Calorimetry Confusion (1 Viewer)

[ubermale]

Just two quick questions! When using the enthalpy equation (ΔH = -mCΔT) to find the molar heat of solution is m the mass of the water ONLY, or the water and the solute combined? Also, is C the specific heat capacity of water (ie.e 4.18) or the specific heat capacity of the solution?

Thanks in advance to anyone that helps.

 

[Aerath]

4.18 is the specific heat capacity of water.

And I THINK that m is the mass of water only.

 

[bored of sc]

Aerath is correct.

Just to clarify.

ΔH = -mCΔT
change in heat energy (kJ) = -mass of water (kg).specific heat capacity of water (4.18kJ/kg/K).change in temperature of solution(K)

If you wanna get mathematical (here's a sort of proof):
ΔH = -mCΔT

kJ = kg.(kJ/kg/K).K <---- Substitute the units in and multiply three sets of units together.
RHS = kg.(kJ/kgK).K <---- Invert fraction and multiply to get kJ/kgK.
= kJ <---- Cancel the kg and K.
= LHS

The specific heat capacity of water is a constant of 4.18kJ/kg/K, which means it doesn't change value. The temperature of the solution is measured by final temperature (after reaction) - intial temperature (before reaction). 1^oCelcius = 1Kelvin (K) by the way.

 

[kaz1]

Aerath is correct.

Just to clarify.

ΔH = -mCΔT
change in heat energy (kJ) = -mass of water (kg).specific heat capacity of water (4.18kJ/kg/K).change in temperature of solution(K)

If you wanna get mathematical (here's a sort of proof):
ΔH = -mCΔT

kJ = kg.(kJ/kg/K).K <---- Substitute the units in and multiply three sets of units together.
RHS = kg.(kJ/kgK).K <---- Invert fraction and multiply to get kJ/kgK.
= kJ <---- Cancel the kg and K.
= LHS

The specific heat capacity of water is a constant of 4.18kJ/kg/K, which means it doesn't change value. The temperature of the solution is measured by final temperature (after reaction) - intial temperature (before reaction). 1^oCelcius = 1Kelvin (K) by the way.

Isn't mass in grams in Chemistry.

 

[tommykins]

Isn't mass in grams in Chemistry.

Not for this formula.

 

[Aplus]

1^oCelcius = 1Kelvin (K) by the way.

I don't think so.

 

[Aerath]

Isn't mass in grams in Chemistry.

Yes, only if you want your answer in joules.

 

[Aerath]

No it doesn't

You know what he meant.

 

[bored of sc]

I don't think so.

You are right. It's not technically the same but the difference between 2 kelvin is the same as the difference between 2^oC if that makes sense. So the temperature change units are interchangable between ^oC and Kelvin.

http://www.answers.com/kelvin&r=67

 

[jellybelly59]

so if you were burning a fuel and you were trying to find which produced most energy - liquid gas or solid... would the mass in the equation be the water or the change in mass of the fuel?

 

[jellybelly59]

bump

 

[kaz1]

so if you were burning a fuel and you were trying to find which produced most energy - liquid gas or solid... would the mass in the equation be the water or the change in mass of the fuel?

If your using the heat capacity of water (4.18) you use the mass of water.