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Posts posted by Primarygun

  1. All stars rise from the east except at the poles.

    However, it is hard to determine whether the moon comes from the south-east or the north-east even when the relative positions of the earth and the moon is given.

    Is there anybody who has a clever method to determine the direction in which the moon rises?

  2. 1. Where is the sun in its ecliptic when it is the hottest period in the northen hemisphere?

    2. Take a look at the picture I upoloaded.

    Why is the time that the sun is at the meridian again 12:30 rather at at noon? How do you explain the wave shape of the curve?


    I'd think that it's because the discrepancies between the sidereal day and the apparent solar day, I don't know if it is the answer

    the time.png

  3. Is the rate constant in the rate equation equivalent to that in the arrhenius equation?

    Does the arrhenius equation describe the overall rate of reaction or can be interpreted to compute the rate of forward reaction and backward reaction respectively? If it could be employed to deal with both sides separately, then I know why the decay of radioactive substance has a constant half-life, otherwise, it still seems to be an enigma to me though after exhausting search in the web.


    Please help

  4. Here are a few questions puzzling me for a long period of time.

    1. If reactant X is converted to product Z through a intermediate Y.

    Then, it is unfair to compare the rate constant of (y to z and x to y) just on the basis of the activation energy for the steps, right?

    arrhenius equation : k=A *exp(Ea/RT)

    What does A mean and under what circumstances does it change.

    2. Recently, my class took a laboratory experiment but I am not sure about the validity of theorem behind.

    In the experiment, four different set-ups with the concentration of the propanone solution as the sole variable factor are used.

    At fixed time interval, the concentration of the remaining iodine concentration was recorded.

    The problem is here,

    We plot the rate of reaction against the concentration of the propanone which are set in different values in four set-ups.

    but the rate of reaction is taken as the slope of the graph of

    the concentration of iodine solution VS the time.

    Aren't the slopes only the average rate of the reaction, I bet initial rate should be taken, right?

  5. In DC circuits you look at the voltage across the capacitor to discern the behavior. But if there were no plates to charge up, the resistance of an open circuit is infinite.[/Quote]

    Do you mean that the capacitor keeps on changing during the discharge of itself in a DC circuit?

  6. What is the resistance of a capacitor? From the data of experiement, I was convinced that the resistance tends to zero. Is this the truth?

    There's a part of empty space between two parallel plates. Isn't the resistance very large as it seems it's hard for electrons to pass through these spaces. How do you reconcile the two facts?

  7. I am not sure what the point of inflection of the titration curve means.

    IN the case of only acid and base titration, say NaOH and HCl,

    I could perceive the term in the way that the point is when the dominating ion changes.( from OH- to H+)

  8. Thanks a lot. By the way, do you know why the curve(the pH against the volume of hydrochloric acid added into the mixture) is rather vertical at the point where the indicator changes its colour?

    I thought it ought to be gradual instead.

  9. There is a solution containing a mixture of carbonate ions and hydrogencarbonate ions.

    A few drops of phenolphthalein indicator is added into the solution, ammonium solution is added drop by drop into the solution following.

    When there's the sign of the permanent change of the colour of the indicator, my teacher calimed that all the hydrogencarbonate ions have been reacted through the reaction

    (HCO3 - + H+ ---> CO2 + H2O)

    My idea is that all carbonate ions did completely reacted instead.

  10. My book said that the gravitational field strength for an object at the equator is smaller than at the poles.

    I think the description is not exactly correct.

    It's because the gravitational field strength depends on the distance from the centre of the earth, doesn't it?

  11. Why is the gravitational field of the earth is larger when one is located at the equator than located at the poles?

    I know that rotation accounts for it, but why does rotation affect the earth's field strength? It doesn't make sense to me when we put a stationary object just above the earth relative to the sun, the gravitational field of the object changes a large amount when compared to the situation that it is in contact, rotating on the earth.

  12. the Arrhenius Equation

    ln K= constant - Delta H/(RT)

    What is K? Equilibrium constant of pressure or equilibirum constant of concentration?

    And I sometimes see that it is employed to find the activiation energy, so how things work?

    Thank you for your attention.

  13. hydrated lime is hydrated calcium hydroxide.

    adding excess acetic acid to dissolve the lime by neutralization.

    Then, I think next you can filter the solution to have a beaker of calcium acetate solution. I lack practical experience so I don't know whether the next step (( evaporating the solution) is correct.

    The size of the crystals is up to you, if you wnat to have large crystals, you should boil off only a small proportion of water, and set the beaker aside to let it cool itself.

  14. About Coulomb's law, it only works for a small test charge.

    I am not perceptive of this part.

    Does it works for a conductive sphere? I found many questions in my book employing this law to calculate the answer where it involves not only a small test charge but also a sphere.

  15. About the drawing the lewis structure, I am puzzled with deciding whether using the dative covalent bond or the double bond.

    What are you guys' very first step in drawing the structure of the covalent molecules?

    May somebody draw the structure of chlorate ion and sulphite ion?

    Is the biggest, most important task in drawing lewis structure is to give the every atom an octet structure and this task cannot be achieved only when there's a noble gas atom,right?

    and for the structure of sulphur dioxide molecule,

    if there is one dative bond and one double bond instead of two double bonds, then sulphur atom can obtain ocetet structure, but in reality, why this is not the case?

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