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Thread: A question on Chem

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    A question on Chem

    This may seem like a weird question (probably is). But what really is the point of balancing chemical equations? Like, what are the real world applications to it? For example, Magnesium + Hydrochloric Acid ---> Magnesium Chloride + Hydrogen - this is represented as:

    Mg + 2HCl ---> MgCl2 + H2

    So what does it mean by '2HCl' or 'MgCl2'? Why do we need to know that a Magnesium atom + 2 Hydrogen and Chlorine atoms will give MgCl2 + H2, how does the accuracy (i.e. we need exactly 2HCl and a single Mg atom to get the product) come up in real life? If we were to mix Magnesium + Hydrochloric Acid, how would we be able to measure the '2HCl' bit and put that in the experiment (actual test tube)? How do we then measure that the product is MgCl2 + H2 (is there a special way of observing different molecules and such)?

    I'm probably missing a very simple point. And also, to the technicality bit of it I mentioned above, my teacher said we don't need to worry about that and just focus on the theory of it, but I'm quite curious as to how Scientists find out how many different individual atoms are needed and SHOWN in a reaction.

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    Junior Member Sp3ctre's Avatar
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    Re: A question on Chem

    I'm not 100% sure but I'm pretty sure both sides of the reaction need to be balanced otherwise the conservation of energy/mass would be violated. In your example if it was only HCl, then you're saying that a H and Cl atom magically appeared. The real world application is basically for every mole of Mg you need 2 moles of HCl for the reaction to completely occur, otherwise there would be an excess of Mg and not enough HCl. You need to know how to balance equations simply because if you don't, then mole calculations won't work because the ratio of your reactants/products will be wrong.
    HSC 2017: English Adv - Maths Ext 1 - Maths Ext 2 - Chemistry - Physics
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    Re: A question on Chem

    Quote Originally Posted by Sp3ctre View Post
    I'm not 100% sure but I'm pretty sure both sides of the reaction need to be balanced otherwise the conservation of energy/mass would be violated. In your example if it was only HCl, then you're saying that a H and Cl atom magically appeared. The real world application is basically for every mole of Mg you need 2 moles of HCl for the reaction to completely occur, otherwise there would be an excess of Mg and not enough HCl. You need to know how to balance equations simply because if you don't, then mole calculations won't work because the ratio of your reactants/products will be wrong.
    I get what you mean, but how did Scientists discover that you need 2 molecules of HCl for every mole of Mg? How do you measure that? Do you just add the atomic mass of each until they balance - because the Law of Conservation of Matter states that "The total mass of reactants in a chemical reaction will equal the total mass of products".
    Last edited by _Anonymous; 26 Sep 2017 at 8:54 PM.

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    Re: A question on Chem

    You need to balance equations properly because in the real world, you need to know what your limiting reagent is and to measure things out in proportion, it is also much easier to compare reactions with each other if you balance all the reactions out.

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    Re: A question on Chem

    Quote Originally Posted by strawberrye View Post
    You need to balance equations properly because in the real world, you need to know what your limiting reagent is and to measure things out in proportion, it is also much easier to compare reactions with each other if you balance all the reactions out.
    Right. But it's not like if you were carrying out an experiment you could measure how many atoms of each molecule you're putting in to get a reaction? For example, it's not like a recipe where a specific amount would mean x amount of atoms in a mole is it? I always thought of chemical equations being some sort of recipe where scientists would measure specific amounts of molecules to get a certain reaction.

    So in this case if you wanted to mix in Mg + 2HCl, how would you get the '2HCl' to add to the mixture? How do scientists get 2 of a compound? This is a really weird question, soz.
    Last edited by _Anonymous; 24 Sep 2017 at 11:07 PM.

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    Re: A question on Chem

    Quote Originally Posted by _Anonymous View Post
    Right. But it's not like if you were carrying out an experiment you could measure how many atoms of each molecule you're putting in to get a reaction? For example, it's not like a recipe where a specific amount would mean x amount of atoms in a mole is it? I always thought of chemical equations being some sort of recipe where scientists would measure specific amounts of molecules to get a certain reaction.

