C5 - Monitoring and Controlling Chemical Reactions

Moles of substance equation

Moles = mass / relative formula mass
n = m/Mr

Avogardo's Law

One mole of any gas occupies the same volume under the same temperature (rt) and pressure conditions (1 atmosphere)

What is rt

20 degrees C

Molar Volume of all gases at rtp

24 dm3/mol

Mole of gas (with volume) equation

Moles = volume (cm^3) or (dm^3) / 24,000 (cm^3 mol^-1) or 24 (dm^3 mol^-1)
n = v/24

Concentration of a solution equation

Concentration = moles/ (v/1000) —-> if volume is in cm^3
C = n/v
ALSO CAN SWAP MOL WITH MASS AND IT WORKS THE SAME BUT UNIT OF CONC WOULD BE g/dm3 NOT mol/dm3

How to change conc in moldm-3 to g/dm3

Multiply conc in mol/dm3 by Mr

Example: Make 250cm3 of a 314g/dm3 solution of sodium chloride

- First work out how many grams of solute you need using the formula:
- m = C x V
So 314 x (250/1000) = 78.5)
- Now weigh out this mass — put an empty beaker on a mass balance and reset it to zero, then add the correct mass.
- Add deionised water to the beaker and stir until all the solute has dissolved.
- use a funnel to carefully pour the solution into a volumetric flask — make sure it's the right size for the volume you're making.
- Rinse the beaker and stirring rod with deionised water and add that to the flask too. (This makes sure there's no solute clinging to the beaker or rod)
- Top the flask up to the correct volume (250 cm3) with more deionised water.
- Make sure the bottom of the meniscus is on the line.
- When you get close to the line, use a dropping pipette to add the last bit of water one drop at a time.
- add a bung to the flask and turn it upside down a few times to mix the solution.

Titration method

1)Use a pipette and pipette filler to add 25 cm3 of dilute sodium hydroxide solution to a clean conical flask.
2)Add a few drops of phenolphthalein indicator and put the conical flask on a white tile.
3)Fill the burette with dilute hydrochloric acid and note the starting volume.
4)Slowly add the acid from the burette to the conical flask, swirling to mix.
5)Stop adding the acid when the end-point is reached (when the colour first permanently changes from pink to colourless). Note the final volume reading.
6)Repeat steps 1 to 5 until you get concordant titres

4 steps to get accurate results in titration

1) Make sure the burette is vertical
2) take the readings from the bottom of the meniscus.
3) rinse the inside of the flask with distilled water
4) add the acid drop by drop.)

Titre meaning

Volume of one reactant needed to react completely with the other reactant in a titration.

Concordant titres meaning

Titres within 0.20 cm³ (or sometimes 0.10 cm³) of each other

What is a titration

the process of adding a known amount of solution of known concentration to determine the concentration of another solution

Phenolphtalein colours

Colourless —> colourless —> pink

Universal indicator colours

Red —> green —> deep purple / blue

Litmus paper colours

Red to blue = alkali
Blue to red = acid

Methyl orange colours

red —> orange —> yellow

Factors that influence rates

- temperature
- surface area
- conc of solutions
- pressure of gases
- catalysts

Rate definition

The change in the concentration of reactant over a given time

Rate equation

Change in conc / time

Unit for rate of reaction

mol dm-3 / s
Or
Cm3/min

What is the study of rates called

Kinetics

What to use to represent conc

- square brackets

Concentration definition

- the number of particles per unit volume

When is a reaction fastest

- At the start
- more frequent collisions

How to find rate using a graph AFTER a certain amount of time vs AT a certain amount of time

For AFTER: you find that time on the x axis and trace up to the y value then do y / x

For At: you find that time on the curve and draw a tangent then calculate the gradient of that tangent

How to calculate the rate of a reaction on a curved graph

1) draw a tangent to the curve
2) Calculate gradient

What does increasing conc do to reaction rate

- increased number of particles in a per unit volume
- more frequent collisions
- rate increases

How does increasing pressure change reaction rate

- increased pressure decreases volume
- increased number of particles per unit volume
- more frequent collisions
- rate increases

How does increasing SA change reaction rate

- more surfaces from reagent to react with
- more frequent collision
- rate increases

