IBDP Chemistry - Yield and Atom Economy

In this topic of IBDP Chemistry, we will be discussing yield and atom economy.

Yield

In IBDP Chemistry, we often utilise different chemical reactions in order to create chemical products. However, during chemical reactions in the real world, there may be things that occur, leading to the formation of less product than predicted. These reasons include:

• Spills and experimental errors
• Incompleted reaction
• Presence of undesirable side reactions etc.

As a result, we will need to determine the success of each reaction - we can do this by determining the percentage yield of an experiment.

• The percentage yield of an experiment is a measure of the success of the reaction by comparing the actual amount of product formed against the theoretical/predicted amount of product that should be formed from the reaction.
• The closer the percentage yield is to 100%, the more successful the reaction.

In order to calculate the percentage yield, you will need two pieces of information - theoretical yield and actual yield.

• Theoretical yield is the maximum amount of product that could have been formed from the reaction, given the amount of each of the reactants you have.
• Actual yield is the actual amount of product that has formed from the reaction.
• This amount is always smaller than the theoretical yield. The value is usually given in the question in the IBDP Chemistry exam.

Once you have values for the theoretical and actual yield, you can substitute these values into the following equation to find the percentage yield of the chemical reaction:

• The actual and theoretical yields can be written using any units (moles, grams, cm³ etc.), so long as they are written in the same units.

Now, we will go through a worked example of how to determine the percentage yield of a chemical reaction - Say you are reacting 0.175mol of silver nitrate with excess sodium chloride. After filtration, 0.160mol of silver chloride was collected. What was the % yield?

• The first step would be to determine the theoretical yield, as we are already given the actual yield (actual yield = 0.160mol of silver chloride).
• To calculate the theoretical yield, we will need to write out the balanced chemical equation in order to see the molar ratio of the reactants and products.
• The chemcial equation is: AgNO₃ (aq) + NaCl (aq) → AgCl (aq) + NaNO₃ (aq)
• From the chemical equation, we can see that there is a 1 to 1 ratio of AgNO₃ to AgCl.
• Given that we have 0.175mol of silver nitrate (AgNO₃), and there is a 1 to 1 ratio of AgNO₃ to AgCl, then we should expect there to theoretically be 0.175mol of AgCl formed from the reaction.
• Thus, the theoretical yield is 0.175mol.
• We can now substitute these values into the equation to calculate % yield:

Atom Economy

As mentioned above, we use chemical reactions to create useful products which we use in everyday life. However, sometimes chemcial reactions will produce products which are not useful, along with other products which are useful. Therefore, chemists work to not only increase the yield of reactions, but to also increase the efficiency of the chemcial process.

To do this, we want to increase the amount of useful products that are created from a chemical reaction. In IBDP Chemistry, this is measured using atom economy.

• Atom economy is a measure of the amount of starting materials that become useful products.
• The higher the atom economy, the more efficient the chemical process, as less waste is created.

To calculate atom economy, you should do the following:

1. Write out the balanced equation for the reaction taking place.
2. Calculate the relative molecular mass of each product, and then determine the mass of each product formed. Use this information to find the total mass of all products formed (note: this should be the same as the total mass of reactants).
3. Find the total mass of the useful/desired products, using the relative molecular mass of the desired product and assuming molar quantities.
4. Divide the total mass of desired products by the total mass of all products, and multiply by 100% to get the atom economy.

We will now do a worked example of finding the atom economy for a industrial reaction - Say you want to extract copper by heating copper oxide with carbon. What is the atom economy of this reaction?

• First, we will need to write out the balanced chemical equation for this reaction.
• The equation is: 2CuO + C → 2Cu + CO₂
• Next, we will have to determine the total mass of the products using the relative molecular mass.
• The relative molecular mass of CO₂ is: 12 + (2 x 16) = 44 g/mol.
• The relative molecular mass of Cu is: 63.55 g/mol (from the periodic table).
• Thus, assuming that we have one mole of CO₂ and two moles of copper, the total mass of products = (1 x 44) + (2 x 63.55) = 171.1g
• Note: in order to make the question easy, simply assume that you have the same number of moles of that substance as the coefficient written in the equation.
• Our desired product is copper, and since we have two moles of copper, the total mass of desired products = 2 x 63.55 = 127.1g
• Now, we can divide the total mass of desired products by the total mass of all products and multiply it by 100% to get the atom economy.

This is the end of this topic.

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