Change in Enthalpy? (Chemistry) (1 Viewer)

[H4rdc0r3]

Does change in H refer to

- the difference in heat content in the reactants and products

or

- does it refer to the change in temperature of the surroundings

Which equation do we use? One book says

(delta H) = mC(delta T)

and another says

- (delta H) = mC(delta T)

EXPLAIN!!

 

[jaychouf4n]

change in H is the change in the energy between the final and beginning substances.

i.e. Triangle H=New energy minus old energy..

It's usually measured in joules.

just let change in T be d

There is an mcd for both substances involved, the one that is releasing energy and the one that is absorbing energy.

Hence mcd for substance 1=-mcd for substance 2, because one is negative since change in T is less than zero.

My explaining is bad :S

 

[bubblesss]

Does change in H refer to

- the difference in heat content in the reactants and products

or

- does it refer to the change in temperature of the surroundings

Which equation do we use? One book says

(delta H) = mC(delta T)

and another says

- (delta H) = mC(delta T)

EXPLAIN!!

delta H refers to specific heat capacity - heat gained or lost by the substance.
if the reaction is exothermic (gives out heat) it is -ve. if it is endothermic ( absorbs heat) it is +ve.

 

[Lyly]

Hi there.

Um, are you sure you've quotes those formulas correctly? Because if my memory serves me correctly, the euqation for delta H should be:

(delta H) = - mc (delta T)

because it's actually measuring the amount of heat released - this is delta H, the heat released when products + reactants react, and is called the enthalphy of that reaction. Thus if you have an endothermic reaction, delta H is negative because heat is actually absorbed! (hence the minus mc in the equation).

However, if you want to measure the quantity of heat transferred, use:

q = mc (delta T).

Hope that helps
(Not sure why your book would use (- delta H) - it was probably referring to endothermic/exothermic reactions, and the sign (delta H) takes on accordingly.)

Let me know if you're still confused, I don't think I've explained it really well

 

[H4rdc0r3]

thanks lyly, the ONLY one who answered the question.

 

[bored of sc]

(delta H) = mC(delta T)

and another says

- (delta H) = mC(delta T)

EXPLAIN!!

As Lyly said the first one measure the quantity of heat absorbed or released (not paying attention to whether the reaction is endothermic or exothermic).

The second equation is correct.

The equation is normally written as:

delta H = -mC(delta T)

All they've done is divide both sides by -1.

- (delta H) = mC(delta T) --> negatives cancel on right hand side of equation.