Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: is3zzz on September 22, 2007, 03:02:36 PM
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True/False Regarding Chemical Reactions:
1. The total moles of reactants must equal the total moles of products.
2. If a gas is evolved, the mass of products must be less that that of reactants.
3. If the reactants are liquids and the products are gases, the density of the system decreases if the container is open to the atmosphere.
4. At least one of the products or reactants must be a solid.
5. Mass is conserved only in biochemical reactions.
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What do you think the answers are?
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1. False (only true if regarding to mass)
2. False (mass must be equal from reactants to products)
3. True (vaporization might cause the density to decrease)
4. False
5. False
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1. False.
It holds good when it is regarding mass, as well as ionic charges.
2. False.
If you can carry out the reaction in a closed system, then and only then, you can find out whether the masses are same or not ... if gases evolve in an open system, then surely, the mass of the reactants will be more than the products. But when everything is managed, in a closed system, the masses are always same of the Reactants and Products.
4. False.
You have various reactions as example to this!
5. False.
When you can have a closed system reaction, mass is conserved.
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As well as what's already been said, I'm not sure 3 is absolutely true; but can't think of any real life examples of a reaction which gives off a gas, and ends up with a more dense liquid than before.
Something along the lines of carbene formation and subsquent dimerisation from chloroform perhaps?
HCCl3 -> HCl (g) + :CCl2
2 :CCl2 -> C2Cl4 ?
but then again both chloroform and 1,1,2,2-tetrachloroethylene are relatively volatile in their own right...
S
(edit: typos)
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I would think 3 is true. I doubt you could find any examples of gases that are denser than the liquids from which they are derived (note the question does not say that the reaction gives off a gas, it says that the products are gases. This to me implies that all of the products are gases).
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FFTFF respectively
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I would think 3 is true. I doubt you could find any examples of gases that are denser than the liquids from which they are derived (note the question does not say that the reaction gives off a gas, it says that the products are gases. This to me implies that all of the products are gases).
Point taken. However, I still have a couple of sneaky points to follow-up (again, these are merely hypothetical, as I can't think of any real life examples, perhaps this is clutching at paper straws to prove a point)
a) If the reactants are liquid, but less dense than the solvent for the reaction, then the density of the solution will gradually increase to that of the bulk solvent as the reactants are converted to products and hence expelled as gases.
b) If there is *no* solvent, and everything is perfectly balanced stoichiometrically, then the *density* of the reaction medium will not change at all.
Feel free to ignore these ramblings ;)
S