1. Percentage Yield Friday, 30 November 2018Friday, 30 November 2018
L.O: To be able to calculate percentage yield, and to understand why high yields are important for sustainable development.

2. Yield
Homework – WebQuest about the Thermite reaction (You will need to use kerboodle to complete this task)

3. Starter
What could percentage yield mean? Answer the questions to fill in the gaps and find out.
ANSWERS:
Amount
Product
Reaction
Percentage
Predicted
QUESTIONS:
What is the name for a quantity of something?
What is the result of a chemical reaction known as?
What is a thing that happens a lot in chemistry?
What is the mathematical term for ‘out of 100’?
What is another word for estimated?
Copy the correct definition into your book!
PERCENTAGE YIELD:
The [1] of [2] formed in a [3], given as a [4] of the [5] [1] of [2].

4. What is percentage yield?
The amount of product you get in a reaction is known as the yield.
The more reactant you start with, the higher the yield will be.
But the percentage yield doesn’t depend on the amount of reactant you started with — it’s a percentage.

5. Percentage Yield
The percentage yield is a comparison between the amount of product you expect to get and the amount of product you actually get.
It is always somewhere between 0% and 100%.
A 100% yield means that you got all the product you expected to get.
A 0% yield means that no reactants were converted into product, i.e. no product at all was made.

6. Calculating Percentage YieldH
Calculating Percentage Yield
The predicted yield of a reaction can be calculated from the balanced reaction equation.
If you know the predicted yield of a reaction and the actual yield of a reaction, you can calculate the percentage yield using the following formula:
Copy into your book!

7. For example:
If you reacted 24 g of calcium hydroxide with an excess of hydrochloric acid, you would expect to make 36 g of calcium chloride.
If you actually only made 28.2 g of calcium chloride, then the percentage yield would be:

8. Why is P.Y. never 100%?
Even though no atoms are gained or lost in reactions, in real life, you never get a 100% yield.
Some product or reactant always gets lost along the way — and that goes for big industrial processes as well as school lab experiments.
There are several reasons for this (as we go through these, make sure to make some notes).

9. 1. The reaction is reversible
Some products are always reacting to turn back into the original reactants.

10. What is a reversible reaction?
A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants.
Reversible reactions can be represented like this:

11. For example…
The breakdown of ammonium chloride into ammonia and hydrogen chloride is a good example of a reversible reaction. Here’s the equation:
The ammonia and hydrogen chloride that form when ammonium chloride breaks down can react with each other to reform the ammonium chloride.

12. In summary…
If a reaction is reversible, it means that the reactants will never be completely converted to products because the reaction goes both ways.
Some of the products are always reacting together to change back to the original reactants. This will mean a lower yield.

13. 2. Product is lost when it’s separated from the reactants

14. Filtering a liquid
When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid.
So, some of the product may be lost when it’s separated from the reaction mixture.
This will also result in a lower yield.

15. 3. Unexpected reactions may be happening

16. ? Unexpected Reactions Things don’t always go exactly to plan.
Sometimes there can be other unexpected reactions happening which use up the reactants.
This means there’s not as much reactant to make the product you want. ?

17. Sustainable Development
Thinking about product yield is important for sustainable development.
Sustainable development is about making sure that we don’t use resources faster than they can be replaced — there needs to be enough for future generations too.

18. Sustainable Processes
There are a few things that can be done to help make industrial processes sustainable:
Use reactions with high percentage yields. Using a reaction with a low percentage yield uses up resources and wastes a lot of chemicals.
Use reactions that don’t require much energy. When you save energy you don’t need to burn as much fuel — this means that you use up less fossil fuels and create less pollution.
Use raw materials that come from renewable sources. The more that we use reactants and fuels from non-renewable sources (like fossil fuels), the faster we’ll run out of them.

19. For example – ethanol…
Ethanol can be made either by hydrating ethene or by fermenting sugar.
Hydrating ethene isn’t very sustainable because the ethene comes from non-renewable crude oil. The reaction is done at a high temperature, so it uses lots of energy. The overall percentage yield is high though.
Fermenting sugar is more sustainable because sugar comes from plants, so it’s a renewable resource. The reaction works at a much lower temperature too, so it uses less energy. But the percentage yield is low.

20. Questions – Fact recall
Answer the following questions…
What is the yield of a reaction?
What is the percentage yield of a reaction?
Give the formula that you would use to calculate the percentage yield of a reaction.
Give three reasons why percentage yields are never 100%.
Explain why high percentage yields are important for sustainable development.
…then answer the questions on the sheet.

21. Answers – Fact Recall
The yield is the amount of product formed in a reaction.
The percentage yield is the amount of product produced in a reaction, given as a percentage of the predicted yield.
E.g. The reaction could be reversible. / Some of the product may be lost when it is separated from the rest of the reaction mixture. / There may be some unexpected reactions happening that are using up some of the reactants.
If the percentage yield is high then fewer resources will be used/less chemicals will be wasted.

22. Answers - Sheet
This reaction is reversible, so the yield will not be 100% because there will always be some product converting back to reactants.
E.g. the new method has a higher percentage yield than the old method, so it will use up less resources/waste less chemicals and produce less waste. The new method also works at a lower temperature, so it will save energy, using less fuel and creating less pollution.
% yield = (28.6 g ÷ 34.6 g) × 100 = 82.7%
% yield = (33.4 g ÷ 41.9 g) × 100 = 79.7%
% yield = (10.3 g ÷ 15.2 g) × 100 = 67.8%
% yield = (4.27 g ÷ 8.45 g) × 100 = 50.5%

23. Homework
Answer the percentage yield questions on the homework sheets I gave you last lesson.
Due in: Next lesson