Limiting Reactants. When the limiting reactant is used up THE REACTION STOPS!

Transcription

1 Limiting Reactants

2 Limiting Reactants The limiting reactant is 1) A reactant (one of the things you start with) and 2) The reactant that makes the least product Or 3) The reactant that is used up first

3 Limiting Reactants When the limiting reactant is used up THE REACTION STOPS!

4 Limiting Reactants THERE IS ONLY ONE LIMITING REACTANT.

5 Excess Reactants Any reactant that is NOT the limiting reactant There can be more than one excess reactant (if for example there are three reactants, 2 will be excess and one will be limiting)

6 Limiting Reactants When we are setting up an experiment or any reaction, we can pick the limiting reactant It can be: The most expensive reactant The most dangerous reactant The one that is hardest to work with

7 Excess Reactants When design the reaction, we can make sure to have a lot of the excess reactant To make sure the limiting reactant gets used up To make sure side reactions don t happen For example, in combustions, we generally want oxygen to be in excess, so carbon monoxide doesn t form.

8 Non-chemistry examples-1 You are making cheeseburgers You have 24 hamburger buns 15 hamburgers 40 slices of cheese What are the most cheeseburgers can you make?

9 Non-chemistry examples-1 Each Hamburger needs 1 bun, 1 hamburger and 1 slice of cheese 24 hamburger buns make 24 cheeseburgers 15 hamburgers make 15 cheeseburgers 40 slices of cheese make 40 cheeseburgers What is the maximum amount of cheeseburgers can you make?

10 Non-chemistry examples-1 Each Hamburger needs 1 bun, 1 hamburger and 1 slice of cheese 24 hamburger buns make 24 cheeseburgers 15 hamburgers make 15 cheeseburgers 40 slices of cheese make 40 cheeseburgers What is the maximum amount of cheeseburgers can you make? 15, if everything goes right. (This is called a Theoretical yield)

11 Non-chemistry examples-2 At the gym, the class wants to play basketball The gym has 5 full courts You have 3 basketballs There are 20 of you who want to play How many games can go on at once?

12 Non-chemistry examples-2 At the gym, the class wants to play basketball The gym has 5 full courts 5 games You have 3 basketballs -3 games There are 20 of you who want to play up to 10 games Each game needs a court, at least 2 players and a ball. How many games can go on at once?

13 Non-chemistry examples-2 At the gym, the class wants to play basketball The gym has 5 full courts 5 games You have 3 basketballs -3 games There are 20 of you who want to play up to 10 games Each game needs a court, at least 2 players and a ball. How many games can go on at once? 3- the number of balls limit the number of games.

14 Non-chemistry examples-3 You have a contract to make centerpieces for a party. Each centerpiece needs 1 vase, 3 flowers and a ribbon You have 31 vases, 78 flowers and 300 ribbons How many centerpieces can you make? What part limits the number of centerpieces you can make?

15 Non-chemistry examples-3 In stoich terms: 1V + 3F + R = C Each centerpiece needs 1 vase, 3 flowers and a ribbon The mole ratios are 1 centerpiece: 1 vase 1 centerpiece: 3 flowers 1 centerpiece: 1 Ribbon

16 Non-chemistry examples-3 1V + 3F + R = C 31 vases x 1centerpiece/1 vase =31 centerpieces 78 flowers x 1centerpiece/3 flowers = 26centepieces 300 ribbons x 1 centerpiece/1 ribbon = 300 centerpieces How many centerpieces can you make? 26 What part limits the number of centerpieces you can make? Flowers

17 The math Hints: Read the whole problem (all the parts) If you are only asked for which reactant is limiting, make a guess and do the reactant to reactant stoich problem If you will be asked to find the amount of a product later, solve for that product.

18 The math Hints: Calculate how much of that product will be formed by each reactant. Whichever reactant makes the least product is limiting. The other(s) are excess.

