Chemistry Paper 2: subject content Watch

A5719
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I have a few questions about chemistry unit 2:
1 - For the process of electrolysis is hydrogen always formed? at the cathode?
2 - How many factors affect rate of reaction ?
3 - How do you work out the empirical formula ?
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Yorrick
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(Original post by A5719)
I have a few questions about chemistry unit 2:
1 - For the process of electrolysis is hydrogen always formed? at the cathode?
2 - How many factors affect rate of reaction ?
3 - How do you work out the empirical formula ?
1- Hydrogen is only formed if:

a) The ionic compound is in solution (i.e. dissolved in water) instead of being molten.
b) The positive ion in the ionic compound is HIGHER on the reactivity series than hydrogen (you get given the reactivity series, so you won't need to remember this)

If both of these are true, then hydrogen forms at teh cathode. It never forms at the anode because hydrogen ions are positive and are attracted to the negative electrode, which is the cathode.

In case it comes up, the half equation for hydrogen (higher only) is

2H+ + 2e- --> H2

This is the only positive ion half equation where the product is a diatomic molecule.

2- Factors that affect rate:

Concentration: More particles dissolved in a volume of water mean more collisions.

Pressure: More gas particles in a space mean more collisions.

Temperature: The particles have more kinetic energy so they move faster, meaning there is more chance of collisions. Also, more particles have energy equal to or above the activation energy, so there are more successful collisions.

Surface area of a solid: More of the solid is exposed to the liquid, so there are more collisions.

Catalyst: The catalyst reduces the activation energy for a reaction, so more particles have the activation energy, so there are more successful collisions.

Here is a great video to help you remember: https://www.youtube.com/watch?v=OttRV5ykP7A

3) For an empirical formula clculation, you will be given the mass of the elements in athe compound. The mass may be given in % or in grams. It doesn't matter what the unit is, you follow the following steps.

1) For each element, use the periodic table to get the relative atomic mass (the top number).
2) For each element calculate the mass divided by the relative atomic mass (this gives the moles of each element).
3) Find the smallest value for the moles.
4) Divide the moles of each element by the smallest value for the moles. This means that the element with the smallest number of moles will have 1 mole.
5) If a value to 4 is almost a whole number, round it to the nearest whole number. If a value has a half, multiply all the numbers by 2 to make them whole.
6) Express as an equation.

I have attached a load of empirical formulae questions for you and there is an exaample here:

http://www.bbc.co.uk/schools/gcsebit...gherrev2.shtml
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