حرارتی کیمیا اور خود بخود ہونے والے عمل

Written by Dr. Ayesha Nadeem (Chemistry)

ایم ڈی کیٹ · کیمیا · 7 سوالات

نام: ____________________ تاریخ: ____________

The standard enthalpy of formation of an element in its standard state is:
- A Negative
- B Always 100 kJ/mol
- C Positive
- D Zero
For an endothermic reaction, the sign of delta H is:
- A Positive
- B It cannot be known
- C Negative
- D Zero
Which change leads to an increase in entropy?
- A A gas dissolving in water
- B Water freezing
- C Ice melting
- D Steam condensing
A reaction is spontaneous at constant temperature and pressure when:
- A Delta G is negative
- B Delta G is positive
- C Delta S is zero
- D Delta H is positive
The heat released by a reaction, 5000 J, is all absorbed by 100 g of water. What is the rise in temperature? (Specific heat of water 4.18 J/g per degree C)
- A 12 degrees C
- B 50 degrees C
- C 5 degrees C
- D 1.2 degrees C
For a reaction, delta H = +40 kJ and T x delta S = 50 kJ. What is delta G?
- A +10 kJ
- B +90 kJ
- C -10 kJ
- D -90 kJ
The heat change of a reaction measured at constant pressure is called the change in:
- A Free energy
- B Entropy
- C Enthalpy
- D Internal energy only

جوابی کلید

1. D, 2. A, 3. C, 4. A, 5. A, 6. C, 7. C

By definition, forming an element from itself involves no change, so its standard enthalpy of formation is zero.
An endothermic reaction absorbs heat, so the system gains enthalpy and delta H is positive.
Melting turns an ordered solid into a more disordered liquid, so entropy increases.
A negative change in Gibbs free energy means the reaction can proceed spontaneously.
Temperature rise = Q / (m x c) = 5000 / (100 x 4.18), which is about 12 degrees C.
Delta G = delta H - T delta S = 40 - 50 = -10 kJ, so the reaction is spontaneous.
Enthalpy change is the heat exchanged at constant pressure.