q = mct

Hello, stupid question but what are the units for each of these things?

q = kj?

m = g or kg?

c = JKg (little -1's after the K and g)

delta t = Kelvin or degrees?

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Reply 1

conroe-killed-the-k8-star

More accurately :

\Delta Q = mc\Delta T

\Delta Q

has units of joules .The base unit of mass is the kilogram - 1 gram is 10^-3 kg and you need to stick to this.

Units of

\Delta T

would be kelvins obviously.

To get a consistent equation where the units on both sides must match up you would need c to have units of

Jkg^{-1}K^{-1}

- you got the units of Q wrong to start off with.

Reply 2

Champagne Supernova

m is in kg.
delta t is in Kelvin.
c is in some really long crap that I can't be bothered to look up at the mo'!

Reply 3

trance addict

c is the specific heat capacity of the liquid

also often youll need to convert that formula to delta H, which in case

Unparseable latex formula:

\dfrac{mct}{moles \x 1000}

will give you kjmol^-1

and note: delta T can be in DEGREES, the change will be exactly the same

Reply 4

BrightGirl

Great, thanks guys!

But, in the question, it says the specific heat capacity of water is 4.18 JK(-1)g(-1)
In a situation like this, do I use the mass in g instead of in Kg?

Reply 5

trance addict

mass should always be in grams

so itd be whatever x 10^3

for the heat capactiy, just stick the number in

Reply 6

BrightGirl

trance addict

mass should always be in grams

so itd be whatever x 10^3

for the heat capactiy, just stick the number in

for the question I posted above, or for everything?

Reply 7

trance addict

sorry just for that question above itd be mass x10^3 because you were given the amount in KG

Reply 8

Malsy

sorry to protrude into this thread

just a quick question

i have multiple questions and answers from my teacher whereby none are converted into Kelvins-these are for reactions involving neutralisation

any guess as to why?

Reply 9

BrightGirl

trance addict

sorry just for that question above itd be mass x10^3 because you were given the amount in KG

gotcha. so basically I use whatever mass type is specified in the specific heat capacity?

Reply 10

trance addict

Malsi101

because the change would be the same

20 - 30 degrees

273K to 283K

still a change of 10

Reply 11

trance addict

BrightGirl

gotcha. so basically I use whatever mass type is specified in the specific heat capacity?

i think youre getting confused

q = mct

m = mass of liquid (so this would be in grams)
t = change in temperature

C is a specific constant to the liquid youre calculating energy for, e.g. for water itd be 4.2 J g^-1 K^-1, you just put the number into the formula in the exam.. infact you can do that for all 3 quantities, you only need to specifiy the unit for the end energy transferred which will always be in joules

cool

trance addict

yeah I think I am too

oh ok so they won't expect me to convert anything?

thanks for helping :smile:

Reply 14

trance addict

BrightGirl

yeah I think I am too

oh ok so they won't expect me to convert anything?

thanks for helping :smile:

c should just go straight in,
t should go straight in as well
g might need to be converted to grams

and then the energy is given in joules and may need to be converted depending on what they want, and np :smile:

Reply 15

BrightGirl

trance addict

c should just go straight in,
t should go straight in as well
g might need to be converted

and then the energy is given in joules and may need to be converted depending on what they want, and np :smile:

great, thank you so much! :biggrin:

Reply 16

pureandmodest

i'm not 100% sure if this is correct but apparently you didn't need the q=mct equation...
you were meant to use c and the other numbers they gave you, rearrange it in some way and you'd get delta S. So then you'd use the G=H - TS equation to work out the temperature...
I got a strange answer when I used q=mct, but my friend told me that they did what I said above and they got 63.9 or something.
I don't really know if that's correct, I just thought i'd put it out there

Reply 17

natninja

Original post by BrightGirl

Hello, stupid question but what are the units for each of these things?

q = kj?

m = g or kg?

c = JKg (little -1's after the K and g)

delta t = Kelvin or degrees?

Q is in Joules or KiloJoules depending on the units you use for C

m must be in Kilograms (unless your C is per gram)

C is in either JKg^-1 OR KJKg^-1

for T it is irrelevant as both have the same step size.

So it depends on the given units for C really what you should use for the others. though one would expect c to be per gram... so m would be grams.

The GOLDEN RULE s to be consistent with the units you use.

(edited 10 years ago)

Reply 18

Big-Daddy

Original post by BrightGirl

gotcha. so basically I use whatever mass type is specified in the specific heat capacity?

Yes. Or whatever else is specified in the specific heat capacity. What you really need, with the specific heat capacity, is the units for whatever value you're given. Then just make sure you are working with that value so that your units reduce down to the unit you want your final answer in.