energetics multiple choice

Question 1

Two reactions of iron with oxygen are shown.
Fe(s) + O2(g) → FeO(s) ΔH = – 272 kJ mol–1
2 Fe(s) + O2(g) → Fe2O3(s) ΔH = – 822 kJ mol–1
What is the enthalpy change, in kJ mol–1
, for this reaction?
2 FeO(s) + O2(g) → Fe2O3(s

2 FeO(s) + O2(g) → Fe2O3(s)
A +550
B –278
C –1094
D –1372

Answer 1

I> 2FeO so do 2 times -272 to get -544 but you are producing this and we want it to be the reactant as shown in the equation given so we go in the endothermic direction +544
> -822+544= -278

Question 2

Some enthalpy change data are shown.
C(s) + 2 H2(g) → CH4(g) ΔH = –75 kJ mol–1
H2(g) → 2 H(g) ΔH = +436 kJ mol–1
What is the enthalpy change, in kJ mol–1
, for the following reaction?
CH4(g) → C(s) + 4 H(g)
A –947
B –361
C +361
D +947

Answer 2

If you reverse and add the equations until you get the desired equation and do the same to the enthalpies, you’ll get the answer.

So, reverse the methane formation equation, to give CH4 -> 2H2 + C with ∆H=75

Double the hydrogen equation to get 2H2 -> 4H with ∆H=872

Add both to give CH4 + 2H2 -> 4H + C + 2H2

The 2H2’s are on both sides of the equation, so cancel them to give CH4 -> 4H + C

Then do the same to the enthalpies as you did to the equations (doubling one, and adding both) to give 872 + 7

Question 3

The temperature changed from 21.8 °C to 19.2 °C during a calorimetry
experiment.
The uncertainty of each reading of the thermometer is ±0.1 °C
What is the percentage uncertainty in the temperature change?
A 0.5%
B 1.0%
C 3.8%
D 7.7%

Answer 3

Calculate the absolute value of the temperature change

∣21.8 ∘ C−19.2 ∘C∣=∣2.6
∘C∣=2.6 ∘ C

0.1 + 0.1 = 0.2

0.2/ 2.6 x100% = 7.7%

Answer 4

enthalpy change of combustion = q/moles of fuel = w x c x delta t / B/mr

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