    So in this case if you wanted to mix in Mg + 2HCl, how would you get the '2HCl' to add to the mixture? How do scientists get 2 of a compound? This is a really weird question, soz.
    In year 11 and 12 chemistry you will go more in depth into moles and how they work, moles are given the definition of the mass of the actual compound or element used (in grams) divided by the molecular mass of the compound or element (given in the periodic table) but to answer your question let me create a scenario.
    Say you have 2 g of magnesium and you wanted to produce magnesium chloride. The way to do this quite obviously is by using the above reaction and react the magnesium with hydrochloric acid. However you dont know how much hydrochloric acid you need to use, you know that by reacting the two reactants you will create your product but if you dont know the exact amount you would just be creating a whole lot of waste which would not be very efficient in the real world or in labs. So you would have to calculate the amount of HCl you would have to use, this is where moles come into play.

    Magnesium has a molar mass of about 24.3 so you can calculate the moles which equals 2/24.3 or 0.08 (correct to one significant figure).
    So now you know the moles are equal to 0.08 then you use the mole ratio of Mg:HCl which is 1:2
    This means that HCl has 0.16 moles (0.08 x 2)

    So now you know the moles you can work backwards and find the mass of HCl required (0.16 x 36.458 = 5.8333g)
    And so now you know that in order to react the complete 2g of magnesium you would require approximately 6g of HCl. Of course scientists wont always be able to replicate the precise amount of grams required so they may use approximations but the moles do serve as a guide as to the exact amount of each reactant require.

    There are alot more ways moles can be used for example if you want to create a specific amount of the product but this is just one example.

    Hope this helped if you have any more questions or confused by anything i said please ask.

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    Re: A question on Chem

    Quote Originally Posted by ichila101 View Post
    In year 11 and 12 chemistry you will go more in depth into moles and how they work, moles are given the definition of the mass of the actual compound or element used (in grams) divided by the molecular mass of the compound or element (given in the periodic table) but to answer your question let me create a scenario.
    Say you have 2 g of magnesium and you wanted to produce magnesium chloride. The way to do this quite obviously is by using the above reaction and react the magnesium with hydrochloric acid. However you dont know how much hydrochloric acid you need to use, you know that by reacting the two reactants you will create your product but if you dont know the exact amount you would just be creating a whole lot of waste which would not be very efficient in the real world or in labs. So you would have to calculate the amount of HCl you would have to use, this is where moles come into play.

    Magnesium has a molar mass of about 24.3 so you can calculate the moles which equals 2/24.3 or 0.08 (correct to one significant figure).
    So now you know the moles are equal to 0.08 then you use the mole ratio of Mg:HCl which is 1:2
    This means that HCl has 0.16 moles (0.08 x 2)

    So now you know the moles you can work backwards and find the mass of HCl required (0.16 x 36.458 = 5.8333g)
    And so now you know that in order to react the complete 2g of magnesium you would require approximately 6g of HCl. Of course scientists wont always be able to replicate the precise amount of grams required so they may use approximations but the moles do serve as a guide as to the exact amount of each reactant require.

    There are alot more ways moles can be used for example if you want to create a specific amount of the product but this is just one example.

    Hope this helped if you have any more questions or confused by anything i said please ask.
    Thank you for your explanation, I understand most of the concept now. Just got a few more questions.

    1) Could you explain why we use moles (I understand we'll learn about them in Year 11 and 12) and how Magnesium having 0.08 means HCl is 2 times 0.08? Aren't they two different things?

    2) How come you did 0.16 x 36.458 to find the mass of HCl? Were you multiplying the mole by the atomic mass of Hydrogen+Chlorine?

    3) I understand that we must balance equations to satisfy the law of conservation of matter. But how come we can't create nor destroy matter?

    4) The end result of Mg + HCl gives Mg + 2HCl ---> MgCl2 + H2. Is there a different way to write the product? That is, how do people find out that adding Mg + HCl would give Magnesium Chloride AND Hydrogen? How do they differentiate between that? So could people write down the product as H2 + MgCl2 or MgH2 + Cl2 or some other thing? How would we know in an exam for example (if question was 'Finish this Chemical equation' or something of that sorts) what Mg + HCl would produce (there is no product mentioned in the question), there's no balancing or anything to do?

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    Re: A question on Chem

    I think it's worth clarifying what it means to balance an equation. In the example you gave (Mg + 2HCl ---> MgCl2 + H2) you are somewhat correct in saying the coefficients are the number of atoms however the coefficients are more typically thought of as the number of moles of a particular reagent.

    The utility of using 'moles' instead of 'atoms' lies in calculating the mass of reagents required. You will notice on a periodic table each element has assigned molar mass/ atomic weight in units of g/mol. This tells us the exact mass needed for 1 mole of the substance. Thus in a synthetic experiment* you could add 24.305 g** of Magnesium (as solid) and 2 x 36.46 g = 72.92 g** of Hydrochloric acid*** (as contained in 1 L of a 2M HCl solution (a 2M solution contains 2 moles of reagent per litre of water)).

