When magnesium metal is burned in air (Figure 3.6 ), two...

When magnesium metal is burned in air (Figure 3.6 ), two products are produced. One is magnesium oxide, $\mathrm{MgO}$. The other is the product of the reaction of $\mathrm{Mg}$ with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. (a) Based on the charge of the nitride ion (Table 2.5 ), predict the formula of magnesium nitride. (b) Write a balanced equation for the reaction of magnesium nitride with water. What is the driving force for this reaction? (c) In an experiment a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of $\mathrm{MgO}$ and magnesium nitride after burning is $0.470 \mathrm{~g}$. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is $0.486 \mathrm{~g}$ of $\mathrm{MgO}$. What was the mass percentage of magnesium nitride in the mixture obtained after the initial burning? (d) Magnesium nitride can also be formed by reaction of the metal with ammonia at high temperature. Write a balanced equation for this reaction. If a 6.3 -g Mg ribbon reacts with $2.57 \mathrm{~g} \mathrm{NH}_{3}(g)$ and the reaction goes to completion, which component is the limiting reactant? What mass of $\mathrm{H}_{2}(g)$ is formed in the reaction? (e) The standard enthalpy of formation of solid magnesium nitride is $-461.08 \mathrm{~kJ} / \mathrm{mol} .$ Calculate the standard enthalpy change for the reaction between magnesium metal and ammonia gas.

When magnesium metal is burned in air (Figure 3.6 ), two products are produced. One is magnesium oxide, $\mathrm{MgO}$. The other is the product of the reaction of $\mathrm{Mg}$ with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas.
(a) Based on the charge of the nitride ion (Table 2.5 ), predict the formula of magnesium nitride. (b) Write a balanced equation for the reaction of magnesium nitride with water. What is the driving force for this reaction? (c) In an experiment a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of $\mathrm{MgO}$ and magnesium nitride after burning is $0.470 \mathrm{~g}$. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is $0.486 \mathrm{~g}$ of $\mathrm{MgO}$. What was the mass percentage of magnesium nitride in the mixture obtained after the initial burning? (d) Magnesium nitride can also be formed by reaction of the metal with ammonia at high temperature. Write a balanced equation for this reaction. If a 6.3 -g Mg ribbon reacts with $2.57 \mathrm{~g} \mathrm{NH}_{3}(g)$ and the reaction goes to completion, which component is the limiting reactant? What mass of $\mathrm{H}_{2}(g)$ is formed in the reaction? (e) The standard enthalpy of formation of solid magnesium nitride is $-461.08 \mathrm{~kJ} / \mathrm{mol} .$ Calculate the standard enthalpy change for the reaction between magnesium metal and ammonia gas.

Key Concepts

Reaction Driving Force

The driving force of a reaction refers to the factors that make the reaction proceed in a particular direction, such as the formation of more stable products or the release of energy. In many reactions, the transformation from a less stable to a more stable chemical species or the evolution of a gas can serve as a key motivating factor.

Standard Enthalpy and Thermochemistry

Thermochemistry deals with the heat changes during chemical reactions, often measured by enthalpy changes. The standard enthalpy of formation provides a reference point to calculate the overall enthalpy change for reactions using Hess's law. Understanding these concepts is essential for evaluating the energetic favorability and spontaneity of chemical processes.

Limiting Reactant Identification

This concept refers to the process of determining which reactant will be completely consumed first in a chemical reaction, thereby limiting the amount of product that can be formed. Recognizing the limiting reactant is critical for predicting yields and for planning efficient chemical reactions.

Chemical Equation Balancing

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is upheld. It involves adjusting the coefficients of reactants and products so that the number of atoms for each element is equal on both sides of the equation, which is crucial for accurate stoichiometric calculations.

Ionic Compound Formation

This concept involves understanding how ions combine based on their charge to form neutral compounds. In general, the total positive charge from the metal cations must balance the total negative charge from the anions. This principle is essential for predicting the formulas of ionic compounds by using the known charges of the constituent ions.

Stoichiometry

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It allows chemists to calculate the amounts of substances consumed and produced by a reaction, often involving conversions between mass and moles, and the determination of mass percentages in mixtures.

Transcript

00:01 In this problem, we're asked to consider burning magnesium in air.

00:07 One product on burning magnesium air is the product of magnesium and oxygen.

00:15 And that product is magnesium oxide.