What does increasing temp do to reaction rate

- Increases KE of particles

- more particles have energy that is greater than Activation energy so there are more successful collisions per second

- ALSO particles move faster so more frequent collisions

- rate increases

What does catalyst do to reaction rate

- provides an alternative route with a lower Ea

- more particles now have a greater energy than the lowered Ea

- more successful collisions per second

- rate increases

How to check that a catalyst was not used up

1) measure it before
2) filter to get catalyst
3) dry in warm oven
4) measure again
- if rate increases but catalyst is unchanged then it is a catalyst

type of catalyst

enzyme/ manganese oxide

Unit of rate of reaction

Mol s^-1

Avogardo's constant definition

Number of atoms in 12g of Carbon 12

Number of particles equation

Number of moles x Avogadro's constant x (number of atoms)
n x L

Mass of one atom equation

Atomic mass / Avogadros constant
Ar / L

Atom economy equation

(Mr of desired product / sum of Mr of all products) x100

% yield equation

(actual mass / theoretical mass) x 100

What is the actual mass of a product

the mass of that product made / formed / obtained / produced in a chemical reaction

Theoretical mass / yield defintion

the maximum mass it is possible to make from a given mass of reactants

What effects % yield ( makes mass less than expected)

- The reaction may be at equilibrium (reversible reactions ) and may not go to completion
- Other side reaction may take place leading to by - products
- The reactants may not be pure . Some of the reactants and products may be left behind in the apparatus used in the experiment . - Separation and purification may result in the loss of some of the product .
(such as filtration , crystalisation , distillation)

Atom economy vs percentage yield

- Atom economy gives information about the efficiency of the reaction whereas percentage yield gives information about how much product is made
- A reaction may have a high percentage yield but a low atom economy .

Addition reactions vs substitution reactons (atom economy)

- Addition reactions have an atom economy of 100 % (only 1 product), whereas substitution reactions are less efficient (not 100%)

benefits of developing chemical processes with a high atom economy

- fewer waste materials
- The higher the atom economy the less waste products

What is atom economy and what does a high AE mean

- a measure of how much of the products are useful
- A high atom economy means that there is less waste this means the process is MORE SUSTAINABLE .

What does a low AE mean

- reaction uses up resources very quickly and make a lot of waste materials that need to be disposed of in some way
- very unsustainable (bc the raw materials will run out and the waste products need to go somewhere)
- not very profitable (raw materials are expensive to buy and waste is expensive to get rid of)
-

What is yield

The amount of product you get from a reaction

What is percentage yield

The overall success of an experiment

What is a catalyst

A substance that increases the rate of a reaction without being used up and lowers Ea and alters the reaction pathway

How to measure rate of reaction

- monitoring the decrease in conc of reactants
- monitoring the increase of conc in products

4 ways to measure the rate of reaction in an experiment

- volume of gas produced
- change in mass
- colour change
- precipitate

Precipitation experiment for rates

- mix 2 see through reactant solutions together in a concical flask and put the flask on top of a white pice of paper with a big black x in the middle
- time how long it takes for the x to disappear as the precipitation forms
- the faster the x disappears, the faster the reaction

Change in mass experiment for rates (when gas is given off)

- carry out the mixture of solutions on a mass balance and observe how quickly the mass drops
- the faster the mass drops, the faster the reaction bc the faster that gas is produced

Why is cotton wool placed into the mouth of the conical flask during the experiment?

To allow gas to escape but stops any liquid or solid escaping the conical flask.

Volume of Gas produced experimente for rates

- attach a gas syringe through the bung on a conical flask and push the syringe all the way in to 0
- gas produced will push the syringe out to measure the volume
- the higher volume in a given time, the faster the reaction

Volume of Gas produced experimente for rates: why is it important to use the right sized gas syringe

- if the reaction is too vigorous it can blow the plunger out of the end of the syringe

Problems with using a measuring cylinder filled with water as an experiment for rates

- if the gas is very soluble in water them some of it might dissolve in the water so the correct volume of water wont be displaced
- it can be difficult to completely fill the cylinder with water and turn it upside down
- - some gas is lost when the bung is removed to mix reactants

Improvements for using a measuring cylinder filled with water as an experiment for rates

