AP chemistry Chapter 6

For a classroom demonstration, a chemistry teacher puts sample of two different pure solid powders in a beaker, The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students' observation

an endothermic chemical change occurred because the temperature of the beaker an the water on the board decreased as heat was absorbed by the reaction

2H2S(g) + 3O2(g)-- 2H2O(l)+2SO2(g) delta H* = -1120 Based on the reaction represented by the chemical equation shown above, what is the amount of heat released when 4mol of H2S(g) reacts with 9mol of O2?

-2240

The cooling curve above shows how the temperature of a sample varies with times as the sample goes through phase changes. The sample starts as a gas, and heat is removed at a constant rate. At which time does the sample contain the most liquid?

t4

At 10 g cube of copper at a temperature T, is placed in an insulated cup containing 10g of water at a temperature T2, if T1 lager than T2, which of the following is true of the system when it has attained thermal equilibrium? The specific heat of copper is 0.385 j/g and the specific heat of water is 4.18 j/g

the temperature of the copper changed more than the temperature of the water

Based on the information for two different reactions given above, which of the following gives the quantities needed to calculate the enthalpy change for the reaction represented by the overall equation above? 2NO(g)+ O2(g)-- N2O4(g）

（-delta H1) + delta H 2

Mg(s)+2HCI (aq)-- MgCI2(aq)+ H2(g) The chemical equation shown above represents the reaction between Mg and HCI. When 12.15g of Mg(s) is added to 500ml of 4M HCI (aq), 95kJ of heat is released, The experiment correct value for the amount of heat released by the reaction?

190

How much energy is required to melt 64h of methane at 90k?

3.8Kj

In an insulated cup of negligible heat capacity, 50g of water at 40degress is mixed with 30g of water at 20 degree. The final temperature of the mixture is closest to

33

which of the following phase changes involves the transfer of heat from the surroundings to the system

NH3(l)-- NH3(g), because NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

2NH3-- 3H2(g)+ N2(g) Delate H* 298= 92kJ/mol *= the small circle, I don't know how to type it According to the information above, what is the standard enthalpy of formation delate H*f, for NH3(g) at 298K

-46kJ/mol

A sample of CHCI3 (s) was exposed to a constant source of heat for a period of time. The graph above shows the change in the temperature of the sample as heat is added. Which of the following best describes what occurs at the particle level that makes segment D longer than segment B?

The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.

NaOH(aq) + HCI(aq) -- NaCI(aq) + H2O(l) To determine the concentration of NaOH(aq) solution, a student titrated a 50ml sample with 0.1M HCI(aq). The reaction is represented by the equation above, The titration is monitored using a pH meter, and the experimental results are pliited in the graph below. A student mixes a 10 ml sample of 1M NaOH (aq) with a 10ml sample of 1.0M HCI(aq) in a polystyrene container. The temperature of the solutions before mixing was 20 degree. If the final temperature of the mixture is 26degree, what is the experimental value of delate H reaction. Assum ethe solution mixture has a specific heat of 4.2 and a density of 1

-50

A 2 mol sample of C2H5OH undergoes the phase transition illustrated in the diagram above. The molar enthalpy of vaporization, delta H vap of C2H5OH is +38.6kj/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram?

-77.2

An equeation representing the dissociation of O2(g) and a table of bond enthalpies are shown above. Based on the information, which of the following is the enthalpy of dissociation for O2(G)?

495

A hot iron ball is dropped into a 200. g sample of water initially at 50 ℃. If 8.4 kJ of heat is transferred from the ball to the water, what is the final temperature fo the water? (The specific heat of water is 4.2 J / (g ℃).)

60

1/2 H2(g)+ 1/2 I2(s)-- HI(g) delta H= 26kJ/ mol 1/2 H2(g)+ 1/2 I2(g)-- HI(g) delta H= 26kJ/ mol Based on the information above, what is the enthalpy change for the sublimation of iodine, represented below

62kJ/mol

CS2(l)+ 2H2O(l)- CO2(g)+2H2S(g) delta H reaction ==?

CS2(l) +3O2(g) -- CO2(g) + 2SO2(g) delta H*= -1075 2H2O(l) + 2SO2(g)-- 2H2S(g)+ 3O2(g) delta H*= 1136

CO(g)+ 2H2(g) -- CH3OH(g) delta H smaller than 0 The synthesis of CH3OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Ko for the reaction at 483K is 14.5 Which of the following statements is true about bond energies in this reaction

The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed.

In the spring, blossoms on cherry trees can be damaged when temperature fall below -2 degree. When the forecast calls for air temperatures to be below -5 degree for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below -2.

The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above -2.

The oxidation of carbon monoxide can be represented by the chemical equation 2 CO(g)+O2(g)-- 2CO2(g) The table above provides the average bond enthalpies for different bond types. Based on the information in the table, which of the following mathematical expressions is correct for the estimated enthalpy change for the reaction?

delta H reaction = {2 (1072kj/mol) + (498kj/mol)] - 4( 799kj/mol)