6. The Rate and Extent of Chemical Change

Collision Theory (using bullet points - )

-Must collide with activation energy to be successful (if not particles bounce off each other with no successful collisions so no reaction occurs)

-RATE OF REACTION depends on two things: how frequently and forcefully particles collide.

-if frequency and forcefulness are changed, RATE OF REACTION will change

Collision Theory explanations: Label A, B, and C in terms of collision theory

*This is accurate for any fizzy reaction

A: many reacting particles, very frequent collisions

B: fewer reacting particles, less frequent collisions

C: no more reacting particles as one reactant has been used up

Label A, B and C (not collision theory)

*This is accurate for any fizzy reaction

A: Reaction fastest (steepest gradient)

B: Reaction slowing (less steep gradient)

C: Reaction stops

Rate of Reaction definition

the change in amount of substance per second

To find mean rate

read off graph and divide y/x

To find mean rate using a tangent

Draw tangent, turn into triangle, divide y/x

Gas Syringe Method definition

The gas syringe is used to collect and measure the amount of gas produced during a reaction from a conical flask.

Gas Syringe Method Advantages

Good Resolution, Gases are not released into the room.

Gas Syringe Method Monitoring Options

Either decide time and measure volume or decide volume and measure time or measure at intervals.

Mass Loss method definition

MEASURES GAS: Use scales to measure how much mass is lost with conical flask and cotton wool ball (only allows gas to escape, not liquid).

Mass Loss method advantages

Excellent resolution: masses measured to the nearest 0.01g

Mass Loss method disadvantages

Gases are released into the room

Mass Loss method monitoring options

Either choose time and measure mass loss or choose mass loss and measure time. Or measure at regular time intervals.

Monitoring Rate of Reactions that don’t produce gas: DEFINITION

If no gas is produced and reaction produces a precipitate or dark colour, time how long it takes for a cross on piece of paper to disappear.

Conical Flask

Precipitate

A solid formed by change in solution.

What factors affect rate

Surface Area, Concentration, Pressure, Temperature

Repeatable

I do it lots and get the same results

Reproducable

Others do it lots and get the same results

In the order Concentration, Surface Area, Pressure, Temperature how do these factors effect rate?

Concentration: Higher concentration, faster rate of reaction
Surface Area: Larger surface area, faster rate of reaction.
Pressure: Increased pressure, faster rate of reaction.

Temperature: Increased temperature increases rate of reaction.

How is pressure caused?

Gas particles colliding with container walls

When does a reaction stop?

When one reactant has been used up so there are no more collisions.

Independent Variable

What I change

Dependent Variable

What I observe

Controlled Variable

What I keep the same

Explain, in terms of particles, why rate of reaction increases with temperature.

-Temperature increase means particles have more energy
-Particles have activation energy
-More frequent collisions

-More forceful collisions

What does a Catalyst do?

Provides a new route for a reaction with lower activation energy so particles can collide more frequently

particles HAVE NOT got more energy!! ONLY

temperature increase can do this

Who are the best catalysts?

Transition metals (specifically manganese oxide) and their compounds

Advantages of using a catalyst + explanations

Reactions done at lower temperatures
-less non-renewable fossil fuels used

-less CO2 emissions, less global warming

-less SO2 emissions, less acid rain

-less NOx emissions, less acid rain

-less particulates, less global dimming and lung diseases

Sustainable

-less non-renewable sources of metal catalyst used

-less mining of metal catalyst, less habit destruction

Disadvantages of using a catalyst + explanations

Fill up landfill sites on disposal:
-they do not decompose/rot
-can contain toxic poisons that enter environment
Eventually stop working so mining of metal catalysts:
-expensive to mine/extract
-rare metals are very expensive to mine/extract

‘floating head reaction’
RP: Sodium Thiosulphate-HCl Reaction
Important information

-no gas produced in this reaction so we cannot use a gas syringe or a balance to monitor it
-A yellow precipitate of sulphur is produced (S(s))
-we measure time taken for the cross to dissapear

‘floating head reaction’
RP: Sodium Thiosulphate-HCl Reaction
What is it used for?

