C5.1 Monitoring Chemical Reactions Flashcards
Theoretical yield
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
Percentage yield
Actual yield
———————— x 100
Theoretical yield
Answer is a percentage
Cannot be more than 100%
What affects percentage yield
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
Atom economy
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
Factors that determine how you make a substance
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
What can you do with byproducts
Byproduct = substance formed in a reaction in addition to desired product
Can sometimes be useful and sold improving atom economy
1 litre in decimetres
1
1 millilitre in cm3
1
Convert dm³ to cm³
Multiply by 1000
Calculate concentrations
. Mass of solute (g)
Concentration in g/dm3 = —————————————
Volume of solution (dm³)
Amount of solute (mol)
Concentration in mol/dm3 = ————————————
Volume of solution (dm³)
Titration
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
Standard solution
Solution with known accurate concentration
Preparation for titration
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
Doing a titration
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
How you can use results from a titration
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
Titration calculation
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
Molar volume
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)
Find moles of gas
Moles of gas = Volume of gas (dm³) / 24 dm³
Moles of gas = Volume of gas (cm³) / 24 cm³
Moles of gas = volume / molar volume
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
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
Measuring volume of a gas
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
Titre
Volume of acid added to an alkali,
to fully neutralise it
Mol/dm³ in g/dm³
Mol/ dm³ x Mr —> g/dm³
