2.1.3 (h) Percentage yields and atom economy

Syllabus

(h) calculations to determine:

(i) the percentage yield of a reaction or related quantities

(ii) the atom economy of a reaction

(i) the techniques and procedures required during experiments requiring the measurement of mass, volumes of solutions and gas volumes.

(j) the benefits for sustainability of developing chemical processes with a high atom economy.

What does this mean?

Percentage yield

This is yet another topic that should be familiar to you.

For numerous reasons it is impossible to make as much product as stoichiometry suggests that you should.

Side-reactions happen, something evaporates or gets stuck in glassware or in a filter etc.

At GCSE they often tell you how much product is made & how much product you think you should have made.

eg.

A student makes 2.4g of Calcium Hydroxide. She had predicted that she would make 3.6g

What is the percentage yield?

(2.4 / 3.6) x 100 = 66.7%

You can safely assume an A level examiner will expect more of you.

A student completely reacts 4.6 g of Sodium with Sulphuric acid. 9.23 g of Sodium Sulphate is made.

What is the percentage yield?

2 Na_(s) + H₂SO_{4 (aq)} → Na₂SO_4(aq) + H₂O_(l)

Moles of Na = Mass/ A_r = 4.6/23 = 0.2

Ratio Na: Na₂SO₄ = 2:1

Moles of Na₂SO₄expected = 0.2/2 = 0.1

M_r(Na₂SO₄) = 23+23+32+64 =142 g/mol

Mass of Na₂SO₄expected = Moles x M_r = 0.1 x 142 = 14.2

% yield = (Actual/Predicted) x 100 = (9.23/14.2) x 100 = 65%

NOTE: The % yield calculation can be carried out in moles or in grams.

Video

Atom Economy

If you covered Atom Economy at GCSE then nothing has really changed.

But people do get confused between this and percentage yield.

Percentage yield measures how well you have carried out your reaction - this might be improved if you altered the temperature or pressure.

Atom Economy takes no account of how well you have carried out the process.

It assumes that everything works perfectly but tries to show what percentage of the products are waste.

In reality, this means that you work out the M_r of the desired product and the M_r of the other products and slot them into the equation above.

So, for our example above: 2 Na_(s) + H₂SO_{4 (aq)} → Na₂SO_4(aq) + H₂O_(l)

Assuming that the desired product is Sodium Sulphate rather than water!

M_r(Desired product) = 23+23+32+64 =142 g/mol

M_r(Water) = 2+16 =18 g/mol

M_r(All products) = 142 + 18 = 160 g/mol

Atom Economy = (142/160) x 100 = 88.75%

If there is a coefficient then you must take it into account

Or: 2Na_(s) + 2 HCl_(aq) → 2NaCl_(aq) + H_2(g)

If the desired product is Sodium Chloride rather than Hydrogen!

M_r(Desired product) = 23+35.5=58.5g/mol

M_r(Hydrogen) = 2 g/mol

M_r(All products) = (2 x 58.5) + 2 = 119 g/mol

Atom Economy = (2 x 58.5 /119) x 100 = 98.3 %

If the desired product is Hydrogen rather than Sodium Chloride!

M_r(Desired Product) = 2 g/mol

M_r(Sodium Chloride) = 23+35.5=58.5g/mol

M_r(All products) = (2 x 58.5) + 2 = 119 g/mol

Atom Economy = (2 /119) x 100 = 1.7 %

Video

Follow-up questions

Likely follow-ups include:

1) Why would a company producing Hydrogen from the following process be unconcerned by a poor atom economy?

Ca + H₂SO_4(aq) → CaSO_4(aq) + H_2(g)

The answer is generally because the other product is also useful and can be sold. So it isn't actually waste.

At which point atom economy is irrelevant & the company would be much more interested in percentage yield.

2) How does developing chemical processes with high atom economy benefit sustainability?

Sustainability is an environmental idea.

Using a high percentage of your raw material to make "dumpable" waste is less sustainable than either

a) Coming up with a process that only produces one product - 100% atom economy

b) Makes several useful products - 100% effective atom economy.

3) Given a process that can be carried out one way with a mediocre atom economy or a second way with a better atom economy why might a company choose the first?

Either because

a) Method 1 produces largely harmless waste, like water, when Method 2 produces less but toxic waste

b) Method 2 may have a terrible percentage yield or a bad rate of reaction or both.

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