SC14 Flashcards
SC14a
1) State what is meant by the theoretical yield of a reaction.
2) State what is meant by the actual yield of a reaction.
3) What is the percentage yield of a reaction and how do you calculate it?
1) Theoretical yield is the maximum possible mass of a product that a chemical reaction can make.
2) The actual yield is the amount of product obtained when your carry out an experiment.
The actual yield is always less than the theoretical yield of a reaction.
3) The percentage yield shows how much product is obtained compared to the maximum possible mass. The equation for percentage yield is: (Actual yield / theoretical yield) x 100
SC14b
1) Recall the formula for calculating atom economy.
2) Explain how atom economy and yield determine the choice of reaction pathway.
3) Explain how the usefulness of by-products determines the choice of reaction pathway.
1) Atom economy = (relative formula mass of the useful product / sum of relative formula mass of all the reactants) x 100%
2) A reaction pathway describes the sequence of reactions needed to produce a desired product. A percentage yield gives no indication of the amount of waste products. A reaction can have a high percentage yield but a low atom economy, meaning that by-products are formed.
Reactions with low atom economies use more raw materials to reduce a certain amount of product, as less of the reactants are converted into useful products.
3) The reaction pathway is chosen where the by-products have the highest use, which increases the the atom economy. One way to improve the atom economy is to find uses for the by-products of the reaction.
Another way to improve the atom economy is to use a more efficient reaction.
Reactions with low atom economies produce more waste materials as more reactants are converted into by-products.
SC14c
1) State the meaning of the term concentration.
2) How do you calculate concentration in g dm⁻³.
3) How do you calculate concentration in mol dm⁻³.
1) The concentration of a solution is the amount of solute dissolved in a stated volume of solution. The units of concentration are usual written as g dm-3. 1 dm3 is the same volume as 1 litre or 1000 cm3.
2) Concentration in g dm⁻³ = mass of solute in g / volume of solution in dm³
3) Concentration in mol/dm3 = number of moles (mol) / volume (dm3)
SC14e
1) Describe what is meant by the molar volume of a gas.
2) Use the molar volume in calculations involving solids and gases in reactions.
3) Recall Avogadro’s law
4) How would you use Avogadro’s law to calculate the volumes of reacting gases.
1) The molar gas volume is the volume occupied by one mole of molecules of any gas. It is 24 dm3 or 24000 cm3 at room temperature and pressure (rtp).
2) To calculate to amount of gas, use this formula: Amount of gas (mol) = volume of gas (dm3)/ molar volume, or volume of gas (dm3) = amount of gas (mol) x 24 (at room temperature and pressure)
3) Avogadro’s law states that, if the temperature and pressure are the same, equal volume of different gases contain an equal number of molecules.
4) First calculate the moles of the substance with a given volume. Then work out the mole ratio, and use this mole ratio of the molar value. Finally, substitute the moles back into the equation to find the volume.
SC14a
1) Describe some reasons why the actual yield is less than the theoretical yield of a reaction.
2) Why is it desirable to obtain as high a percentage yield as possible?
1) The actual yield is always less than the theoretical yield of a reaction. The reaction could be reversible so may not have gone to completion, the products may have been lost during the reaction, or there may have been other unwanted reactions.
2) To reduce waste, to increase sustainability and to reduce cost.
SC14d
1) Convert concentration in g dm⁻³ into concentration in mol dm⁻³.
2) Convert concentration in mol dm⁻³ into concentration in g dm⁻³.
3) What is the equation involving concentration in moles, relative mass and volume?
1) Concentration in mol dm⁻³ = concentration in g dm⁻³ / relative formula mass of solute
2) concentration in g dm⁻³ = relative formula mass of solute x concentration in mol dm⁻³
3) Concentration in mol dm⁻³ = relative formula mass of solute x volume of solution in dm³
SC14d - Core Practical
1) What is the aim for the acid-alkali titration?