19 Chemistry Example 1 You have 5 moles of hydrogen and 5 moles of nitrogen. Which is the limiting reactant? The directions for making a product are in the balanced chemical equation. Hydrogen plus nitrogen make ammonia 3H 2 + N 2 2NH 3

20 Chemistry Example 1 You have 5 moles of hydrogen and 5 moles of nitrogen. Which is the limiting reactant? 3H 2 + N 2 2NH 3 Make a guess- it looks like you need 3 times as many moles of hydrogen as you do nitrogen. Check the math: 5 molh 2 x 1 moln 2 3 molh 2 = 1.67molN 2

21 Chemistry Example 1 You have 5 moles of hydrogen and 5 moles of nitrogen. Which is the limiting reactant? What if you guessed nitrogen? Check the math: 5 moln2 x 3 molh 2 1 moln 2 = 15mol H 2 Since you have 5 mol of N 2, its in excess and H 2 is limiting (you used ALL of it)

22 Chemistry Example 1 You have 5 moles of hydrogen and 5 moles of nitrogen. How much ammonia can you make? 5 moles H 2 x 2molNH 3 3 mol H 2 = 3.33molNH 3 5 moles N 2 x 2molNH 3 1 mol N 2 = 15molNH 3 The limiting reactant makes less product, so H 2 is limiting

23 Chemistry Example 2 You have 5 grams of hydrogen and 5 grams of nitrogen. Which is the limiting reactant? 3H 2 + 1N 2 2NH 3 If you guessed H 2 : 5gH 2 x 1 moleh gH 2 x 1mole N 2 3 molh 2 x 28.02gN 2 1 moln 2 = 23.1g N 2 You need 23.1g of N 2 and you have 5, so N 2 is limiting and H 2 is excess

24 Chemistry Example 2 You have 5 grams of hydrogen and 5 grams of nitrogen. Which is the limiting reactant? 3H 2 + 1N 2 2NH 3 If you guessed N 2 : 5gN 2 x 1 molen gN 2 x 3mole H 2 1 moln 2 x 2.02gH 2 1 molh 2 = 1.1g H 2 You need 1.1g of H 2 and you have 5, so H 2 is excess and N 2 is limiting

25 Chemistry Example 2 You have 5 grams of hydrogen and 5 grams of nitrogen. How much ammonia can you make? 3H 2 + 1N 2 2NH 3 If you had found the limiting reactant, you would only calculate the ammonia formed by nitrogen 5gN 2 1 moln 2 x x 2molNH 3 x 17.04gNH 3 = 6.1gNH 28gN 2 1 moln 2 1molNH 3 3

26 Chemistry Example 2 You have 5 grams of hydrogen and 5 grams of nitrogen. How much ammonia can you make? 3H 2 + 1N 2 2NH 3 5gN 2 x 1 moln 2 28gN 2 x 2molNH 3 1 moln 2 x 17.04gNH 3 1molNH 3 = 6.1gNH 3 5gH 2 x 1 molh gH 2 x 2molNH 3 3 molh 2 x 17.04gNH 3 1molNH 3 = 28.1gNH 3 If you had not found the limiting reactant, you would choose the smaller amount of product, because once you use all the nitrogen, the reaction stops.

27 Chemistry Example 3 You have 12 grams of hydrogen and 20 grams of nitrogen. Which one is the excess reactant? How much nitrogen would you need to use all the hydrogen? 3H 2 + 1N 2 2NH 3 12gH 2 x 1 moleh gH 2 x 1mole N 2 3 molh 2 x 28.02gN 2 1 moln 2 = 55.5gN 2

28 Chemistry Example 3 You would need 55.5g of N 2 to use all the H 2, but you only have 20g of N 2, so it limits the reaction. Nitrogen is the limiting reactant.

29 Chemistry Example 3 You have 12 grams of hydrogen and 20 grams of nitrogen. Which one is the excess reactant? What if you guessed that nitrogen was the limiting reactant? How much hydrogen would you need to use all the nitrogen? 3H 2 + 1N 2 2NH 3 = 4.32gH 2 20gN 2 x 1 molen gN 2 x 3 molh 2 1 moln 2 x 2.02gH 2 1 molh 2

30 Chemistry Example 3 You would need 4.32g of H 2 to use all the N 2, and you have 12g of H 2, so it does not limit the reaction. Hydrogen is the excess reactant.

31 Chemistry Example 3 Which Reactant would you use to calculate the theoretical yield of ammonia? Nitrogen

32 Chemistry Example 3 How much ammonia would you expect to produce? 20gN 2 x 1 molen gN 2 x 2 molnh 3 1 moln 2 x 17.04gNH 3 1 molnh 3 = 24.33gNH 3