    Ultimately ensuring the reagents are properly balanced limits the problem of having a lot of leftover reagent at the end of a reaction. Think about what would happen if only one mole of HCl was used in the above reaction (hint: only half of the Mg reagent would be converted to product).

    *This type of reaction would never be done as a synthetic experiment - however it makes for a straightforward example.
    **The mass and volume of reagents in this example is extremely excessive and such a reaction would be scaled down to more managable quantities considering cost/ hazards associated with using large amounts of dangerous reagents.
    ***In reality HCl would be added in slight excess as it is difficult to measure accurately the amount of acid in solution.

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    Re: A question on Chem

    Quote Originally Posted by _Anonymous View Post
    Thank you for your explanation, I understand most of the concept now. Just got a few more questions.

    1) Could you explain why we use moles (I understand we'll learn about them in Year 11 and 12) and how Magnesium having 0.08 means HCl is 2 times 0.08? Aren't they two different things?

    2) How come you did 0.16 x 36.458 to find the mass of HCl? Were you multiplying the mole by the atomic mass of Hydrogen+Chlorine?

    3) I understand that we must balance equations to satisfy the law of conservation of matter. But how come we can't create nor destroy matter?

    4) The end result of Mg + HCl gives Mg + 2HCl ---> MgCl2 + H2. Is there a different way to write the product? That is, how do people find out that adding Mg + HCl would give Magnesium Chloride AND Hydrogen? How do they differentiate between that? So could people write down the product as H2 + MgCl2 or MgH2 + Cl2 or some other thing? How would we know in an exam for example (if question was 'Finish this Chemical equation' or something of that sorts) what Mg + HCl would produce (there is no product mentioned in the question), there's no balancing or anything to do?
    Not OP but:

    1) 0.08 is referring to the number of moles of Mg. The reaction equation specifies a mole ratio of 1 mole Mg to 2 moles HCl. Thus if there is only 0.08 moles of Mg available then 0.16 moles of HCl fits the mole ratio (1:2) defined by the reaction equation.

    Remember that moles are a standardised quantity as the varying mass of elements is accounted for. 1 mole of any element is 6.02x10^23 atoms of that element. https://chem.libretexts.org/Core/Phy...#39;s_Constant

    2) yes, atomic mass or molecular weight (MW) is the mass (in grams) of 1 mole of that element.
    given the number of moles (n) the equivalent mass (m) can be found by: m=n*MW eg. 0.16 moles*36.46 g/mole=5.83 g
    given the mass (m) the equivalent number of moles (n) can be found by: n=m/MW eg. 5.83 g/ 36.46 g/mole= 0.16 moles

    3) In the context of this chemical reaction (Mg + 2HCl ---> MgCl2 + H2), creating matter would be like being able to produce MgCl2 without putting the appropriate reagents in the reaction vessel. Destroying matter would be like putting the Mg and HCl in a sealed reaction vessel only to have them disappear with no reagents or products left. Thus you can see both of these scenarios are quite impossible.

    4) technically MgH2 and Cl2 are viable compounds in their own right but in the context of this reaction a certain understanding of what products are favourable is required to determine the products of a reaction. In general terms elements from opposite sides of the periodic table have a higher affinity for one another.

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    Re: A question on Chem

    Quote Originally Posted by acemer View Post
    Not OP but:

    1) 0.08 is referring to the number of moles of Mg. The reaction equation specifies a mole ratio of 1 mole Mg to 2 moles HCl. Thus if there is only 0.08 moles of Mg available then 0.16 moles of HCl fits the mole ratio (1:2) defined by the reaction equation.

    Remember that moles are a standardised quantity as the varying mass of elements is accounted for. 1 mole of any element is 6.02x10^23 atoms of that element. https://chem.libretexts.org/Core/Phy...#39;s_Constant

    2) yes, atomic mass or molecular weight (MW) is the mass (in grams) of 1 mole of that element.
    given the number of moles (n) the equivalent mass (m) can be found by: m=n*MW eg. 0.16 moles*36.46 g/mole=5.83 g
    given the mass (m) the equivalent number of moles (n) can be found by: n=m/MW eg. 5.83 g/ 36.46 g/mole= 0.16 moles

    3) In the context of this chemical reaction (Mg + 2HCl ---> MgCl2 + H2), creating matter would be like being able to produce MgCl2 without putting the appropriate reagents in the reaction vessel. Destroying matter would be like putting the Mg and HCl in a sealed reaction vessel only to have them disappear with no reagents or products left. Thus you can see both of these scenarios are quite impossible.

