Unit 4 Chemistry Questions

[horizon]

Hi,

1) For the oxidation of Cl- to be preferred what is its required concentration?
2) For heat of combustion value should be put the negative sign or not?

Thank you!

[LukeHigham]

Despite being a slightly weaker reductant than water, even at standard conditions (1M, 25oC), the oxidation of Cl- to Cl2 occurs to a significant amount because its reaction rate is faster than the oxidation of water.

Concentrations above 1 M will favour oxidation of Cl-.

[krith]

2) For heat of combustion value should be put the negative sign or not?

Yes, as the combustion reactions release energy, characteristic of exothermic reactions which are negative (they are involve an increase in temperature) and hence you need a negative value for the delta H calculation

[sin0001]

No, you do not need the negative sign when writing the heat of combustion, because it's defined as the "energy released when a specified amount of substance burns in oxygen," therefore it should already be assumed that delta H is negative, as energy is always released in combustion reations

[LeviLamp]

It should be included anyway - better to be safe than sorry, and since questions can ask for so many different things there's zero advantage in leaving the sign off, even if including it doesn't attract a penalty.

[krith]

No, you do not need the negative sign when writing the heat of combustion, because it's defined as the "energy released when a specified amount of substance burns in oxygen," therefore it should already be assumed that delta H is negative, as energy is always released in combustion reations

When asked to write a thermochemistry equation like for the complete combustion of methane, for example, you need to put a negative sign (you can look through the examiners reports for such questions and they all have negative signs, I'm not sure whether you'd still be awarded the mark without it) whereas if you're asked how much energy is released you don't need the sign, but if it's asking for a delta H value, I'm fairly certain you need the sign and yeah as LeviLamp said, even if it's not imperative, better to be safe than sorry

[sin0001]

I thought you only needed the negative sign for the delta H, not heat of combustion? But won't they take a mark off for including the neg. sign when you didn't have to include it?

[Nobby]

If you look at the back of the data booklet, all "heat of combustion" values are positive, and I think they will take marks off for a negative sign.

[charmanderp]

If you look at the back of the data booklet, all "heat of combustion" values are positive, and I think they will take marks off for a negative sign.

From memory they're actually negative.

[Reckoner]

They're negative in the data booklet, but are called "molar enthalpies of combustion". Heat of combustion is positive though (I think...). 

[Nobby]

If you look at the back of the data booklet, all "heat of combustion" values are positive, and I think they will take marks off for a negative sign.

From memory they're actually negative.

oh balls, what the hell am i thinking about

[joseph95]

Guys think about what combustion means - the process of burning a substance
Won't burning something always release energy, and thus always be negative/exothermic?
You definitely have to have the negative sign

[Reckoner]

I know delta H is negative, and when you write a thermochemical equation you include the negative sign, but if the question asks explicitly for "heat of combustion",it's asking for the heat evolved, so positive (I think...).

[d3stiny]

Heat of combustion - positive
Molar enthalpy - negative  (which is in data book)

Straight from the chief assessor.