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University of Maine

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Problem 110

The following scenes represent a chemical reaction between $\mathrm{AB}_{2}$ and $\mathrm{B}_{2}$ :

(a) Write a balanced equation for the reaction. (b) What is the limiting reactant? (c) How many moles of product can be made from 3.0 $\mathrm{mol}$ of $\mathrm{B}_{2}$ and 5.0 $\mathrm{mol}$ of $\mathrm{AB}_{2} ?(\mathrm{d})$ How many moles of excess reactant remain after the reaction in part (c)?

Answer

a) $2 \mathrm{AB}_{2}+\mathrm{B}_{2}-2 \mathrm{AB}_{3}$

b) $A B_{2}$

c) $\mathrm{n}\left(\mathrm{AB}_{3}\right)=\mathrm{n}\left(\mathrm{AB}_{2}\right)=5 \mathrm{mol}$

d) Excess $\mathrm{B}_{2}=3-2,5=0,5 \mathrm{mol}$

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## Discussion

## Video Transcript

we can use this picture representing a chemical reaction to look at both with the overall reaction is as well as the idea of a limiting an excess free agent. When we start, we have two different react. It's on this side. If we call our green circles be, we can see that there are molecules of B two. The's react with molecules that have to bees in an A, which will call a need to. So our reactions are a B two plus B two, and then when we look at the product side, we see would make a new compound A with three bees. There are also some B two's still on the side. We can count how many of each there are. So there's 123456 12345 Btu's. And on this side we have a total of six A B threes and to be twos. We can simplify this by combining the B two since they appear on both sides or the equation ist six a B two plus three B two make six a B three, and then finally we can reduce it to the lowest whole number ratio or dividing everything by three to a B two plus B two makes two, maybe three. So this is the overall equation for the chemical reaction that we see to identify which react and is limiting. We can also look at which one we have extra and you can see that we start off with B two and they're still be, too, on the other side. So that means that that's not all reacted where we say it's an excess. So what that means is the one determining our amount of product is a be two or limiting, and we can verify this river calculation using the mole ratios from the equation. If I have six moles of a B two, I should produce according to the equation two moles of a B three for every two moles of a B two or six moles of product. If I have five moles of be to the other reactor, I know that I'll produce two moles of a B three for every one mole of B two from the coefficients of the balanced equation, or I'll produce 10 moles, maybe three. Since six is less than 10 that means that a B two is the limiting re agent. We can also use the information for different quantities besides those given in the picture. So if, for example, I have three moles of B two and five moles of a B two, I can predict how much product for each one I predict. I calculate the product like I did above so three moles of B two, and I know that there are using the mole ratio two moles of a B three for every one mole would be to. So I should produce six malls. Be to if I have Dr Moles of a B two. According to my balanced equation, there will be two moles of a B three for every two moles of a B two or produce. Five moles have a B three. Since five is less than six, that's the amount of product that I'll produce. That means that I have excess free to. So if I were to produce five moles of a B three, I only need half assed much be too or 2.5 malls be to, but I have three, so three minus 2.5 indicates that I'll have 0.5 moles be to in excess were left over

## Recommended Questions

The scene below represents a mixture of A2 and B2 before they react to form $\mathrm{AB}_{3}$

(a) What is the limiting reactant?

(b) How many molecules of product can form?

Consider the following reaction at a certain temperature

$$

\mathrm{A}_{2}+\mathrm{B}_{2} \rightleftharpoons 2 \mathrm{AB}

$$

The mixing of 1 mole of $\mathrm{A}_{2}$ with 3 moles of $\mathrm{B}_{2}$ gives rise to $x$ mole of $\mathrm{AB}$ at equilibrium. The addition of 2 more moles of $\mathrm{A}_{2}$ produces another $x$ mole of $\mathrm{AB}$. What is the equilibrium constant for the reaction?

Consider the following reaction at a certain temperature

$$

\mathrm{A}_{2}+\mathrm{B}_{2} \rightleftharpoons 2 \mathrm{AB}

$$

The mixing of 1 mole of $\mathrm{A}_{2}$ with 3 moles of $\mathrm{B}_{2}$ gives rise to $x$ mole of $\mathrm{AB}$ at equilibrium. The addition of 2 more moles of $\mathrm{A}_{2}$ produces another $x$ mole of AB. What is the equilibrium constant for the reaction?