    4) technically MgH2 and Cl2 are viable compounds in their own right but in the context of this reaction a certain understanding of what products are favourable is required to determine the products of a reaction. In general terms elements from opposite sides of the periodic table have a higher affinity for one another.
    Thanks for your response, just got a few more questions.

    If you were to add Mg + HCl in a Chemical reaction, would it give MgCl2 + H2 as the product? How did the Mg + HCl turn into 2HCl?

    And also, could you explain point 4 again?

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    Re: A question on Chem

    Quote Originally Posted by _Anonymous View Post
    If you were to add Mg + HCl in a Chemical reaction, would it give MgCl2 + H2 as the product? How did the Mg + HCl turn into 2HCl?
    You are effectively saying Mg + HCl ---> MgCl2 + H2, which is correct in that those are the chemical species generated - however the equation is not correctly balanced.

    To balance the equation, the quantities of each atom must be the same on each side. Thus, Mg + 2HCl ---> MgCl2 + H2 and 1/2Mg + HCl ---> 1/2MgCl2 + 1/2H2 would both be acceptable ways of balancing the equation. Remember that the coefficient is best thought of as the number of moles of that chemical species, so essentially a ratio.


    I think you have the idea by now, however sometimes it helps to think about this from another point of view. Consider a bicycle manufacturer:

    A bicycle is composed of a frame and two wheels, represented by the following equation:
    frame + (2 x wheel) ---> bicycle
    The manufacturer must ensure the ratio of frames to wheels remains at 1:2 to avoid having leftover parts.
    For example if the manufacturer wishes to produce 100 bicycles, they will require 100 frames and 200 wheels.

    Imagine the manufacturer instead followed the below equation:
    frame + wheel ---> bicycle
    The manufacturer wanted to produce 100 bicycles following this (fundamentally incorrect) equation they would use 200 frames and 200 wheels to make the same number of bicycles as before. Thus, there would be 100 leftover bicycle frames - not a very efficient business model! If the manufacturer were somehow able to create 100 bicycles from 100 frames and 100 wheels, this would be a good example of a violation of conservation of mass, since an additional 100 wheels must have come out of thin air to be able create 100 complete bicycles.
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    Re: A question on Chem

    Quote Originally Posted by _Anonymous View Post
    4) The end result of Mg + HCl gives Mg + 2HCl ---> MgCl2 + H2. Is there a different way to write the product? That is, how do people find out that adding Mg + HCl would give Magnesium Chloride AND Hydrogen? How do they differentiate between that? So could people write down the product as H2 + MgCl2 or MgH2 + Cl2 or some other thing? How would we know in an exam for example (if question was 'Finish this Chemical equation' or something of that sorts) what Mg + HCl would produce (there is no product mentioned in the question), there's no balancing or anything to do?
    The example given, Mg + 2HCl ---> MgCl2 + H2, is reaction of a metal (Mg) with an acid (HCl) to produce a salt/ ionic compound (MgCl2) and hydrogen gas (H2).

    Note that a salt is an ionic compound bound by attraction of opposite charges (in this case Mg2+ and 2 x Cl-). The charge is determined by the group number from the periodic table https://chem.libretexts.org/Textbook...ds/3.2%3A_Ions . Ionic compounds should have neutral overall charge.

    All reactions of reactive metals with acids will follow the same form. See if you can predict the products for the following reaction (and balance the equation if necessary).
    Zn + H2SO4 →

    There a quite a number of different reaction classes with different general forms. It would take some time to cover them, but in general intimate knowledge of each reaction type is not essential as you will develop an intuiton for what should be produced or - as in more advanced reactions - determine the product by providing a mechanism.

    Some things to look out for in simpler reactions:
    - opportunities to form diatomic molecules (eg. H2, O2, N2, Cl2), which are gases. The formation of gasses is thermodynamically favourable. Note the formation of H2 gas in the above reaction.
    - ionic compounds formed will be neutral (overall charge of zero). Pay attention to charges and form the ionic compound according to the number of particular atoms that result in a neutral overall charge. Note MgCl2 produced above is neutral due to +2 charge from Mg and two -1 charges from two chlorine atoms.
    Last edited by acemer; 29 Sep 2017 at 2:31 AM.
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