Law of Conservation?

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Brendan Duong 1I
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Law of Conservation?

Postby Brendan Duong 1I » Mon Oct 05, 2020 10:59 am

Referencing the Week 1 Monday lecture for this question:
Once we balance the equation
2Na+ 2H2O -> 2NaOH +H2
why do we say it is
2 moles + 2 moles --> 2 moles +1 mole when 4 is not equal to 3. Isn't there supposed to be the law of conservation?
I understand that if we actually multiply out the coefficients with the subscripts, the number of atoms of each element is equal on both sides, but it seems we ignore this step and only pay attention to the coefficients?
Can someone explain?
Time stamp 34:22 on Lecture 10/05/2020

Sophia Hu 1A
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Re: Law of Conservation?

Postby Sophia Hu 1A » Mon Oct 05, 2020 11:03 am

The law of conservation is referring to the law of conservation of mass which indicates total mass before = total mass after. This is also the same as the total ATOMS left (reactants) = total atoms right (products).

It is definitely confusing with how it was written with moles, but the law of conservation does not refer to the number of moles for the different compounds. Instead, it focuses on the conservation of the number of atoms of each element. The coefficients are not an indicator of the law of conservation, but you have to go through the process that you explained above about multiplying out the coefficients with the subscripts to determine the number of atoms.

Hope this helps!

Sandy Lin 1L
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Re: Law of Conservation?

Postby Sandy Lin 1L » Mon Oct 05, 2020 11:04 am

Law of Conservation is the idea that the total mass is the same before and after. This leads to the amount of atoms having to be the same values in order to add up to equal masses. The moles are only used in order to balance/adjust the number of atoms so they do not have to be equal as long as the masses are equal.

Gina Spagarino 3G
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Re: Law of Conservation?

Postby Gina Spagarino 3G » Mon Oct 05, 2020 11:05 am

The law of conservation of mass refers to the total number of atoms in the reactants and products; so, by rearranging these atoms into different compounds, the total moles of the individual pieces of the the reactants/compounds are not always the same.
Last edited by Gina Spagarino 3G on Mon Oct 05, 2020 11:06 am, edited 1 time in total.

Cecilia Cisneros 1F
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Re: Law of Conservation?

Postby Cecilia Cisneros 1F » Mon Oct 05, 2020 11:06 am

I believe when we say:
"2 moles + 2 moles --> 2 moles + 1 mole"
we are providing the molar ratio within the chemical equation of its reactants and its products.
However, we are not directly adding the number of moles to say that 4 = 3 because this statement is not true.
I hope this helps!

Marisa Gaitan 2D
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Re: Law of Conservation?

Postby Marisa Gaitan 2D » Mon Oct 05, 2020 11:06 am

As long as the numbers of moles of ELEMENTS are the same, then we are safe to say the equation is balanced. If you add up the number of moles each element in the equation, we get 2 Na atoms on the left, 2 on the right; 4 H atoms on the left, 4 on the right; and 2 O atoms on the left 2 on the right.

Nicole Huang 3F
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Re: Law of Conservation?

Postby Nicole Huang 3F » Mon Oct 05, 2020 11:07 am

Because each element has a different molar mass, the moles do not have to be 4=4 (the coefficients are there to balance out the differences in molar mass if that makes sense). To follow the law of conservation of mass, you would have to convert the molecules from moles to mass.
So for the example he used:
2Na+ 2H2O -> 2NaOH +H2 would roughly end up being:
2mol(23g/mol)+2mol(18g/mol) -> 2mol(40g/mol)+1mol(2g/mol)
46g+36g=80g+2g
82g=82g

The total mass before is equal to the total mass after. Nothing is created nor destroyed, therefore following the law of conservation of mass. Hope this helps!

anikamenon2H
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Re: Law of Conservation?

Postby anikamenon2H » Mon Oct 05, 2020 11:09 am

The moles of each atom are equal on either side of the equation, that's what matters. The stoichiometric coefficients are used to balance the moles of each atoms but they themselves do not equal each other on both sides of the equation. There are 2 moles of Na on either side, 2 moles of H and 2 moles of O meaning that no mass was created or destroyed. The main idea here is that stoichiometric coefficients are used to balance out the moles of each atom but to find the total moles of each side, you must multiply it out.

