Question

1.The value of ΔH° for the reaction below is -126 kJ. How much energy is release when 80 g of NaOH is formed in the reaction?   2Na2O2 (s) + 2H2O →(l) 4NaOH (s) + O2 (g)

A. 63 kJ

B. 252 kJ

C. 3.9 kJ

D. -126 kJ

2. If the specific heat of iron is 0.450 J/g-K, how many joules of energy is required to raise the temperature of a 35.2 g samples of iron from 23.7 °C to 29.5 °C?

A. 38.2 J

B. 91.9 J

C. 355 J

D. 826 J

Answer 1

(1) Given reaction is

2Na2O2 (s) + 2H2O(l) -------> 4NaOH (s) + O2 (g)

∆H°=-126 kJ.

Given mass of NaOH=80 g and molar mass of NaOH=40 g/mol

Therefore moles of NaOH=mass/molar mass=80 g/40 g/mol=2 mol.

In the reaction, the moles of NaOH produced=4 mol.

Therefore for 4 mol of NaOH production -----> ∆H°=-126 kJ,

Then for 2 mol of NaOH production -------> ∆H°=?

∆H°=(2 molx-126 kJ)/4 mol= -63 kJ.

The energy released=63 kJ.

Option (A) is correct.

(2) We know that energy, q=mC∆T

Where m=mass, C=specific heat, ∆T=T2-T1=temperature change, T1=initial temperature, T2=final temperature.

Given for Iron, m=35.2 g, C=0.45 J/g-K,

T1=23.7°C=23.7 +273 K=296.7 K,

T2=29.5°C=29.5+273 K=302.5 K.

Therefore energy,

q=(35.2 g) x (0.450 J/g-K) x (302.5 K-296.7 K)

q=35.2 g x 0.450 (J/g-K) x 5.8 K

q=91.87 J ~ 91.9 J.

Option (B) is correct.

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