00:21 The second product is the reaction of magnesium and nitrogen.

00:32 And that product is magnesium nitrite.

00:41 Excuse me.

00:45 When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas.

01:14 Okay, so here's all of our equations.

01:17 I'm going to go ahead and balance these.

01:18 I think we have to do these later, but i can't take not having them balanced.

01:25 And here we have a three, a two, and a three.

01:34 I think that should be good.

01:37 Okay.

01:37 So part a says based on the charge of the nitride ion, write the formula for magnesium nitride, which i already did.

01:47 Magnesium is a 2 plus, nitride is a 3 minus, so the formula is mg3n2.

02:04 Part b says write the balanced equation for, oh, i already did this, here is the balanced equation for part b.

02:19 What is the driving force for this reaction? looking at this, i'm going to say the driving force for this reaction is probably the formation of nh3, which is a gas.

02:38 I'm going to say that that's going to be a driving force for this reaction.

02:44 And then c, i believe, is a long one, so i'm going to get started with c on another page.

02:51 C might actually take two pages.

02:55 So in an experiment, a piece of magnesium ribbon is burned in air.

02:58 So we're burning magnesium ribbon.

03:02 The mass of magnesium oxide and magnesium nitrite is 0 .470 grams after the reaction.

03:12 And this is a mixture of mgo and mg3n2.

03:22 Water is added to the crucible and we're all finished.

03:26 We add water, we heat it to dryness, and we get 0 .4.

03:33 Eight six grams of m -g -o only.

03:41 What's the mass percentage of m -g -3 and 2 in the 0 .470 grams? that's what we're going to be looking for.

04:06 Okay, so let's figure this out.

04:12 I want to get to all of the different things here.

04:16 So i set this up by a bunch of numbered steps.

04:18 I'm just going to go ahead and number my steps.

04:22 One, my initial mass of magnesium is going to be equal to magnesium oxide mass times, i'm going to call this the molar mass of magnesium divided by the molar mass of mgo.

04:56 Substitute our values in here, and we get magnesium.

05:03 I'm going to call it initial mass.

05:05 Just abbreviate initial, equals 0 .486 grams times the molar mass of magnesium is 24 .31 grams.

05:27 And the molar mass of magnesium oxide is 40 .31 grams per mole.

05:35 When i do my work here, i got 0 .293 grams.

05:43 And that was my mass of magnesium.

05:46 And we'll need that further on in the problem.

05:49 For the second thing, i got ready.

05:53 I'm going to say let x equal the mass of the mg mass in mgoo, and let y equal the mg mass in mg 3 and 2.

06:18 We know that x plus y has to equal my total mass of my product, which we said was 0 .470.

06:26 Oops, that's grams.

06:28 My bad.

06:31 0 .293 grams of mg.

06:39 I'm going to tell you a little later, we're going to solve this for x, so i'm just going to go ahead and write x equals 0 .293 grams minus y.

06:57 Let's do our third step, which my third step was find the masses of mgo and mg3 and 2.

07:14 So my mgo mass, magnesium oxide mass, i just took that equal to x from above times the molar mass of magnesium oxide divided by the molar mass of magnesium.

07:49 So i'm going to go ahead and just write those down.

07:52 That was 40 .31 grams and 24 .31 grams.

08:01 Actually, grams per mole, but those cancel out.

08:10 And my mg3n2 mass, it's called the y, same thing here, except it's molar mass of mg3n2, and the molar mass of mg3 times 3.

08:33 So in this case, that is 100 .9.

08:42 I think i wrote 929, divided by 3 times 20.

08:47 4 .31.

08:53 Okay.

08:56 If i add for number four, i'll keep the same color, if i add 3a plus 3b, and i'm going to go back a moment, let's call this 3a and 3b, if i add those two, i'm going to get my mass of product, which we were given.

09:29 So i'm just going to rewrite this quickly here, mass of product equals my mgo mass, which is x times 40 .31 over 24 .31 plus y times 100 .929 times 3 times 24 .31.

10:15 Now back over here i have this that we were going to substitute in for x.

10:22 So let's go ahead and substitute this in for x.

10:24 I'll switch colors.

10:27 And i'm also going to figure out these values.

10:31 My mass of my product was 0 .470 grams.

10:38 We're going to substitute for x 0 .293 grams.

10:47 Whoops...

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