CHEM191 Made Easy! (Tutorial)- Stoichiometry and Limiting Agents/ Reactants

Brynn OngCHEM191, Health Science First YearLeave a Comment

STOICHIOMETRY = RECIPE

Think of stoichiometry as a recipe. To make a yummy meal from Jamie Oliver’s newest recipe book, you need to follow all the instructions clearly. You also need the correct ratio of ingredients. For chemical reaction to happen, you need the correct ratio of reactants.

For example,

500g flour + 250g water + 100g butter –>  1 cake

To make 1 cake, you need 500g flour, 250g water and 100g butter.

 

Let’s use a chemical reaction as an example this time,

2H+ SO42- –> H2SO4

The number in front of each reactant represent the ratio for the recipe. To make 1 mol of H2SO4you need 2 mol of Hand 1 mol of SO42-

 

Now back to our recipe, 500g flour + 250g water + 100g butter –>  1 cake

What if this time, you only have half the amount of ingredients? 250g flour, 125g water and 50g butter. Can you still make a cake? Yeah you can. But you will only get half a cake.

What if you have double the amount of ingredients? 1000g flour, 500g water and 200g butter. You guessed it- 2 cakes.

 

Okay, I hope you are with me so far. Let’s look at our chemical equation again but with different amount (mols) this time.

2H+ SO42- –> H2SO

So we know 2 mols of Hand 1 mol of SO42- gives us 1 mol of  H2SO4. 

Say we have 4 mols of  Hand 2 mol of SO42-, we will get 2 mols of H2SO4

Say we have 1 mols of  Hand 0.5 mol of SO42-, we will get 0.5 mols of H2SO4

Make sense?

COMMON MISTAKES

4 mols of  Hand 2 mols of SO42- gives 6 mols of H2SO4

1 mols of  Hand 0.5 mol of SO42- gives 1.5 mols of H2SO4

See above? A lot of students think of stoichiometry as a direct addition of all the mols. It is not, stoichiometry tells us the ratio of reactants needed for the reaction!

 


LIMITING AGENTS

Limiting agents are also known as limiting reactants. Basically, they are reactant/ reactants that limit the full potential of the reaction.

Back to the cake recipe example.

500g flour + 250g water + 100g butter –>  1 cake

You go to the kitchen- you found 500g flour, 250g water but only 50g butter. What happens now?- you don’t have enough butter! It is half less than what you need. Can you still make the cake? Yes! Except now you only have to use 250g flour and 125g water to make the cake and you will only get to make half a cake.

Remember we want to make sure that the ratio is right. We cannot mix all 500g flour + 250g water with only 50g butter. Your cake will taste wrong.

From above, it becomes clear that the butter is limiting the full potential of the recipe. Hence, the butter is the limiting reactant and you have excess flour and water

 

The same applies for chemical reaction.

  • 2H+ SO42- –> H2SO

Again, you need 2 mols of Hand 1 mol of SO42- to form 1 mol of  H2SO4

If you only have 1 mol of Hand 1 mol of SO42-, then the H+ becomes the limiting agent. The reaction will still go on, except this time 1 mol H+ will react with only 0.5 mol of  SO42- and form only 0.5 mol H2SO4

 

COMMON MISTAKES

4 mols of  Hand 4 mols of SO42- = no limiting agent found.

  • WRONG, with 4 mols of H+, you can only use 2 mols of SO42- for the reaction. Therefore, you will have an excess of 2mols of SO42- and H+ is the limiting reactant

4 mols of  Hand 3 mols of SO42- = SO42- is the limiting agent.

  • WRONG, with 4 mols of H+, you can only use 2 mols of SO42- for the reaction. Therefore, you will have excess  SO42- and H+ is the limiting reactant

Please remember, you are following a recipe and its ratio. You cannot compare directly unless they are all 1:1 ratio.

Hence, your H+ is limiting the full potential of the reaction and you have excess SO42-

 

 

I hope this makes sense. If you have questions or comments, please e-mail me brynn1991@yahoo.com

We will go further in depth and apply the concepts of limiting agent to actual chemistry questions. Till then, take care!

 

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