Question

1.a) If ΔE > 0 then energy is being absorbed by the system.

True

False

1.b) If ΔE_system is positive then ΔE_surrounding must be:

		negative
		none of these
		positive
		zero 1.c) Consider the following chemical equation and calculate how much heat is released when 77.90 grams of C2H6 reacts. 2 C2H6 + 7 O2 --> 4 CO2 + 6 H2O                 ΔH = -385 kJ - answer should read out to one place after the decimal - do not include units 1.d) A 27.4 g cube of copper initially at 74.6 ºC is submerged into a cup filled with 121.4 grams of water at 13.6 ºC. Determine the final temperature of both substances at thermal equilibrium. HINTS:      Specific heat of copper = 0.385 J/g × ºC    Specific heat of water = 4.184 J/g × ºC - your answer should contain four significant figures. 1.e) Calculate the heat (in kJ) associated if 0.0107 kg of CH4 was used for the reaction: CH4(g) + 2 O2(g)  --> CO2(g) + 2 H2O(g) ΔHrxn = -890 kJ -report answer using three sig figs

Answer 1

1. (a) Here energy of product is more than energy of reactant. That means energy is absorbed during the reaction.

The statement is true.

(b ) The answer is: negative

(c) Molar mass of C2H6 is: 30 g/ mole

Moles of C2H6 = 77.90/ (30g/mole) = 2.60 moles

Energy released = - 385 kJ × (2.60 mole/ 2 mole) = - 500.5 kJ

(d) suppose the final temperature is X ^o C

Heat gained by water =H1 = mass of water × specific heat of water × ( Final temperature - Initial temperature) = 121.4 g × 4.184 J/ g ^o C × ( X - 13.6) ^o C = 508 × ( X - 13.6) joules

Heat released by copper = mass of copper × specific heat of copper × ( Final temperature - Initial temperature) = H2 = 27.4 g × 0.385 J/ g ^o C × ( X - 74.6) ^o C = 10.6 × (X - 74.6) joules

At thermal equilibrium; H1 = - H2

508× (X -13.6) = - 10.6× ( X -74.6)

Solving the equation; we get:

518.6 X = 7699.6

So; X = 14.85

So ; final temperature is 14.85 ^o C

(e) Molar mass of CH4 is 16 g/ mole

Mass of CH4 is 0.0107 kg = 10.7 g ( 1 kg = 1000 g)

Moles of CH4 = 10.7 g/ (16 g/ mole) = 0.69 mole

So; heat associated : - 890 kJ × (0.69 mole/ 1 mole) = - 614 kJ