2) What is the method for the acid-alkali titration?
1) Carry out an acid-alkali titration, using a burette, a pipette and a suitable indicator.
2) Method
A. Rinse a burette with hydrochloric acid, then fill the burette with the acid, making sure the jet below the tap is also full.
B. Record the initial volume of acid in the burette.
C. Rinse a pipette with sodium hydroxide solution, then fill the pipette to the 25.0 cm3 mark and empty the solution into a conical flask.
D. Add a few drops of methyl orange indicator to the flask and place the flask on a white tile under the burette.
E. Add the acid to the sodium hydroxide solution while swirling the flask.
F. When the indicator starts to change colour, rinse the tip of the burette and the sides of the flask with a small amount of distilled water from a wash bottle to ensure that all the acid is in the mixture, then add the acid drop by drop until the end-point is reached.
G. Record the final volume of acid in the burette.
H. Repeat the experiment, apart from the initial rinsing of the burette and pipette, until concordant results are obtained.
SC14d - Core Practical
1) What are the safety precautions for the titration core practical?
2) Explain the purpose for carrying out a ‘rough’ first titration
3) State what is meant by the ‘end-point’ of the titration
1) -Eye protection must be worn
- Sulfur hydroxide is an irritant
- Hydrochloric acid is an irritant
2) A rough titration saves time, and allows you to find out the volume that you should slow down and add drop by drop in repeat titrations.
3) The end point of a titration is when the acid or alkali in the conical flask has been neutralised.
SC14d - Core Practical
1) Describe the effect of acids and alkalis of the 3 common indicators: blue litmus paper, methyl orange and phenolphthalein
2) What point do you measure from in the burette of a titration?
1) Blue litmus paper: acidic - red, neutral - purple, alkaline - blue
Methyl orange: acidic - red, neutral - yellow, alkaline - yellow
Phenolphthalein: acidic - colourless, neutral - colourless, alkaline - pink
2) The bottom of the curve is known as the meniscus and this point is what you should use to take your measurements.
SC14d - Core Practical
1) Calculate the number of moles of solute in a given volume of solution.
2) How would you deduce the mole ratio of acid to alkali from a balanced equation?
1) Number of moles of solute = concentration in mol dm-3 x volume of solution in dm3
2) The mole ratio is the ratio of moles in the substances in a balanced equation. The mole ratio is given by the big numbers in front of the compound or molecule in the equation. Eg. The mole ratio of 2HCl to 3NaOH will be 2:3
SC14d - Core Practical
1) How would you calculate the concentration of a solution using the results of an acid-alkali titration.
2) How would you calculate the volume of solution required in an acid-alkali titration, given the concentrations of both the acid and the alkali.
3) What are the products of a neutralisation reaction?
1) First, calculate the number of mole of the solution of know volume and concentration. The, use the balanced equation to work out the umber of moles of alkali that reaches. Finally, calculate the concentration of sodium hydroxide solution.
2) First, calculate the number of moles of the solution of known volume and concentration. Then, use the balanced equation and mole ratio to work out the number of moles of acid that reacted. Finally, calculate the volume of solution needed.
3) An acid and alkali will neutralise each other and produce a salt and water. This is called a neutralisation reaction.
SC14d - Core Practical
1) Why should the burette and pipette be rinsed before they are filled?
2) What should you do after filling the burette with acid and before taking the initial reading?
3) Why is universal indicator not suitable for a titration
1) To remove any water or other solution left in them from previous use.
2) Make sure that the section below the tap is filled with acid, make sure there are no air bubbles in the burette and check that the tap is not leaking.
3) When using universal indicator, there is no sharp change in colour.
SC14d - Core Practical
1) State what is meant by concordant results
2) Give a reasons for using a burette instead of a measuring cylinder to measure the volume of acid
1) Results that are the same or very close to each other.
2) A burette is better because it is more precise than a measuring cylinder. A burette has a higher resolution.
