Percentage Yield & Atom Economy Flashcards
Reasons for not getting a 100% conversion of reactants to products when oxidising ethanol & ethanoic acid
REACTION TIME
REACTION TIME:
- Not all the ethanol / partial oxidation product ethanal had reacted.
- due to insufficient time when refluxing
Percentage yield & formula
- The amount of product made by the end of a reaction
- Tells us how much of the reactants has been converted to products ( efficiency of the reaction)
actual amount of product (mol) x100
theoretical amount of product (mol)
Other general reasons for not getting 100% conversion of reactants to products
- The reactants may not be PURE
- The reaction may not go to COMPLETION~ applies to equilibrium reactions.
Reasons for not getting a 100% conversion of reactants to products when oxidising ethanol & ethanoic acid
DISTILLATION
- A type of purification process which SEPARATES the product from other products and reagents.
- Some of the ethanoic acid may not have been fully distilled
- Leaving it in the round bottom flask or the joints of the apparatus.
Reasons for not getting a 100% conversion of reactants to products when oxidising ethanol & ethanoic acid
SIDE REACTIONS LEADING TO BY-PRODUCTS
Ethanol, in the presence of HEAT and c.H2SO4 , can be DEHYDRATED :
- Produces ETHENE
CH3CH2OH —– H2C=CH2 + H2O
Unreacted ethanol, in the presence of heat and c.H2SO4, can react with ethanoic acid :
- Produces ETHYL ETHANOATE
CH3CH2OH + CH3COOH —- CH3COOCH2CH3 + H2O
Procedure for working out % yield
- Write a balanced EQUATION for the reaction.
- Work out the MOLES of each REACTANT
- Determine the LIMITING REAGENT and use this to calculate the THEORETICAL MOLES of the product using the reaction equation.
- Calculate the ACTUAL MOLES of the product made in the reaction.
- Calculate the PERCENTAGE YIELD using the equation.
Fermentation of glucose
- An alternative method to the hydration of ethene to make ETHANOL.
C6H12O6(aq) — 2CH3CH2OH(aq) + 2CO2(g)
Reaction Conditions:
- Yeast
- 37 degrees
- Absence of oxygen
Atom economy
molecular mass of desired product/molecular mass of all products x 100
Percentage yield and its disadvantages
Percentage yield :
- measures how wasteful the reaction is based on how much product is lost.
- Often occurs due to the reaction being incomplete or losses during collection and purification procedures.
DISADVANTAGES:
- Does not measure how wasteful the reaction actually is.
- A reaction can have a 100% yield but still have a lot of the reactants end up as BY-PRODUCTS , rather than the desired product.
Why is atom economy better than percentage yield?
- Measure the EFFICIENCY of a reaction
- Based on the PROPORTION of reactant atoms which become part of the DESIRED PRODUCT, rather than the by-products.
- The calculation is based on balanced symbol equation.
- In an ideal chemical process, a use would be found for all the products and hence the atom economy would be 100%.
Problems & solution to a LOW atom economy
- A reaction with a low atom economy means that most of the reactants turn into WASTE.
- Particularly bad when FINITE MATERIALS are used and cannot be preserved.
- Waste has to be DISPOSED of ~ COSTLY and poses ENVIRONMENTAL PROBLEMS.
SOLUTIONS:
- Sold off
- recycled
- used to make other products
Sustainability
- Developing processes to PREVENT THE DEPLETION of natural resources
- e.g crude oil & wood
LOW atom economy = process has LOW sustainability
The sustainability of a chemical process can be IMPROVED by:
REDESIGN PRODUCTION METHOD:
- Different/less hazardous starting materials are used
MILDER REACTION CONDITIONS:
- Better catalysts
- Less hazardous solvents
RECYCLING
SELLING BY-PRODUCTS
- Using production methods with FEWER STEPS and HIGHER ATOM ECONOMIES.
Calculating atom economy
- Write EQUATION for the reaction
- Calculate the MOLECULAR MASS of DESIRED product.
- Calculate the MOLECULAR MASS of ALL the products.
- Use FORMULA to calculate ATOM ECONOMY.
