Chemistry help please - on Enthalpy - for Higher coursework.?
"A polystyrene cup containes 50cm3 of water at 21.2*C.
2.53 g of of powdered ammonium chloride (NH4CL) was added and dissolved with stirring.
Calculate the lowest temperature which the solution should reach given that the enthalpy of solution of NH4CL is +15 KJ mol-1."
I know the answer is 17.8*C. BUT I don't know how to do the sum.
Can you help please help?
Just show me the how you worked it out.
Answer:
The first step is to convert the NH4Cl to moles. Divide the 2.53g by the molecular mass. Now multiply the moles by the enthalpy of solution of 15,000 J / mol.
This gives you the heat absorbed (it is endothermic) by the dissolving salt.
Now you have the total joules transfered.
The next formula for the water is q = m Cp delta T.
You already have the q. The Cp of water is 4.184 J / gram and your mass is 50 grams (1cm3 = 1 gram).
solve for the delta T and subtract it from the starting temp. of 21.2 C
OK...since the enthalpy of solution is positive, it means it's endothermic, so the energy must be supplied. The amount of energy is 15 kJ.mol-1
No of mol = mass / molar mass = 2.53 / 53.5 = 0.0473 mol
Enthalpy required = 15 * 0.0473 = 0.709 kJ = 709 J
Now...the energy will be supplied by loss of thermal energy of water, which can be expressed as mc(delta theta)
assuming density of water to be 1 g.cm-3, then mass of 50cm3 of water is 50g
So: 709 = 50*4.2*(delta theta) [where 4.2 Jg-1deg-1 is the specific heat capacity of water]
delta theta = 709 / 50*4.2 = 3.38 deg
So if starting T is 21.2 deg C, and water decreases in temperature to supply energy, final T will be 21.2 - 3.38 = 17.8 deg C.
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