Q: A coffee-cup calorimeter is used to measure the enthalpy change when 7.50 g of NH$$_4Cl is $$dissolved in 70.0 g of water, cooling the solution from 30.00$$^\circC to 10.27$^\circC. $$What is the correct formula to calculate $$q_{sol}$$ for this process?

$$q_{sol} = c_s \times m \times \Delta T$$

Question 1

Which of the following substances has a standard enthalpy of formation, $$\Delta H^\circ_f$$, equal to zero at 298 K?

Accepted Answer

Cl$$_2(g)$$

Question 2

A coffee-cup calorimeter is used to measure the enthalpy change when 7.50 g of NH$$_4Cl is $$dissolved in 70.0 g of water, cooling the solution from 30.00$$^\circC to 10.27$^\circC. $$What is the correct formula to calculate $$q_{sol}$$ for this process?

Accepted Answer

$$q_{sol} = c_s 	imes m 	imes \Delta T$$

Question 3

How much heat (in kJ) is required to raise the temperature of 5.00 kg of water from 40.0$$^\circC to 90.0$^\circC$$? (Specific heat of water = 4.184 J/g-$$^\circC)$$

Accepted Answer

2007 kJ

Question 4

A system absorbs 122 kJ of heat and does 572 kJ of work on the surroundings. What is $$\Delta E$$ for the system, and is the process endothermic or exothermic?

Accepted Answer

$$\Delta E = -694 kJ$$, endothermic

Question 5

Given the following data: C$$_2H$_6(g$$) $$\Delta H^\circ_f = -84.68 kJ/$$mol, O$$_2(g$$) $$\Delta H^\circ_f = 0 kJ/$$mol, C$$_2H$_4(g$$) $$\Delta H^\circ_f = 52.3 kJ/$$mol, H$$_2O(g$$) $$\Delta H^\circ_f = -241.82 kJ/$$mol. Calculate $$\Delta H^\circ_{rxn}$$ for the reaction: 2 C$$_2H$_6(g) + O$_2(g$$) $$\rightarrow 2 C$_2H$_4(g) + 2 H$_2O(g).$$

Accepted Answer

$$\Delta H^\circ_{rxn} = -274.2 kJ/$$mol

Question 6

A fuel is burned in a cylinder equipped with a piston. The initial volume is 50.0 L, and the final volume is 30.0 L. If the piston compresses against a constant pressure of 4.50 atm, how much work (in kJ) is done? ($$1 L-$$atm $$= 101.3 J)$$

Accepted Answer

11.8 kJ

Question 7

Use Hess’s law and the following reactions to calculate $$\Delta H$$ for: 2 N$$_2O$_5(g$$) $$\rightarrow 2 N$_2(g) + 5 O$_2(g).

2 H$_2(g) + O$_2(g$$) $$\rightarrow 2 H$_2O(l$$) $$\Delta H^\circ = -571.7 kJ 
2 $$HNO$$_3(l$$) $$\rightarrow N$_2O$_5(g) + H$_2O(l$$) $$\Delta H^\circ = 92.0 kJ 
2 $$HNO$$_3(l$$) $$\rightarrow N$_2(g) + 3 O$_2(g) + H$_2(g$$) $$\Delta H^\circ = 348.2 kJ$$

Accepted Answer

$$\Delta H = -315.5 kJ/$$mol

Question 8

Which of the following represents the standard enthalpy of formation for CO$$_2(g$$)?

Accepted Answer

The enthalpy change for the reaction C(s) + O$$_2(g$$) $$\rightarrow CO$_2(g)$$

Question 9

Which statement about chemical kinetics is FALSE?

Accepted Answer

The instantaneous reaction rate is always equal to the average rate.

Question 10

For the reaction BF$$_3(g) + NH$_3(g$$) $$\rightarrow F$_3$$BNH$$_3(g$$), which is second order in NH$$_3$$ and zero order in BF$$_3$$, what happens to the rate if [BF$$_3$$] is increased by a factor of 3 while [NH$$_3$$] remains constant?

Accepted Answer

The reaction rate will not be affected.

Question 11

Given the mechanism: Step 1: N$$_2O(g$$) $$\rightarrow N$_2(g) + O(g) ($$slow), Step 2: N$$_2O(g) + O(g$$) $$\rightarrow N$_2(g) + O$_2(g) ($$fast), what is the overall rate law?

Accepted Answer

Rate = k[N$$_2O$$]

Question 12

For the reaction 2 NO(g) + 2 H$$_2(g$$) $$\rightarrow N$_2(g) + 2 H$_2O(g$$), which species is an intermediate in the following mechanism? Step 1: P + Q $$\rightarrow R + S$$; Step 2: S + Q $$\rightarrow P + T$$

Accepted Answer

S

Question 13

A student determines a reaction is zero order using the graph shown. What are the correct labels for the x and y axes?

Accepted Answer

x axis = time, y axis = [A]

Question 14

Which of the following reactions is an example of heterogeneous catalysis?

Accepted Answer

2SO$$_2(g) + O$_2(g$$) $$\xrightarrow{NO(g)} 2SO$_3(g)$$

Question 15

For the elementary process NO(g) + Cl$$_2(g$$) $$\rightarrow$$ NOCl$$_2(g$$), what is the molecularity and rate law?

Accepted Answer

Bimolecular, rate = k[NO][Cl$$_2$$]

Question 16

In the industrial conversion of ammonia to nitrogen monoxide: N$$_2F$_4(g) + 5 H$_2(g$$) $$\rightarrow 4 NH$_3(g) + 4 HF(g$$), if H$$_2 is $$consumed at 1.00 M/s, what is the rate of production of HF(g)?

Accepted Answer

Question 17

Which of the following will NOT result in an altered rate constant, k?

Accepted Answer

Doubling the concentration of reactants

Question 18

According to collision theory, the rate of a reaction depends on all of the following EXCEPT:

Accepted Answer

The energy difference between the reactants and products

Question 19

Which statement correctly describes an endothermic chemical reaction?

Accepted Answer

The products have higher potential energy than the reactants, and $$\Delta H \u003e 0$$

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