In the combustion example of butane, on the right the stoichiometric coefficients add up to 15 while the on the left they add up to 18. Same idea here.

Hope this helps!

Emma Chang 1G
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Re: Law of Conservation?

Postby Emma Chang 1G » Mon Oct 05, 2020 11:27 am

I believe he said in his example of 2Na + 2H2O --> 2NaOH + H2 that chemists will more commonly refer to the reactants and products of a chemical equation in terms of moles, rather than the elements themselves. For example, he referred to 2Na as 2 moles of Na rather than 2 Na atoms. He just rephrased the equation as 2 moles + 2 moles --> 2 moles + 1 mole. The rearrangement of the elements during the chemical reaction will result in different moles of compounds produced on the other side. As long as the total mass before equals the total mass after, and the moles of individual elements are the same on both sides, I think the chemical equation will be balanced. Hope this helps!

Joey_Okumura_1E
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Re: Law of Conservation?

Postby Joey_Okumura_1E » Mon Oct 05, 2020 11:29 am

The amount of moles of each individual element is conserved, but the amount of moles of the compounds aren't necessarily reserved.

2Na+ 2H2O -> 2NaOH +H2 does not exactly translate to 2 moles + 2 moles --> 2 moles + 1 moles. It translates to 2 moles Na + 4 moles H + 2 moles O--> 2 moles Na + 2 moles O + 2 moles H + 2 moles H. (To find the amount of moles of each atom, multiply the coefficients by the subscript.) Notice that both the reactants and products have 2 moles Na, 4 moles H, and 2 moles O.

505352202
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Re: Law of Conservation?

Postby 505352202 » Mon Oct 05, 2020 11:30 am

Chemical equations obey the law of conservation of mass (matter cannot be created or destroyed). That is why we balance the equation to get the same number of atoms of each element on each side of a chemical equation. I think you might be confusing it with the number of moles.

Hope this helps!

JonathanSung_2G
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Re: Law of Conservation?

Postby JonathanSung_2G » Mon Oct 05, 2020 12:27 pm

Even if the coefficients do not add up to the same value on the left and right hand side of the equation, the law of conservation of mass is still upheld. Even if the moles of molecules are not equal on both sides (4 vs. 3 in your example) we can still see that there are 2 moles of Na, 4 moles of H, and 2 moles of O on both sides of the equation. Thus because there is an equal amount of moles of elements on the left and right, the number of atoms involved in the reaction doesn't change. Hope this helps!

Charisma Arreola 2F
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Re: Law of Conservation?

Postby Charisma Arreola 2F » Mon Oct 05, 2020 1:38 pm

When looking at the conservation of mass, we want to ensure we have balanced atoms on both sides. The easiest way for me to do this is to write out each element next to each other so I can see how many atoms there are on the reactants and products individually. I then balance is and rewrite the equation using that!

Ryan Laureano 3I
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Re: Law of Conservation?

Postby Ryan Laureano 3I » Mon Oct 05, 2020 4:44 pm

Law of conservation relates to mass. Moles does not equate to mass, but rather the amount of atoms in that molecule. Since there are a variety of elements in the molecule that have a variety of molar masses, the chemical equation can have different mole amounts on each side of the equal sign.

Nina Tartibi 1F
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Re: Law of Conservation?

Postby Nina Tartibi 1F » Mon Oct 05, 2020 6:20 pm

When dealing with balancing equations, you do not need to worry about having an equal number of stoichiometric coefficients on both sides of the equation. Balancing equations just means having an equal number of each substance on either side, therefore the mass is conserved.

Samiha Molla 3G
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Re: Law of Conservation?

Postby Samiha Molla 3G » Mon Oct 05, 2020 6:24 pm

the stoichiometric coefficients represent the number of moles (number of atoms) as you noted, but not to the masses. the masses on both sides of the equation should still be equivalent according to the law of conservation!

Alessia Renna 1D
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Re: Law of Conservation?

Postby Alessia Renna 1D » Wed Oct 07, 2020 1:46 pm

The law of conservation states that the mass of the reactants will always equal the mass of the products. The moles of each compound can change as long as the moles of each element are preserved.


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