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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: lazaward on February 21, 2018, 05:36:54 pm

Title: State of water in combustion reactions:
Post by: lazaward on February 21, 2018, 05:36:54 pm: Hello,

There are cases throughout the textbook where the combustion of methane or ethanol results in water in gas state and also sometimes it’s written in liquid state. I asked my teacher and she said to keep it in gas state but I’m really confused as to why it can be both and which one is actually right. Thank you
Title: Re: State of water in combustion reactions:
Post by: Yertle the Turtle on February 21, 2018, 05:44:40 pm: Quote from: carolinsale-17 on February 21, 2018, 05:36:54 pm
Hello,

There are cases throughout the textbook where the combustion of methane or ethanol results in water in gas state and also sometimes it’s written in liquid state. I asked my teacher and she said to keep it in gas state but I’m really confused as to why it can be both and which one is actually right. Thank you
I think that it depends on the energy released by the reaction. In a high energy release reaction the water will be in a gas state, but in some it will be liquid. Think of the exhaust of a car that has just been turned on, it drips water. This is because the engine is not yet warm, and the water vapour condenses, but when the engine is warmed up, the water is only water vapour. In general I think just use gas.
Title: Re: State of water in combustion reactions:
Post by: lovelyperson on February 21, 2018, 05:53:42 pm: Idk, but I was told by my teacher to put it in liquid because in the data book, it says that heat of combustion is calculated at SLC (wherein water is liquid).
Title: Re: State of water in combustion reactions:
Post by: applesauce on February 21, 2018, 11:20:45 pm: The phase of water is determined by the temperature and pressure at which the reaction occurs. Most reactions occur at room temperature at which water is a liquid. If the temperature is greater than the boiling point, then the water will be a gas.

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Mod edit (insanipi): removed link
Title: Re: State of water in combustion reactions:
Post by: ExamPro on February 22, 2018, 10:42:20 am: Quote from: carolinsale-17 on February 21, 2018, 05:36:54 pm
Hello,

There are cases throughout the textbook where the combustion of methane or ethanol results in water in gas state and also sometimes it’s written in liquid state. I asked my teacher and she said to keep it in gas state but I’m really confused as to why it can be both and which one is actually right. Thank you

Here is Thushan's commentary on water as a gas/liquid in thermochemical equations, excerpted from ExamPro Units 3&4 Chemistry:

(http://i.gyazo.com/a0a08c53108c5c26ccd5df4057dc4e24.png)

tl;dr don't worry about it, either liquid or gas is fine and nobody is going to split hairs on the state of water in your balanced thermochemical reaction.

However, it will matter if the state of water is explicitly enquired about in a question.
Title: Re: State of water in combustion reactions:
Post by: pha0015 on February 22, 2018, 10:46:38 am: It's fine to put water as either gas or liquid state, so long as there's no additional info such as temperature rose over 100 degrees Celsius, in which water would then have to be a gas. Usually though, putting liquid is the most commonly accepted answer, since most experiments are conducted at SLC, which is at 25 degrees Celsius
Title: Re: State of water in combustion reactions:
Post by: MAGGOT on February 27, 2018, 10:56:02 am: my chem teacher talked about that in the jacaranda chem 34 book and said to leave it in gas form or the book is wrong as, the shc could affect the enthalpy
Title: Re: State of water in combustion reactions:
Post by: Sine on February 27, 2018, 01:29:05 pm: definitely no need to memorise states at which temperature and generally both will be accepted (even though obviously one of them is wrong).

Even in past exams, most notably 2016 the state of CO2 hasn't been required and answers including (aq) or (g) were accepted