backwards vs forwards)

COLOURS

How to remove reactants/products

Force a precipitate reaction (eg. add Ag+)

Temperature energy profile diagram

When something added/ removed - special consideration

even though partial opposition, does not go back to original

• Therefore, when discussing changes in conc. of each species → make sure you still say that [X] is higher than before (only partially opposed)

Temp equilibrium graph

1. Gradual increase/decrease → not as sudden

2. Kc1 is initial equilibrium → Kc2 is second equilibrium

3. See if concentration of products increases or decreases and discuss Kc value changes in terms of that

Pressure equilibrium graph

Although system partially opposes change, the overall colour will still be lighter/darker (original change), as will not return to initial concentrations

• KC VALUE DOESNT CHANGE

Catalyst equilibrium graph

DOES NOT AFFECT EQUILIBRIUM

Adding catalyst increases rate of both forward and backward reaction at same rate.

• reaction will reach equilibrium faster if catalyst added during adjustment period

Changing pressure shortcuts

• If volume of container increases, then pressure decreases, therefore, conc of reactants and products decreases

• If volume of container decreases, then pressure increases, therefore, conc of reactants and products increases

Changing temp shortcut

Temp increase = endo favoured

Temp decrease = exo favoured

Yield

AKA “extent of reaction”

• how much product is formed

Conflict (rate vs yield)

High temperatures favour the ROR, low temps favour yield in EXO reactions

Solution: use moderate temperature and add catalyst to increase rate

Conflict 2 (rate vs yield)

High pressure (bc. conc) favours rate of reaction and low pressures favour yield when forward reaction produces more particles

Solution: moderate pressure (Eg. 100kPa SLC) and add catalyst to increase rate

Describe change in conc vs change in moles

Conc: go by graph (is the flat line higher or lower than original flat line?)

Moles: go by direction (which direction is being favoured → more products or reactants?)

ICE Table to calc Kc of gases

1. Same process and can use c = n/v even if gas (no need for pv = nRT)

2. SHOW EQUILIBRIUM EXPRESSION BEFORE SUBBING IN VALUES

3. DON’T FORGET UNITS AND TEMP IF GIVEN

COD - Volume decrease

1. Decreasing the volume increases the pressure of gas mixture

2. System partially opposes by decreasing pressure so favours side with least number of particles.

3. Therefore, F/B reaction is favoured

When drawing pressure incr/decr - how long are the initial vertical lines?

If not given an amount just either double each line or half it

• it’s the ratios that matter

• eg. if volume doubles, pressure halves, line drops by half the height of the element.

Whenever colour given!

MUST MENTION COLOUR CHANGE AS PART OF COD

Whenever volumes mentioned in questions

check is it doubled/halved? - any special number that conc changes by

Why no pressure greater than 250atm (even though that's not good for rate or equilibrium)

If smth not good for rate or yield in industrial setting

• ALWAYS TALK ABOUT ECONOMICAL ASPECT

  • 1. Although higher pressure means higher yield at faster rate, it’s expensive to maintain high machinery etc.

  • 2. Therefore, more ECONOMICAL to use moderate pressure for moderate yield and rate

Why temp of only 400 degrees when lower means higher yield

CONFLICT

• higher temp = faster rate

• lower temp = higher yield

• therefore moderate temp = reasonable yield at acceptable rate

To draw effect of a catalyst on energy profile

Whenever it asks for Kc

UNITS + TEMP