C5.1 - Monitoring Chemical Reactions Flashcards
What is the difference between the theoretical yield of a product and the yield of a product?
The yield of a product is the mass of the product made in a chemical process whereas the theoretical yield is the maximum mass it is possible to make from a given mass of reactants.
What three things do you need to know to calculate theoretical yield?
- The mass of the limiting reactant.
- The relative formula masses of reactants and
products. - The balanced equation for the reaction.
How do you calculate the theoretical yield of a product?
- Work out the number of moles of the limiting
reactant. - Work out the number of moles of the product.
- Work out the mass of the product
OR
Theoretical yield = (mass of limiting reactant/sum of Mr of limiting reactant) x Sum of Mr for the product
What is the actual yield and how do you calculate percentage yield?
- The actual yield is the mass of product you
actually make in a chemical reaction. - Percentage yield = (Actual yield / Theoretical
yield) x 100
What factors affect percentage yield?
- The reactants may react in a different way than
expected. - The reaction may not go into completion - some
reactants present may not react - (often in
reversible reactions). - Some of the product may be lost when you
separate it from the reaction mixture and purify it.
What is the atom economy for a reaction and how do you calculate it?
- Atom economy is a measure of the proportion
of atoms in the reactants that end up in the
desired product. - Atom Economy = (Sum of Mr of desired product
/ Sum of Mr of all products) x 100
What are the factors to be considered when choosing a reaction pathway?
- Yield of the product.
- Atom economy of the reaction.
- Usefulness or otherwise of by-products.
- Rate of the reaction.
- Equilibrium position (if it a reversible reaction).
What is a by-product can you do with by-products?
- A by-product is a substance formed in a reaction in
addition to the desired product. - Can be sold - Improves the atom economy of the
process as the by-product becomes a desirable
product.
How do you convert between cm3 and dm3?
- Divide by 1000 to convert from cm3 to dm3.
- Multiply by 1000 to convert from dm3 to cm3.
What is the equation to calculate concentration?
concentration in g/dm3 = mass of solute in g / volume of solution in dm3
concentration in mol/dm3 = mass of solute in mol / volume of solution in dm3
What an easy way to convert between mol/dm3 to g/dm3?
Multiply by the molar mass and vice versa if you wanted to convert the other way.
What happens in a titration?
- Known volume and concentration of alkali is
added to a conical flask. - Indicator added such as phenolphthalein which is
pink in alkaline and colourless in acidic solutions. - Acid added from a burette (a long graduated glass
tube with a tap at the bottom). - Acid is stopped being added once the indicator
changes colour indicating the end point.
How do you measure out the acid or alkali to put inside a conical flask for a titration and why?
You use a volumetric pipette as it is accurate.
How do you obtain a repeatable titre in a titration?
- Swirl the flask during a titration to mix its contents.
- Your first titration is usually a rough run, done
quickly so that you get an idea of what the titre is. - Later runs you can add acid quickly until you within
get a few cm3 of the rough titre then add the acid
drop by drop.
- Repeat the titration until you obtain at least two - concordant titres (titres that are within 0.10cm3.
- You can then calculate a mean with higher
precision and repeat without the indicator.
How should you record your readings of your titres?
- To 2 decimal places.
- Record from the bottom of the meniscus.
