Advanced Chemistry: Thermodynamics

1. The system absorbs 72 J of heat while 35 J of work is done on it. What is the change in internal energy of the system.

107 J

37 J

-37 J

-107 J

None of these.

If heat is absorbed then q is positive. If work is done to the system then work is also positive. If q and w are positive then and ΔE = q + w = 107 J

Explanation

If heat is absorbed then q is positive. If work is done to the system then work is also positive. If q and w are positive then and ΔE = q + w = 107 J

2. You have two pieces of different metals but they have the same mass. If you subjected both of them to the same amount of energy, what would happen?

Their temperatures would both increase at the same rate.

The metal with the lower heat capacity would undergo a lower temperature change.

The metal with the higher heat capacity would undergo a higher temperature change.

The metal with the lower heat capacity would undergo a higher temperature change.

None of these.

Heat capacity and temperature change have an indirect relationship.

Explanation

Heat capacity and temperature change have an indirect relationship.

3. Which of the following are not a form of fossil fuels?

Coal

Natural gas

Diesel fuel

Propane

CO2

I highly recommend that you look over the fossil fuels section in your packet.

Explanation

I highly recommend that you look over the fossil fuels section in your packet.

4. How much heat would be required to heat 150. g of water at 25.0 C to boiling point? The specific heat of water is 4.18 J/g C

4.70 kJ

47.0 kJ

470 J

1.57 kJ

15.7 kJ

Since water boils at 100 C the heat needed to change the temperature to boiling is = q = smΔT where change in temperature is 75 C.

Explanation

Since water boils at 100 C the heat needed to change the temperature to boiling is = q = smΔT where change in temperature is 75 C.

5. A 150.0 g sample of metal at 75.0 C is added to 150.0 g of H2O at 15.0 C. The temperature of the water rises to 18.3 C. Calculate the heat capacity of the metal, assuming that all the heat lost by the metal is gained by the water. The heat capacity of water is 4.18 J/g C

0.22 J/ g C

0.25 J/ g C

0.23 J/ g C

0.48 J/ g C

0.43 J/ g C

The heat absorbed by the water = q= smΔT = 2100 J. Heat released from the metal is = heat gained by the water. Heat released = -q = smΔT.

Explanation

The heat absorbed by the water = q= smΔT = 2100 J. Heat released from the metal is = heat gained by the water. Heat released = -q = smΔT.

6. Which of the following is not considered an alternate energy source?

Hydrogen gas

Ethanol

Gasohol

Biodiesel

Methanol

All of these are considered alternate energy sources.

I highly suggest you look over the new energy sources section in your packet and pay particular attention to the Hydrogen as a Fuel section.

Explanation

I highly suggest you look over the new energy sources section in your packet and pay particular attention to the Hydrogen as a Fuel section.

7. Which of the following statements are correct?

ΔE is positive if work is done by the system and heat flows from the system

ΔE is only positive if q is positive.

ΔV is positive when work done by the system

ΔV is positive only when q is negative

W is negative when ΔV is positive

ΔE = q + w. q is positive when heat flows into the system and negative when heat flows out of the system. w is positive when work is done to the system and negative when work is done by the system. ΔE would be positive if q and w are positive or if heat or work into the system is more than what is leaving the system. w=-PΔV. When ΔV is positive (expansion) then work is negative because work is leaving the system. When ΔV is negative (compression) then work is positive because work is done to the system.

Explanation

ΔE = q + w. q is positive when heat flows into the system and negative when heat flows out of the system. w is positive when work is done to the system and negative when work is done by the system. ΔE would be positive if q and w are positive or if heat or work into the system is more than what is leaving the system. w=-PΔV. When ΔV is positive (expansion) then work is negative because work is leaving the system. When ΔV is negative (compression) then work is positive because work is done to the system.

8. If the heat of combustion of H₂ = -572 kJ. What is the change in enthalpy of the reaction if you have 46.1 g of hydrogen gas?

1.31 x 10^4 kJ

-1.31 x 10^4 kJ

2.61 x 10^4 kJ

-2.61 x 10^4 kJ

None of these.

Heat of combustion assumes 1 mole of the substance. 46.1g of H2 = 22.82 mols of H2. 22.82 mols x (-572 kj/mol) = 1.31 x 10^4 kJ

Explanation

Heat of combustion assumes 1 mole of the substance. 46.1g of H2 = 22.82 mols of H2. 22.82 mols x (-572 kj/mol) = 1.31 x 10^4 kJ

9. What does the First Law of Thermodynamics mean and how does the system change its internal energy?

The overall energy of the universe is constant and the change in energy of the system is due to energy flowing in and out of the system in the form of heat and work.

Explanation

The overall energy of the universe is constant and the change in energy of the system is due to energy flowing in and out of the system in the form of heat and work.

10. When calculating the heat of a reaction, you have to know the heat of formations of the products and reactants. Some of these substances always have a heat of formation of zero. Why is this?

Elements standing alone always have a heat of formation of zero.

Compounds, such as water, in their standard state are always zero.

Gases are always zero because they are freely formed in the atmosphere.

Elements standing alone in their standard state are always zero.

Water is the only one that is zero.

When elements are in their standard state, they exist in their most stable form and do not require any energy to be formed. Therefore, their heat of formation is considered to be zero. This is because elements in their standard state have no bonds to break or form, and their energy content is already at its minimum.

Explanation

When elements are in their standard state, they exist in their most stable form and do not require any energy to be formed. Therefore, their heat of formation is considered to be zero. This is because elements in their standard state have no bonds to break or form, and their energy content is already at its minimum.

11. The combustion of methane (CH₄) releases 891 kJ of energy. The heat of formations of CO₂ = -393 kJ/mol and H₂O_(l) = -286 kJ/mol. What is the heat of formation of methane?

-1570 kJ

-1865

-74 kJ

74 kJ

212 kJ

The change in enthalpy of a reaction = (sum of the formation of the products) - ( sum of formation of the reactants. In this case, the enthalpy of the reaction is -891 kj = (-393 kJ (CO2) + 2x-286 (H2O)) - ( x + 0 kJ). Note that there are 2 moles of water produced when methane is combusted.

Explanation

The change in enthalpy of a reaction = (sum of the formation of the products) - ( sum of formation of the reactants. In this case, the enthalpy of the reaction is -891 kj = (-393 kJ (CO2) + 2x-286 (H2O)) - ( x + 0 kJ). Note that there are 2 moles of water produced when methane is combusted.