Question

A) A sample of solid copper is heated with an electrical coil. If 72.4 Joules of energy are added to a 11.4 gram sample and the final temperature is 38.3°C, what is the initial temperature of the copper? Answer:_________________ °C.

B)When NO(g) reacts with H₂(g) to form N₂(g) and H₂O(l) , 376 kJ of energy are evolved for each mole of NO(g) that reacts.

Write a balanced thermochemical equation for the reaction with an energy term in kJ as part of the equation. Note that the answer box for the energy term is case sensitive.

Use the SMALLEST INTEGER coefficients possible and put the energy term (including the units) in an appropriate box. If a box is not needed, leave it blank.

________+_________+_________->__________+_________+________

Answer 1

1)For copper metal,

Specific heat at constant pressure (Cp) = 0.39 kJ.Kg^-1.K^-1= 0.39 x 10³ J.Kg^-1.K^-1 =390 J.Kg^-1.K^-1

Heat energy supplied (H) = 72.4 J

Mass of copper = 11.4 g = 11.4 x 10^-3 Kg = 1.14 x 10^-2 Kg

Final temperature (Tf) = 38.3 ⁰C = 38.3 + 273.15 = 311.45 K

Initial temperature (Ti) = ?

We know the relation,

H = m x Cp x ΔT

H = m x sp.heat x (Tf-Ti)

72.4 = (1.14 x 10-2) x (390) x (311.45-Ti)

72.4 = 4.446 x (311.45 –Ti)

72.4 / 4.446 =311.45 – Ti

16.28 = 311.45 –Ti

Ti = 311.45 – 16.28

Ti = 295.17 K

Ti = 295.17 – 273.15

Ti =22.02 K

Initial temperature of copper was 295.17 K.

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2) Given that, heat is evolved in the transformation per mole of NO₂ gas.

Hence it’s an exothermic reaction. So heat evolved can either written added on product side or subtracted from reactant side.

Hence the balanced thermochemical equation with energy term for given chemical transformation is,

2NO(g) + 2H2(g) < --------->   N2(g) + 2H2O(l)

i.e. 2 moles of NO gives 1 mole of N2 gas hence the energy term to be written in balanced thermochemical equation needed to be multiply by 2

i.e. ΔE = 2 x 376 kJ

ΔE = 752kJ

Hence the balanced thermochemical equation with energy term for given chemical transformation is,

2NO(g) + 2H2(g) + 752 kJ < --------->   N2(g) + 2H2O(l)

Or

2NO(g) + 2H2(g) < --------->   N2(g) + 2H2O(l) – 752 kJ.

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