Consider the reaction

$$

2 \mathrm{~A}+\mathrm{B} \longrightarrow \mathrm{C}

$$

(a) In the diagram here that represents the reaction, which reactant, A or $\mathrm{B}$, is the limiting reagent?

(b) Assuming complete reaction, draw a molecularmodel representation of the amounts of reactants and products left after the reaction. The atomic arrangement in $\mathrm{C}$ is $\mathrm{ABA}$.

Consider the following generic reaction:

$$

\mathrm{A}_{2} \mathrm{B}_{2}+2 \mathrm{C} \longrightarrow 2 \mathrm{CB}+2 \mathrm{A}

$$

What steps and information are necessary to perform the following determinations assuming that $1.00 \times 10^{4}$ molecules of $\mathrm{A}_{2} \mathrm{B}_{2}$ are reacted with excess $\mathrm{C}$ ?

a. mass of CB produced

b. atoms of A produced

c. moles of C reacted

d. percent yield of CB

Consider the following generic reaction:

$$\mathrm{A}_{2} \mathrm{B}_{2}+2 \mathrm{C} \longrightarrow 2 \mathrm{CB}+2 \mathrm{A}$$

What steps and information are necessary to perform the following determinations assuming that $1.00 \times 10^{4}$ molecules of $\mathrm{A}_{2} \mathrm{B}_{2}$ are reacted with excess $\mathrm{C} ?$

a. mass of CB produced

b. atoms of A produced

c. moles of C reacted

d. percent yield of CB

Consider the equation $3 \mathrm{A}+\mathrm{B} \rightarrow \mathrm{C}+\mathrm{D} .$ You react 4 moles of A with 2 moles of $\mathrm{B}$ . Which of the following is true?

a. The limiting reactant is the one with the higher molar mass.

b. A is the limiting reactant because you need 6 moles of A and have 4 moles.

c. B is the limiting reactant because you have fewer moles of B than A.

d. B is the limiting reactant because three A molecules react with each B molecule.

e. Neither reactant is limiting.

Justify your choice. For those you did not choose, explain why they are incorrect.

For each of the following unbalanced chemical equations, suppose that 10.0 mg of each reactant is taken. Show by calculation which reactant is limiting. Calculate the mass of each product that is expected.

a. $\mathrm{CO}(g)+\mathrm{H}_{2}(g) \rightarrow \mathrm{CH}_{3} \mathrm{OH}(l)$

b. $\mathrm{Al}(s)+\mathrm{I}_{2}(s) \rightarrow \mathrm{AlI}_{3}(s)$

c. $\mathrm{Ca}(\mathrm{OH})_{2}(a q)+\mathrm{HBr}(a q) \rightarrow$

$\quad \mathrm{CaBr}_{2}(a q)+\mathrm{H}_{2} \mathrm{O}(l)$

d. $\mathrm{Cr}(s)+\mathrm{H}_{3} \mathrm{PO}_{4}(a q) \rightarrow \mathrm{CrPO}_{4}(s)+\mathrm{H}_{2}(g)$

A chemical reaction in which reactants A and B form the product C is studied in the laboratory. The researcher carries out the reaction with differing relative amounts of reactants and measures the amount of product produced. Examine the given tabulated data from the experiment and answer the questions.

a. For which experiments is A the limiting reactant?

b. For which experiments is B the limiting reactant?

c. The molar mass of A is 50.0 g>mol, and the molar mass of B is 75.0 g>mol. What are the coefficients of A and B in the balanced chemical equation?

d. For each of the experiments in which A is the limiting reactant, calculate the mass of B remaining after the reaction has gone to completion. Use the molar masses and coefficients from part c.

e. The molar mass of C is 88.0 g>mol. What is the coefficient of

C in the balanced chemical equation?

f. Calculate an average percent yield for the reaction.

The reaction $\mathrm{A}_{2}+\mathrm{B}_{2} \rightleftharpoons 2 \mathrm{AB}$ has an equilibrium constant $K_{c}=1.5 .$ The following diagrams represent reaction mixtures containing $\mathrm{A}_{2}$ molecules (red), $\mathrm{B}_{2}$ molecules (blue), and AB molecules, (a) Which reaction mixture is at equilibrium? (b) For those mixtures that are not at equilibrium, how will the reaction proceed to reach equilibrium? [Sections 15.5 and 15.6$]$