C5.1 Monitoring Chemical Reactions

Question 1

Theoretical yield

Answer 1

Maximum mass it is possible to make from a given mass of reactants

Mass limiting reactant (g)
———————————— x sum of Mr for products
Limiting reactant Mr

Question 2

Percentage yield

Answer 2

Actual yield
———————— x 100
Theoretical yield

Answer is a percentage
Cannot be more than 100%

Question 3

What affects percentage yield

Answer 3

Varies from 0% to 100%.
Less than 100% because:
Reactants can react in a different way than expected
Reaction may not go to completion
Some of the reactants present do not react
Often happens in reversible reactions
You may lose some of the product when you separate it from the reaction mixture and purify it, such as filtration

Question 4

Atom economy

Answer 4

Sum of Mr of desired products
——————————————— x 100
Sum of Mr of all products

Answer as a percentage
Cannot be more than 100%

Percentage of reactants turned into useful products

Question 5

Factors that determine how you make a substance

Answer 5

Manufacturers in the chemical industry must profit from producing substances or they go out of business
Chemical engineers must consider many factors choosing a reaction pathway:
Yield of product
Atom economy of the reaction
Usefulness of by products
Rate of reaction
Equilibrium position if is a reversible reaction

Question 6

What can you do with byproducts

Answer 6

Byproduct = substance formed in a reaction in addition to desired product
Can sometimes be useful and sold improving atom economy

Question 7

1 litre in decimetres

Answer 7

1

Question 8

1 millilitre in cm3

Answer 8

1

Question 9

Convert dm³ to cm³

Answer 9

Multiply by 1000

Question 10

Calculate concentrations

Answer 10

. Mass of solute (g)
Concentration in g/dm3 = —————————————
Volume of solution (dm³)
Amount of solute (mol)
Concentration in mol/dm3 = ————————————
Volume of solution (dm³)

Question 11

Titration

Answer 11

Add a KNOWN volume of alkali in a conical flask and few drops of SINGLE indicator like phenolphthalein
Then add acid to flask from a burette bit by bit

At end point when the alkali is neutralised and the indicator first changes colour stop adding acid

Question 12

Standard solution

Answer 12

Solution with known accurate concentration

Question 13

Preparation for titration

Answer 13

Use measuring cylinder to measure 25 cm³ of alkali solution for the conical flask, but a volumetric pipette is more accurate
(Need a pipette filler to use a volumetric pipette safely)
Burette must be clamped vertically so you obtain accurate readings
When you have filled the burette with acid, and added a few drops of indicator to the alkali in the conical flask, you are ready
Eye protection should be worn

Question 14

Doing a titration

Answer 14

Take an initial burette reading and a final burette reading at the end-point
Difference between these two readings is the titre is volume of acid added to the alkali in the flask
Record your readings to two decimal places, ending in 0 if the bottom of the meniscus is on a burette line, or 5 if the meniscus is between two lines

To obtain a repeatable tire you need to use following techniques:
Swirl the flask during a titration to mix its contents
First titration is usually a rough run, done quickly so that you get an idea of what the titre is
In later runs can quickly add the acid to within a few cm³ of the rough titre, then add the acid drop by drop

Repeat titration until you obtain at least two concordant titres: titres that are within 0.1 cm³ of each other
allows you to calculate a mean tire that has high precision

Eye protection should be worn

Question 15

How you can use results from a titration

Answer 15

When you have carried out a titration, you will know:
The two reactants used (an acid and an alkali) the volume and concentration of one of the reactants the volume, but not the concentration, of the other reactant
With this information, and the balanced equation, you can calculate the unknown concentration

Question 16

Titration calculation

Answer 16

Example: In a titration, 25.0 cm3 of 0.040 mol/dm3 barium hydroxide solution reacted with 21.6 cm3 of hydrochloric acid. Find the concentration of the hydrochloric acid in mol/dm3.
Balanced equation: Ba(OH)₂(aq) + 2HCl(aq) → BaCl₂(aq) + 2H2O(l)

Create table of information: (information already known from the question in 𝕥𝕙𝕚𝕤 font) then fill it in using information.
Ba(OH)₂ 2HCl
Volume(cm³) 𝟚𝟝.𝟘 𝟚𝟙.𝟞

Volume(dm³) 25.0 / 1000 = 0.025 21.6 / 1000 = 0.0216

Concentration (mol/dm³) 𝟘.𝟘𝟜𝟘 Moles = concentration x volume
0.002 = concentration x 0.0216
0̳.̳0̳9̳̳̇2̳̳̇5̳̳̇ ̳m̳o̳l̳ ̳/̳ ̳d̳m̳³̳ ̳=̳ ̳c̳o̳n̳c̳e̳n̳t̳r̳a̳t̳i̳o̳n̳ ̳

Moles (in titration N = 0.04 x 0.025 1 : 2 ratio of Ba(OH)₂ : 2HCl so,
questions use N = 0.001 mol 0.002 mol
formula:
N = conc x vol

Question 17

Molar volume

Answer 17

1 mole of any substance in gas state occupies the same volume at the same temperature and pressure
At room temperature and pressure: 24 dm³/mol
(24 000 cm³/mol)

Question 18

Find moles of gas

Answer 18

Moles of gas = Volume of gas (dm³) / 24 dm³
Moles of gas = Volume of gas (cm³) / 24 cm³

Moles of gas = volume / molar volume

Question 19

1.31g of zinc reacts with excess sulfuric acid:
Zn(s) + H2SO4 (aq) -> ZnSO4 (aq) + H2 (g)
Calculate the volume of hydrogen produced at room temperature

Answer 19

Step 1: Calculate the amount of limiting product:
Ar = 65.4

Moles of zinc = mass (g) / Ar
= 1.31 g / 65.4 g/mol
= 0.02 mol

Step 2: Use balanced equation to calculate the amount of hydrogen produced
1 mol Zn produces 1 mol H2 so 0.02 mol of Zn produces 0.0200 mol of H2

Step 3: Use your answer to Step 2, and Vm to calculate the volume of hydrogen:
volume in dm3 = amount in mol × 24 dm³/mol
= 0.0200 mol × 24 dm³/mol = 0.48 dm³

Question 20

Measuring volume of a gas

Answer 20

1 Fill the measuring cylinder with water and while keeping its mouth underwater turn upside down
2 Clamp the measuring cylinder securely
3 Mix the reactants in a conical flask and attach a delivery tube
4 Measure the volume of gas produced in the reaction by recording the end reading on the measuring cylinder

In this investigation, you need to keep the mouth of the measuring cylinder underwater to stop air getting in or water getting out
Either of these would affect the volume reading on the measuring cylinder
Eye protection should be worn throughout this practical

Or just use a gas syringe

Question 21

Titre

Answer 21

Volume of acid added to an alkali,
to fully neutralise it

Question 22

Mol/dm³ in g/dm³

Answer 22

Mol/ dm³ x Mr —> g/dm³