Chemistry homework

[c0d0ps]

"You are boiling 1.2dm^3 tea water. (Assume that 1.0dm^3 water have the mass 1.0kg) in a "water-boiler". The effect is 1.5kW

The water is at 18 degrees celsius when you start och of course 100 degrees when it's boiling.

How long time does it take for water to boil up. Assume that C(H2O) = 4,18 KJ*kg^-1*K^-1

Also assume that the operating "speed" is at 100%

Answer in seconds."

 

This is the assignment/question and I don't know what to do.

It's late and I am not the best at Chemistry  

(This is translated from Swedish)

[T.Vengeance]

Use the equation q = mc delta T.

 

kW = killojoules/sec

 

Find out how much energy it takes to boil the water and then solve for time.

[Castdeath97]

That's actually mostly physics, apply thermal capacity formula I guess.

[SniperDan]

Seems more like a physics question to me, but

q = m c dt

Total Energy = mass of the liquid  x specific heat capacity x change in temperature

[T.Vengeance]

Seems more like a physics question to me, but

q = m c dt

Total Energy = mass of the liquid  x specific heat capacity x change in temperature

Happened in our school system too. I have no clue why, but when I was in highschool, thermal stuff was taught in chemistry. I guess it's easier when you can combine it with moles, and other stuff too for change of states. But I agree, it does seem more physics than chem.

[Delusional Phil]

Happened in our school system too. I have no clue why, but when I was in highschool, thermal stuff was taught in chemistry. I guess it's easier when you can combine it with moles, and other stuff too for change of states. But I agree, it does seem more physics than chem.

That's because it is ... This stuff appeared in Physics for me ... 

Is the answer 3,64 seconds ? I was confused whether the Effect 1,5kW is from 1 kg of tea water or 1,2 tea water ... I'm guessing it's 1,2 ... 

 

You change kilojules into jules that divide that number with the energy it would take to boil the water then you're done right ? 

[T.Vengeance]

That's because it is ... This stuff appeared in Physics for me ... 

Is the answer 3,64 seconds ? I was confused whether the Effect 1,5kW is from 1 kg of tea water or 1,2 tea water ... I'm guessing it's 1,2 ... 

 

You change kilojules into jules that divide that number with the energy it would take to boil the water then you're done right ? 

I got 270 seconds. And yeah in the end, everything is physics. But chemistry is a more general way of looking at physics instead of considering the interactions of every individual molecule.

 

[Delusional Phil]

I got 270 seconds. And yeah in the end, everything is physics. But chemistry is a more general way of looking at physics instead of considering the interactions of every individual molecule.

 

 

Aw crap ...  now I know why I'm wrong .. I didn't convert Celcius to Kelvin ... Oopsy daisy  

[c0d0ps]

1. Use the equation q = mc delta T.

 

2. kW = kilojoules*/sec

 

3. Find out how much energy it takes to boil the water and then solve for time.

1. Never heard of it.

2. I thought it was kW= Kilowatts.

3. Yes, that is the question.

[c0d0ps]

That's actually mostly physics, apply thermal capacity formula I guess.

Can you write down the formula?

[c0d0ps]

I got 270 seconds. And yeah in the end, everything is physics. But chemistry is a more general way of looking at physics instead of considering the interactions of every individual molecule.

 

SNIP

I got this from a friend. He said that the answer was ca 274sec

" Delta T = (100-18) = 82K. E = KJ/kg*K = Kg*K = KJ*kg*K/kg*K = KJ = 4,18*1,2*82 = 411,312KJ. T = E/P = 411,312KJ/1,5KJ/s = 274,208

Answer: Ca 274 seconds.

[c0d0ps]

That's because it is ... This stuff appeared in Physics for me ... 

Is the answer 3,64 seconds ? I was confused whether the Effect 1,5kW is from 1 kg of tea water or 1,2 tea water ... I'm guessing it's 1,2 ... 

 

You change kilojules into jules that divide that number with the energy it would take to boil the water then you're done right ? 

I think It was kept at Kilojoules all the time (when a friend did it).

I think the effect is from the tea boiler and not the water itself.

The answer ended up being ca 274 seconds (we think).

[T.Vengeance]

I think It was kept at Kilojoules all the time (when a friend did it).

I think the effect is from the tea boiler and not the water itself.

The answer ended up being ca 274 seconds (we think).

