2.1 Chemical change

How to calculate the amount of moles when only one volume is given but it is said that the conditions are STP

n= v/vm

How to determine the limiting reactant

1. Calculate the number of moles of each reactant.

2. Divide the number of moles of each reactant by their coefficient in the reaction. Compare the two numbers – the smaller number is the limiting reactant.

3. Use the original moles of limiting reactant, and the mole ratio to calculate the yield of the product in focus.

4 reasons why the experimental yield is possibly lower than the theoretical yield

• Loss of product: some of the product may evaporate or simply be spilled. It may also be lost when isolating or purifying the product.

• Incomplete reaction: there may be impurities present or the reaction may not go to completion

  due to insufficient time.

• Alternative reactions: an unexpected side reaction may occur which results in a different product.

• One or more of the reactants is impure.

When is the theoretical yield lower than the experimental yield?

When there are impurtities present in the prodyct

when there are impurities present in the product, such as the presence of moisture or a solvent if the drying of a

precipitate

 or solid product was incomplete. It may also occur if the product undergoes subsequent reactions with its surroundings, such as oxidising or decomposing over time

How is percentage yield calculated?

percentage yield = experimental yield/theoretical yield×100

Determine the percentage yield for the reaction between 3.74 g of Na and excess O₂ if 5.34 g of Na₂O₂ is recovered:

2Na + O₂ → Na₂O₂

Step 1: Determine the limiting reactant and calculate the maximum mass of product which could theoretically be obtained.

The question states that oxygen is in excess so sodium must be the limiting reactant.

Step 2: Identify theoretical and experimental yield.

What is atom economy?

A measure reaction efficiency which compares the total mass of reactant atoms which end up as desired products, represented as a percentage.

Formula for atom economy

% atom economy= (Molar mass of desired product/Molar mass of all reactants)×100

What type of reaction has 100% atom economy?

when there is only one product

What type of relationship is there between atom economy and wastage in industrial processes.

an inverse relationship. The higher the atom economy the lower the waste.