Any reaction that produces a precipitate or a dark colour

‘floating head reaction’
RP: Sodium Thiosulphate-HCl Reaction

Variables

IV: Concentration of ‘thio’
DV: Time for cross to disappear
CV: Concentration of HCl
Volume of HCl

Total volume of ‘thio’
Constant Temperature

‘floating head reaction’
RP: Sodium Thiosulphate-HCl Reaction
Accuracy

To improve accuracy
-use pipette or burette to measure vol

-use light sensor connected to a data logger to measure when light intensity drops to a certain value (as precipitate forms)
-use water bath set at fixed temp to keep contents in a flask at a constant temperature

Calcium Carbonate (marble) or Magnesium HCl reaction
Methods

-Measure gas volume with a gas syringe

-Measure mass loss using a balance

Calcium Carbonate (marble) or Magnesium HCl reaction
Variables

IV: Concentration of HCl
DV: Time for cross to disappear
CV: Total voume of HCl
Same mass of solid
Constant room temperature

Calcium Carbonate (marble) or Magnesium HCl reaction
Accuracy

To improve accuracy:
-use pipette or burette to measure volume

-use water bath set at fixed temp to keep contents at a constant temp.

How to prove a Catalyst can not be used up

-weigh catalyst

-filter mix

-dry catalyst

-reweigh cat.

Endothermic

reactants absorb heat energy from the surroundings to form products

Exothermic

releases heat, causing the temperature of the immediate surroundings to rise

Volume/Mass Loss vs time graph

Lower Concentration: Slower (less steep) and less gas produced
Halving concentration: halves volume/ mass gas produced

Concentration mathematical link

-Rate directly proportional to concentration (rate ∝ concentration)
e.g if doubling conc doubles rate
twice as many reacting particles
twice as many frequent collisions

Temperature mathematical link

-Rate is NOT directly proportional to temperature
-Rate approximately doubles per 10 degree rise

More diluted solution means

Less gas produced (less steep line, finishes later)

Change in surface area

Does not change how much gas is produced, just gives a much faster reaction

Surface area mathematical link

-rate is directly proportional to surface area

-e.g doubling surface area doubles rate
twice as many reacting solid particles available

twice as many frequent collisions

Pressure mathematical link

-rate directly proportional to pressure
-e.g doubling pressure doubles rate
same number of particles in half the volume
twice as many frequent collisions

Straight line through origin

Directly proportional

Change in temperature

Does not change amount of gas produced, just gives a faster reaction

Change in temperature graph

Not straight line through the origin:

Rate is not directly proportional to temperature
Pattern is called ‘exponential’

‘exponential’

increase one thing by a certain amount each time (eg 10 degrees)

other ‘thing’ increases by more and more each time
(eg by 2, then 7, then 25 etc)

The most common reactions that produce gas

Adding metal to an acid to produce H2 gas
for example:

Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen

Adding a carbonate to an acid to produce CO2 gas
for example:
Calcium Carbonate + Hydrochloric Acid → Calcium Chloride + Carbon Dioxide + Water

The rate of any reaction that produces a gas

can be measured by mass loss or gas volume methods

A successful collision means

bonds are broken in reactants

the particles separate from each other

they combine differently

new bonds form between them to make the products

Making catalysts even better

-Powdered Catalysts: Large Surface Area, Reactions go even faster
-Nanocatalysts: Huge surface area to volume ratio (huge surface area compared to volume)

Catalysts and rate

Hydrogen Peroxide solution (aq) breaks down very slowly. Can be sped up using a catalyst.

What factors affect rate

Surface Area, Concentration, Pressure, Temperature

Repeatable

I do it lots and get the same results

Reproducable

Others do it lots and get the same results

Collision Theory

Many reacting particles, more frequent collisions

Concentration

Higher concentration, faster rate of reaction.

Surface Area

Larger surface area, faster rate of reaction.

Pressure

Increased pressure, faster rate of reaction.

How is pressure caused?