- use a bung with 2 holes to add a funnel to drop reactant in so less gas is lost
- put a vile in the flask and tip over the flask to start the reaction
- use a gas syringe

Advantage of using a gas syringe for experiment on rates

- it monitors the reaction continuously without interfering

DisAdvantage of using a gas syringe for experiment on rates

- the syringe cans tick and give unreliable results
- the syringe might be too small

Advantage of using mass balance for experiment on rates

- it monitors the reaction continuously without interfering

DisAdvantage of using a mass balance for experiment on rates

- need a 3 d.p balance to be precise
- could release harmful gases into the room

How to show a faster rate on a graph

- steeper gradient

Rate relationship with time

-inversely proportional
- as rate becomes faster, there is less time required for the reaction to go on

What is an irreversible reaction

A reaction that stops when all of the limiting reagent has been used up and the products can not react again to form the reactant

What is a reversible reaction

a reaction where the reactants form products, which react together to give the reactants back

Open system meaning

- where matter and energy are allowed in or out

Matter meaning in a reaction

- the reactants and products

Example of reactions catalysed by enzymes

- respiration
- photosynthesis
- protein synthesis

How to show a reaction is reversible

- two way arrowsl

What is a closed system

- where matter can not enter or leave but energy can

What is dynamic equilibrium

- a closed system in which the concentration of reactants and products remain constant AND the rare if forward reaction is equal to the rate of backwards reaction

How to show dynamic Eqm on a graph of concentration against time

- the point where both lines for products and reactants begin to level off is the dynamic eqm

How to show dynamic Eqm on a graph of rate against time

- the point where the lines for rate of fwd reaction and bwd reaction meet is the dynamic equilibrium

What us Le Chatelier's principle?

- when a reaction is at dynamic equilibrium is subject to a change in concentration, temp or pressure, the position of eqm will move to counteract that change

Factors affecting eqm position

- temperature
- pressure
- concentration

Why do catalysts not affect eqm position

- it would increase the rate of forward reaction and backwards reaction equally

Why can equilibrium only be reached in a closed system?

Because none of the matter can escape

What does it mean if the eqm position is more to the right

-lots of product and not much reactant
- conc of products is greater that conc of reactant

What does it mean if the eqm position is more to the left

- lots of reactant and not much product
- conc of reactants is greater that conc of products

How does a positive enthalpy affect eqm position (increasing temperature)

- if enthalpy is positive then the forward reaction is endothermic
- increasing temp will shift eqm position in endothermic direction to the LHS
- less product is made

How does a negative enthalpy affect eqm position (increasing temperature)

if enthalpy is negative then the forward reaction is exothermic
- increasing temp will shift eqm position in endothermic direction to the LHS
- more reactants made

How does a positive enthalpy affect eqm position (decreasing temperature)

- if enthalpy is positive then the forward reaction is endothermic
- decreasing temp will shift eqm position in exothermic direction to the RHS
- more product is made

How does a negative enthalpy affect eqm position (decreasing temperature)

if enthalpy is negative then the forward reaction is exothermic
- decreasing temp will shift eqm position in exodothermic direction to the RHS
- less product is made

Why does increasing temp shift eqm position to endothermic direction

- to absorb extra heat

How to predict eqm position with increasing pressure (only for gases)

- count how many gaseous molecules are on each side of reaction using the big coefficient at the front of each of the molecules
- If you increase pressure, eqm position will shift to side with fewer gaseous molecules to reduce pressure

(either RHS or LHS)

How to predict eqm position with decreasing pressure (only for gases)

- count how many gaseous molecules are on each side of reaction using the big coefficient at the front of each of the molecules
- If you decrease pressure, eqm position will shift to side with more gaseous molecules to increase pressure

(either RHS or LHS)

Why does decreasing temp shift eqm position to exothermic direction

- to produce more heat

Effect on eqm position if you increase conc

- eqm position will shift to the opposite side of the reaction to decrease the added concentration

How to predict eqm position for change in pressure when there are equal numbers of gaseous molecules

- change in pressure will have no effect on eqm position as there are equal numbers of gaseous molecules on each side of reaction

Effect on eqm position if you decrease conc

- eqm position will shift to the opposite side of the reaction to increase the removed concentration