No. of moles = Mass/ Mr or Mass/ Ar
No. of moles = Volume (dm3) x Concentration (mol/dm3)
No. of moles = Volume (dm3) / 24 dm3
Home page
Explain, using structure and bonding, why ethanol cannot conduct electricity.
Ethanol has a simple covalent structure. It exists as discrete molecules in the different states. There are no mobile ions or delocalised electrons to conduct electricity.
Running Distance and Time
ratio m:n:p
Double-Pivot Push-up
Why is there a need to recycle metals?
Metals are finite resources.
With the increasing demand for metals, our metal reserves will not last much longer.
Hence, there is a need to recycle metals to conserve the metal resources.
Recycling metals also use up less energy and produce fewer pollutants than extracting metals from their ores.
What are the observations when Group I metals react with cold water?
– Metal piece darts about quickly around the surface of the water
– Effervescence observed (due to hydrogen gas produced in water)
– Hissing sound heard (due to the vigorous release of hydrogen gas)
– Metal piece becomes smaller in size
– Flame observed (for some metals)
– Lithium (no flame)
– Sodium (yellow flame)
– Potassium (lilac flame)
– pH of solution increases from pH7 to pH14 (due to the formation of a strong alkali)
Describe the effects of absorbing electromagnetic waves.
Heating Effect
– Infrared heating
Fires and hot bodies emit infra-red radiation. Our skin absorbs the infra-red rays, making us feel warm.
– Microwaves heating
Microwaves are of a non-ionizing character. However, microwaves, being more penetrative, can cause more severe burns to the skin, especially when skin has high water content.
Ionisation Effect
UV rays, X-ray, Gamma ray can cause breakdown of DNA, and ionization of molecules and atoms in living matter. The mutation of these cells and atoms can lead to formation of cancerous cells.
Explain, in terms of oxidation state, why the following reaction is a redox reaction.
PbS (s) + 4H2O2 (aq) -> PbSO4 (s) + 4H2O (l)
PbS has been oxidized to PbSO4.
The oxidation state of sulfur increased from -2 in PbS to +6 in PbSO4.
H2O2 has been reduced to H2O.
The oxidation state of oxygen decreased from -1 in H2O2 to -2 in H2O.
Since oxidation and reduction occurred simultaneously in the reaction, it is a redox reaction.
Format for explanation in terms of oxidation state
(The reactant substance) has been oxidized / reduced to (the product substance).
The oxidation state of (the element) increased / decreased from (o.s. no.) in (the reactant substance) to (o.s. no.) in (the product substance).
Examples:
Explain whether the underlined substance has been oxidized or reduced, in terms of oxidation state.
1. ZnO (s) + C (s) -> Zn (s) + CO (g)
Zinc oxide has been reduced to zinc.
The oxidatation state of zinc decreased from +2 in zinc oxide to 0 in zinc.
2. PbS (s) + 4H2O2 (aq) -> PbSO4 (s) + 4H2O (l)
Lead(II) sulfide has been oxidized to lead(II) sulfate.
The oxidation state of sulfur increased from -2 in lead(II) sulfide to +6 in lead(II) sulfate.
3. PbO (s) + H2 (g) -> Pb (s) + H2O (g)
Lead(II) oxide has been reduced to lead.
The oxidation state of lead decreased from +2 in lead(II) oxide to 0 in lead.
4. 2FeCl2 (aq) + Cl2 (g) -> 2FeCl3 (aq)
FeCl2 has been oxidized to FeCl3.
The oxidation state of iron increased from +2 in iron(II) chloride to +3 in iron(III) chloride.