GCSE Chemistry - What is a Limiting Reactant? Limiting/Excess Reactants Explained #27

Summary

This video explains the concept of limiting reactants in chemical reactions, using the example of hydrochloric acid reacting with calcium carbonate. The reaction produces calcium chloride, water, and carbon dioxide, with the calcium carbonate being the limiting reactant. The video emphasizes that the amount of product depends on the limiting reactant, while the excess reactant does not affect the outcome. It also walks through a calculation of sodium oxide production when sodium burns in air, highlighting the process of determining the limiting reactant, balancing equations, and calculating the mass of the product produced.

Takeaways

• 😀 Limiting reactants are the substances that run out first in a chemical reaction, stopping the reaction.
• 😀 The amount of product formed in a reaction is determined by the limiting reactant.
• 😀 An excess reactant is a substance that remains after the reaction has stopped because the limiting reactant is used up.
• 😀 The reaction between hydrochloric acid and calcium carbonate is a good example of how a limiting reactant works.
• 😀 If the reaction stops after all of the calcium carbonate has been used, it is the limiting reactant.
• 😀 In excess reactants, such as hydrochloric acid, more of the substance would remain even after the reaction ends.
• 😀 A real-world example involves sodium reacting with oxygen to produce sodium oxide, with oxygen being in excess and sodium as the limiting reactant.
• 😀 To calculate the mass of a product, start by writing and balancing the chemical equation for the reaction.
• 😀 The mole ratio from a balanced equation helps determine how many moles of product can be formed from the moles of limiting reactant.
• 😀 The mass of the product is calculated by multiplying the number of moles of product by its molar mass.
• 😀 In the example given, 115 grams of sodium will produce 155 grams of sodium oxide, assuming oxygen is in excess.

Q & A

What is a limiting reactant?

A limiting reactant is the substance in a chemical reaction that is completely consumed first, limiting the amount of product that can be produced. In the example with hydrochloric acid and calcium carbonate, calcium carbonate is the limiting reactant because it is used up first, stopping the reaction.

How can you tell when a limiting reactant has been used up in a reaction?

You can tell when the limiting reactant is used up when the reaction stops. For instance, in the reaction between hydrochloric acid and calcium carbonate, once the calcium carbonate has fully reacted, the fizzing stops, indicating that it is the limiting reactant.

What does it mean when a reactant is in excess?

A reactant is in excess when there is more of it present than is needed for the reaction to go to completion. In the example with hydrochloric acid and calcium carbonate, hydrochloric acid is in excess because there is still some left over after all the calcium carbonate has been used up.

Why is the limiting reactant important in calculating the mass of a product?

The limiting reactant is important because it determines how much product can be formed in a chemical reaction. The amount of product produced depends entirely on the amount of limiting reactant available.

What happens if you add more of the limiting reactant?

If you add more of the limiting reactant, the reaction will continue until that new amount of limiting reactant is used up, potentially producing more product.

What happens if you add more of a reactant in excess?

Adding more of a reactant in excess will not affect the reaction once the limiting reactant is used up because there is no more of the limiting reactant for it to react with.

How do you determine which reactant is the limiting reactant in a combustion reaction?

In a combustion reaction, such as burning sodium in air, oxygen is in excess because it is abundant in the atmosphere. Therefore, the limiting reactant is typically the substance being burned, in this case, sodium.

What is the first step in calculating the mass of a product in a chemical reaction?

The first step is to write out and balance the chemical equation for the reaction to understand the molar ratios between the reactants and products.

How do you calculate the number of moles of a reactant?

To calculate the number of moles of a reactant, use the formula: moles = mass / molar mass (mr). For example, to calculate the moles of sodium, divide the mass of sodium by its molar mass (23 g/mol).

How do you calculate the mass of a product from moles of reactant?

To calculate the mass of a product, first determine the number of moles of the product using the molar ratio from the balanced equation. Then use the formula: mass = moles × molar mass (mr) to find the mass of the product.

What is the molar mass of sodium oxide (Na2O) and how is it calculated?

The molar mass of sodium oxide (Na2O) is calculated by adding the atomic masses of its components: 2 × 23 (for sodium) + 16 (for oxygen), which gives a total of 62 g/mol.