Gas particles colliding with container walls

Temperature

Increased temperature, faster rate of reaction.

GRAPHS: Fast (steep gradient)

Many reacting particles, very frequent collisions

GRAPHS: Slower (less steep gradient)

Fewer reacting particles, less frequent collisions.

When does a reaction stop?

When one reactant has been used up so there are no more collisions.

Gas Syringe Method definition

The gas syringe is used to collect and measure the amount of gas produced during a reaction from a conical flask.

Gas Syringe Method Advantages

Good Resolution, Gases are not released into the room.

Gas Syringe Method Disadvantages

Very fast reaction could make the plunger shoot out and break.

Gas Syringe Method Monitoring Options

Either decide time and measure volume or decide volume and measure time or measure at intervals.

Mass Loss method definition

MEASURES GAS: Use scales to measure how much mass is lost with conical flask and cotton wool ball (only allows gas to escape, not liquid).

Mass Loss method advantages

Excellent resolution: masses measured to the nearest 0.01g

Mass Loss method disadvantages

Gases are released into the room

Mass Loss method monitoring options

Either choose time and measure mass loss or choose mass loss and measure time. Or measure at regular time intervals.

Monitoring Rate of Reactions that don’t produce gas: DEFINITION

If no gas is produced and reaction produces a precipitate or dark colour, time how long it takes for a cross on piece of paper to disappear.

Conical Flask

Precipitate

A solid formed by change in solution.

Magnesium + Hydrochloric Acid →

Mg(s) + 2HCl(aq) →

Magnesium Chloride + Hydrogen

MgCl2(aq) + H2(g)

Magnesium + Sulphuric Acid →

Mg(s) + H2SO4(aq) →

Magnesium Sulphate + hydrogen

MgSO4(aq) + H2(g)

Calcium Carbonate + Hyrdrochloric Acid →

CaCO3(s) + 2HCl(aq) →

Calcium Chloride + Carbon Dioxide + Water

CaCl2(aq) + CO2(g) + H2O(l)

Collision Theory Definition

For reactions to occur, particles must collide successfully.

What is a successful collision?

Particles must collide, with sufficient energy, and with the correct orientation.

Activation Energy (E with subscript A)

Minimum energy particles must have to react

Rate of Reaction

How frequently particles collide and how hard they hit each other (collision energy)

To find mean rate

Read off graph at time given and then divide y/x

To find rate using a tangent

Draw tangent, turn into triangle, divide y/x

Reversible reaction

products can react to produce original reactants

direction can be changed by changing conditions

same amount of energy transferred each way (one way energy lost, one gained)

If reaction is exothermic one way..

endothermic the other

Equilibrium

When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.

Le Chatelier’s law

an equilibrium will react to oppose changes made to it

if substance added to equilibrium,

if we add something to one side, it gets used up

so more of the other side forms (the opposite side is favoured)

new equilibrium established

Equilibrium: Adding extra A or B

-A or B is used up

-forward reaction is favoured so more C, D & E will form

(equilibrium has moved to RHS)

-a new equilibrium is established

Equilibrium: adding extra C, D, E

-C/D/E is used up

-reverse reaction is favoured so more A & B will form

(equilibrium has moved to LHS)

-a new equilibrium is established

removing substances from an equilibrium

if we remove something from one side, it is brought back

so the side we remove from is favoured

equilibrium: removing A or B

-reverse reaction (LHS) is favoured so C, D & E react

-to restore A/B (the equilibrium has moved to LHS)

-a new equilibrium is established

equilibrium: removing C, D or E

-forward reaction (RHS) is favoured so A & B react

-to restore C/D/E (the equilibrium has moved to RHS)

-a new equilibrium is established

changing temperature of an equilibrium

cool an equilibrium: more ‘hot side’ is made

warm an equilibrium up: more ‘cold side’ is made

The forward reaction is exothermic (Z gives out heat)

increase temperature

-increasing temperature favours endothermic (reverse) reaction (LHS)

-more X &Y (less Z) forms (equilibrium moved to LHS)

-a new equilibrium is established