Should be 270 seconds because of significant figures. But if your teacher doesn't care about that, then go all out.

 

Aw crap ...  now I know why I'm wrong .. I didn't convert Celcius to Kelvin ... Oopsy daisy  

Actually since it's a temperature delta, taking Celcius or Kelvin would make no difference. They use the same increments but Celcius is just offset by 270-something degrees . In the end the number would still the same but it would not cancel with the provided specific heat capacity.

[Delusional Phil]

Should be 270 seconds because of significant figures. But if your teacher doesn't care about that, then go all out.

 

Actually since it's a temperature delta, taking Celcius or Kelvin would make no difference. In the end the number is still the same but it's just your units won't cancel with the provided specific heat capacity.

Yup ... My fault totally for having 1,5kJ divide 411,312 kJ instead of the other way around 

[c0d0ps]

Should be 270 seconds because of significant figures. But if your teacher doesn't care about that, then go all out.

 

Actually since it's a temperature delta, taking Celcius or Kelvin would make no difference. They use the same increments but Celcius is just offset by 270-something degrees . In the end the number would still the same but it would not cancel with the provided specific heat capacity.

I haven't sent it in yet.

But if it makes a difference then I suppose I could change it ^^

[T.Vengeance]

I haven't sent it in yet.

But if it makes a difference then I suppose I could change it ^^

Well it's a minor thing. I guess you guys haven't covered significant figures yet? Might be called something else in Swedish but basically your final answer shouldn't "lie" about its precision. By saying it's 274 seconds instead of 270, you're saying that you know it's definitely at least 274, which isn't the case because your answer was limited to the precision of your data (which the least precise measurement was 2 significant figures). So it's more of a technicality than not.

[c0d0ps]

Well it's a minor thing. I guess you guys haven't covered significant figures yet? Might be called something else in Swedish but basically your final answer shouldn't "lie" about its precision. By saying it's 274 seconds instead of 270, you're saying that you know it's definitely at least 274, which isn't the case because your answer was limited to the precision of your data (which the least precise measurement was 2 significant figures). So it's more of a technicality than not.

I think my Chemistry teacher have talked a tiny bit about it.

But we didn't lose any points or something if we didn't do it.

 

I think 274 gives a more precise answer, because it's closer to the answer than 270 ..?

(274,208 Seconds)

[T.Vengeance]

I think my Chemistry teacher have talked a tiny bit about it.

But we didn't lose any points or something if we didn't do it.

 

I think 274 gives a more precise answer, because it's closer to the answer than 270 ..?

(274,208 Seconds)

Well science isn't like math where you can get an "exact" answer. Nothing is exact in science because the answers you get from equations are based off your measurements. For example, if you measure a table and you get 1.7 meters, that number is different in precision compared to 1.700000000 meters. The 2nd measurement tells you that it's precise to the nanometer. But the first just says you know it's 1.7 meters, but could it be 1.70002 meters? 1.70005 meters? Etc, both of those can still be "1.7m" because your ruler can't measure down to the micrometer. So we say that 1.7 meters has 2 significant figures, and 1.7000000000 has 11 sig. figs. The significant figures tells you what the maximum precision of your answer coming from your data should have. So let's say you're calculating the perimeter of that desk. The perimeter can't be precise to more than 2 sig figs. So the perimeter can be 4.6 meters. But it can't be 4.61025 m.

 

(hopefully what I said makes sense lol).

[c0d0ps]

Well science isn't like math where you can get an "exact" answer. Nothing is exact in science because the answers you get from equations are based off your measurements. For example, if you measure a table and you get 1.7 meters, that number is different in precision compared to 1.700000000 meters. The 2nd measurement tells you that it's precise to the nanometer. But the first just says you know it's 1.7 meters, but could it be 1.70002 meters? 1.70005 meters? Etc, both of those can still be "1.7m" because your ruler can't measure down to the micrometer. So we say that 1.7 meters has 2 significant figures, and 1.7000000000 has 11 sig. figs. The significant figures tells you what the maximum precision of your answer coming from your data should have. So let's say you're calculating the perimeter of that desk. The perimeter can't be precise to more than 2 sig figs. So the perimeter can be 4.6 meters. But it can't be 4.61025 m.

 

(hopefully what I said makes sense lol).

I think I understood most of it.

 

I never seen such an informational comment here before.

I wish I could favorite this..