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pychu [463]
3 years ago
9

Calculate the enthalpy change, ∆H in kJ, for the reaction H2O(s) → H2(g) + 1/2O2(g). Use the following information: : +279.9 kJ

+291.9 kJ –279.9 k –291.9 kJ
Chemistry
1 answer:
Len [333]3 years ago
5 0

Answer:

+ 291.9 kJ

Solution:

The equation given is as;

H₂O ₍s₎ → H₂ ₍g₎ + 1/2 O₂ ₍g₎ ΔH = ?

First, as we know the heat of formation of H₂O ₍l₎ is,

H₂ ₍g₎ + 1/2 O₂ ₍g₎ → H₂O ₍l₎ ΔH = - 285.9 kJ

Now, reversing the equation will reverse the sign of heat as,

H₂O ₍l₎ → H₂ ₍g₎ + 1/2 O₂ ₍g₎ ΔH = + 285.9 kJ

Also, we know that,

H₂O ₍s₎ → H₂O ₍l) ΔH = + 6.0 kJ

Now, adding last two equations,

H₂O ₍l₎ → H₂ ₍g₎ + 1/2 O₂ ₍g₎ ΔH = + 285.9 kJ

H₂O ₍s₎ → H₂O ₍l) ΔH = + 6.0 kJ

-----------------------------------------------------------------------------

H₂O ₍s₎ → H₂ ₍g₎ + 1/2 O₂ ₍g₎ ΔH = + 291.9 kJ

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A second- order reaction of the type A + B -->P was carried out in a solution that was initially 0.075 mol dm^-3 in A and 0.0
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Answer:

a) 16.2 dm^3/mol*h

b) 6.1 × 10^3 s, 2.5 × 10^3 s (it is different to the hint)

Explanation:

We can use the integrated rate equation in order to obtain k.

For the reaction A+ B --> P the reaction rate is written as

Rate = -\frac{dC_A}{dt} = -\frac{dC_B}{dt} = \frac{dC_P}{dt} = kC_AC_B

If C_{A0} and C_{B0} are the inital concentrations and x the concentration reacted at time t, so C_A=C_{A0} -x and C_B=C_{B0} -x and the rate at time t is written as:

-\frac{dx}{dt} =-k(C_{A0} -x)(C_{B0}-x)

Separating variables and integrating

\int\limits^x_0 {\frac{1}{(C_{A0}-x)(C_{B0}-x)} } \, dx = \int\limits^t_0 {k} \, dt

The integral in left side is solved by partial fractions, it can be used integral tables

\frac{1}{C_{B0}-C_{A0}}(ln\frac{C_{A0}}{C_{A0}-x}-ln\frac{C_{B0}}{C_{B0}-x}) =kt

Using logarithm properties (ln x - ln y = ln(x/y))

\frac{1}{C_{B0}-C_{A0}}(ln\frac{C_{A0}C_{B}}{C_{A}C_{B0}}) =kt

Using the given values k can be calculated. But the data seems inconsistent since if the concentration of A changes from 0.075 to 0.02 mol dm^-3 it implies that 0.055 mol dm^-3 of A have reacted after 1 h, so according to the reaction given the same quantity of B should react, and we only have a C_{B0} of 0.05 mol dm^-3.

Assuming that the concentration of B fall to 0.02 mol dm^-3 (and not the concentration the A). So we arrive to the answer given in the Hint.

So, the values given are t= 1, C_{A0}=0.075, C_{B0}=0.05, C_{B}=0.02, it implies that the quantity reacted, x, is 0.03 and C_{A}=0.075-x = 0.045. Then, the value of k would be

kt = \frac{1}{0.05-0.075}(ln\frac{0.075*0.02}{0.045*0.05})

k = 16.21 \frac{dm^3}{mol*1h}

b) the question b requires calculate the time when the concentration of the specie is half of the initial concentration.

For reactant A, It is solved with the same equation

\frac{1}{C_{B0}-C_{A0}}(ln\frac{C_{A0}C_{B}}{C_{A}C_{B0}}) =kt

but suppossing that C_A= C_{A0}/2=0.0375 so  C_B=C_{B0}- C_{A0}/2=0.0125, k=16.2 and the same initial concentrations. Replacing in the equation

t=\frac{1}{16.2(0.05-0.075)}(ln\frac{0.075*0.0125}{0.0375*0.05})

t=1.71 h = 1.71*3600 s = 6.1*10^3 s  

For reactant B, C_B= C_{B0}/2=0.025 so  C_A=C_{A0}- C_{B0}/2=0.05, k=16.2 and the same initial concentrations. Replacing in the equation

t=\frac{1}{16.2(0.05-0.075)}(ln\frac{0.075*0.025}{0.05*0.05})

t=0.71 h = 0.7*3600 s = 2.5*10^3 s  

Note: The procedure presented is correct, despite of the answer be something different to the given in the hint, I obtain that result if the k is 19.2... (maybe an error in calculation of given numbers)

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-2.5089 kJ is the amount of heat is released by the combustion of the naphthalene.

<h3>How we calculate the released heat?</h3>

Released amount of heat is calculated by using the below formula:

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c = specific heat of calorimeter = 4.13 kJ/°C

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Hence, - 2.5089 kJ heat is released.

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Explanation:

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