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Levart [38]
2 years ago
11

We might think of a porous material as being a composite wherein one of the phases is a pore phase. Estimate upper and lower lim

its for the room-temperature thermal conductivity of a magnesium oxide material having a volume fraction of 0.10 of pores that are filled with still air.
Chemistry
1 answer:
eduard2 years ago
8 0

Answer:

The upper and lower limits for the room-temperature thermal conductivity of a magnesium oxide material having a volume fraction of 0.10 of pores that are filled with still air are

Ku = 38.252 W/mK

K lower = 0.199 W/mK

Explanation:

As we know  

Ku = Vp * Kair + Vmagnesium * K metal  

Ku = 0.10 *0.02 + (1-0.25) * 51

Ku = 38.252 W/mK

The lower limit  

K lower = Kmetal* Kair/( Vp * Kmetal + Vmetal * K air)

K lower = (0.02*51)/(0.10*51 + 0.90 * 0.02)

K lower = 0.199 W/mK

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Suppose a laboratory wants to identify an unknown pure substance. The valence electrons of the substance's atoms feel an effecti
zalisa [80]

Answer:

  • The answer is the third option in the list:<em> It would have smaller atomic radii than Si and higher ionization energies than Si.</em>

Explanation:

The<em> effective nuclear charge</em> is that portion of the total nuclear charge that a given electron in an atom feels.

Since, the inner electrons repel the outer electrons, t<em>he effective nuclear charg</em>e of a determined electron is the sum of the positive charge (number of protons or atomic number) that it feels from the nucleus less the number of electrons that are in the shells that are are closer to the nucleus than the own shell of such (determined) electron.

Mathematically, <em>the effective nuclear charge (Zeff)</em> is equal to the atomic number (Z) minus the amount (S) that other electrons in the atom shield the given (determined) atom from the nucleus.

  • Zeff = Z - S.

Since, the valence electrons are the electrons in the outermost shell of the atom, you can find certain trend for the value Zeff.

Let's look at the group to which Si belongs, which is the group 14. This table summarizes the relevant data:

Element   Z   Group   # valence electrons     S                      Zeff = Z - S

C              6      14                      4                     6 - 4 = 2             6 -  2 = +4

Si             14     14                      4                     14 - 4 = 10         14 - 10 = +4

Ge           32     14                     4                     32 - 4 = 28       32 -28 = +4

Sn           50     14                     4                     50 - 4 = 46       50 - 46 = +4

Pb           82     14                     4                     82 - 4 = 78        82 - 78 = +4  

With that, you have shown that the valence electrons of the unknown substance's atoms feel an effective nuclear charge of +4 and you have a short list of 4 elements which can be the unknown element: C, Ge, Sn or Pb.

The second known characteristic of the unknown substance's atoms is that it has a <em>higher electronegativity than silicon (Si)</em><em>.</em>

So, you must use the known trend of the electronegativity in a group of the periodic table: the electronegativity decreases as you go down in a group. So, three of the elements (Ge, Sn, and Pb) have lower electronegativity than Si, which has left us with only one possibility: the element C. The valence electrons of carbon (C) atoms feel an effective nuclear charge of +4 and it carbon has a higher electronegativity than silicon.

Other two periodic trends attending the group number are the <em>atomic radii and the ionization energy</em>.

The atomic radii generally increases as you go from top to bottom in a group. This is because you are adding electrons to new higher main energy levels. So, you can conclude that the originally unknwon substance (carbon) has a smaller atomic radii, than Si.

The ionization energies generally decreases as you go from top to bottom in a group. This os due to the shielding effect: as seen, the effective nuclear charge of the atom's valence electrons remains constant, while the distance of the electrons from the nucleus increases (the valence electrons are farther away from the nucleus), which means the upper the element in a given group, the larger the ionization energy of the atoms.

With this, our conclusions about the unnkown substance are:

  • Since it has a higher electronegativity value than silicon (Si), it is right up of Si, and there is on only element possible element than can be (C).

  • Since, it is upper than silicon (Si), it would have smaller atomic radii.

