Measuring Enthalpy Changes

OP

boromir9111

what's the specific heat capacity used here? never mind, i think it's about 0.74(c that is).....

we're using this

E = mc\Delta\theta

and c being specific heat capacity of water

The specific heat capacity used is 4.18Jg-1K-1.
But how can i work out the specific heat capacity of water.

I have tired to work out the number of moles for NaCl.
Moles of NaCl = 5.73 / 58.5 = 0.0979 mol.

Then i used

E = mc\Delta\theta

.
q = ? x 4.18 x 2.6
But not sure what to do to work out specific heat capacity of water. :s-smilie:

Reply 3

danhirons

15

You don't need to it's just a known fact which obviously can be worked out some way but you just need to remember the number.

Reply 4

boromir9111

17

Mehwish-S

The specific heat capacity used is 4.18Jg-1K-1.
But how can i work out the specific heat capacity of water.

I have tired to work out the number of moles for NaCl.
Moles of NaCl = 5.73 / 58.5 = 0.0979 mol.

Then i used

E = mc\Delta\theta

.
q = ? x 4.18 x 2.6
But not sure what to do to work out specific heat capacity of water. :s-smilie:

i thought m = mass?

Reply 5

OP

boromir9111

i thought m = mass?

Yep.
But Mass of Water.

Reply 6

danhirons

15

N = 5.73/(23 + 35.5) N = 0.0979 moles of NaCl
enthalpy change per 0.0979 moles = 4.18 x 2.6 x 100
= 1086.8J per 0.0979 moles

therefore for 1 mole = 11095.6 j mol-1
= 11.1kj mol-1

Reply 7

OP

danhirons

You don't need to it's just a known fact which obviously can be worked out some way but you just need to remember the number.

Oh Yehhh..
Pure Water is 1.0 gcm-3.

Thank You. :smile:

Reply 8

boromir9111

17

Mehwish-S

Yep.
But Mass of Water.

we have the volume of the solution of water,

100cm^{3}

how can you convert this to mass? if it is mass of water

edit - nevermind, you figured it out :smile:

Reply 9

Q=m*c*DeltaT (Where Q=energy released in joules, c=4.186Jg^-1K^-1, and DeltaT=temperature change in degrees/kelvins).

Q=100(1cm³=1g)*4.186*(22.4-19.8)
Q=1088.36J
Q=1.088KJ

Moles=mass/M_r
Moles=5.73/58.5
Moles=0.098

Enthalpy change=1.088/0.098=11.1KJ mol^-1

Reply 10

OP

boromir9111

we have the volume of the solution of water,

100cm^{3}

how can you convert this to mass? if it is mass of water

Its 1.0 gcm-3.

Thank You For The Help. :smile:

Reply 11

OP

LearningMath

Q=m*c*DeltaT (Where Q=energy released in joules, c=4.186Jg^-1K^-1, and DeltaT=temperature change in degrees/kelvins).

Q=100(1cm³=1g)*4.186*(22.4-19.8)
Q=1088.36J
Q=1.088KJ

Moles=mass/M_r
Moles=5.73/58.5
Moles=0.098

Enthalpy change=1.088/0.098=11.1KJ mol^-1

:O
That Was SO Easy.

Thank You. :smile:

Reply 12

OP

LearningMath

Q=m*c*DeltaT (Where Q=energy released in joules, c=4.186Jg^-1K^-1, and DeltaT=temperature change in degrees/kelvins).

Q=100(1cm³=1g)*4.186*(22.4-19.8)
Q=1088.36J
Q=1.088KJ

Moles=mass/M_r
Moles=5.73/58.5
Moles=0.098

Enthalpy change=1.088/0.098=11.1KJ mol^-1

BTW.. i was wondering why is it +11.1KJ mol^-1.
How do we know its an endothermic reaction.?

Reply 13

Mehwish-S

BTW.. i was wondering why is it +11.1KJ mol^-1.
How do we know its an endothermic reaction.?

They just asked for the change, so ignore signs.

Reply 14

danhirons

15

Mehwish-S

BTW.. i was wondering why is it +11.1KJ mol^-1.
How do we know its an endothermic reaction.?

It is an endothermic reaction because the temperature of the water decreases. If it was an exothermic reaction the temp would increase as heat is released!

Reply 15

OP

LearningMath

They just asked for the change, so ignore signs.

The exercises that am doing are all asking for the change.
But some of the answers are negative.

Is this one positive because were breaking bonds which means that it needs to absorb energy. :s-smilie:

Reply 16

Mehwish-S

The exercises that am doing are all asking for the change.
But some of the answers are negative.

Is this one positive because were breaking bonds which means that it needs to absorb energy. :s-smilie:

As dan says, 23.4oc to 19.8oc, temperature decrease. So it is an endothermic reaction!

Reply 17

Mehwish-S

Is this one positive because were breaking bonds which means that it needs to absorb energy. :s-smilie:

Be wary of this line of thought because you're also forming ion-dipole bonds which release energy, it just so happens in this case that less energy is released from their formation than put in to break the lattice. But the favourable entropy change overrides this.

Reply 18

OP