  • Due to the shielding effect, it would have larger ionization energies.

  • The answer is the third option in the list: It would have smaller atomic radii than Si and higher ionization energies than Si.

6 0
3 years ago
Rank the compounds in each set in order of increasing acid strength.
Viktor [21]

Answer:

See explanation

Explanation:

For this question, we have to remember the effect of an atom with high <u>electronegativity</u> as "Br". If the "Br" atom is closer to the carboxylic acid group (COOH) we will have an <u>inductive effect</u>. Due to the electronegativity of Br, the electrons of the C-H bond would be to the Br, then this bond would be <u>weaker</u> and the compound will be more acid (because is easier to produce the hydronium ion H^+).

With this in mind, for A in the last compound, we have <u>2 Br atoms</u> near to the acid carboxylic group, so, we will have a high inductive effect, then the C-H would be weaker and we will have <u>more acidity</u>. Then we will have the compound with only 1 Br atom and finally, the last compound would be the one without Br atoms.

In B, the difference between the molecules is the <u>position</u> of the "Br" atom in the molecule. If the Br atom is closer to the acid group we will have a <u>higher inductive effect</u> and more <u>acidity</u>.

See figure 1

I hope it helps!

5 0
3 years ago
A closed container contains 0.40 moles of argon gas at 25 °C and a pressure of 740 torr. The container is heated to 125 °C and t
Ksivusya [100]

The number of moles of argon that must be released in order to drop.

Solution:

Initial Temperature = 25°c = 298 K

Final Temperature =125 °c = 398 K

Initial Moles (n1) = 0.40 mole

Now,  Using the ideal gas law,

n1T1 = n2T2

0.400×298 = n2 × 398

n2 = 0.299 mol

Moles of Argon released

= 0.400-0.299

= 0.100 mol.

Pressure and force are related. That is using the physical equations if you know the other, you can calculate one using pressure = force/area. This pressure can be reported in pounds per square inch, psi, or Newtons per square meter N/m2. Kinetic energy causes air molecules to move faster. They hit the walls of the container more often and with greater force. The increased pressure inside the can may exceed the strength of the can and cause an explosion.

Learn more about The temperature here:-brainly.com/question/24746268

#SPJ1

7 0
1 year ago
Classify each of these reactions. Ba(ClO3)2 → BaCl2 3 O2 double replacement combination single replacement decomposition CaO CO2
faltersainse [42]

Answer:

Ba(ClO₃)₂ → BaCl₂ +  3 O₂ <em>Descomposition. </em>

CaO + CO₂ → CaCO₃ <em>Combination. </em>

NaNO₂ +  HCl → NaCl + HNO₂ <em>Double replacement. </em>

Mg + ZnSO₄ → MgSO₄ + Zn <em>Single replacement. </em>

Explanation:

A combination reaction is defined as a reaction in which two or more substances combine to form a single new substance.

A + B → AB

A descomposition reaction is defined as a reaction in which a compound breaks down into two or more simpler substances.

AB → A + B

A double replacement is a chemical reaction were the positive and negative ions of two ionic compounds exchange places to form two new compounds.

AB + CD → AD + CB

A single replacement is another type of reaction were one element replaces a similar element in a compound.

A + BC → AC + B

Thus, with this information it is possible to classify these reactions as:

Ba(ClO₃)₂ → BaCl₂ +  3 O₂ <em>Descomposition. </em>One single molecule breaks down into two or more molecules

CaO + CO₂ → CaCO₃ <em>Combination. </em>Two substances are combined to form one single molecule

NaNO₂ +  HCl → NaCl + HNO₂ <em>Double replacement. </em>Na and H are exchange places to form two new compounds

Mg + ZnSO₄ → MgSO₄ + Zn <em>Single replacement. </em>Mg is replacing Zn.

I hope it helps!

6 0
3 years ago
Explain why changes of state are physical changes.
kaheart [24]

Solution: Change in state of matter is a physical change because of the physical condition and appearance changes but not the chemical composition.

6 0
3 years ago
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