Energy and Phase Change Quiz

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Questions and Answers

What is the heat of fusion?

  • Energy required to convert a gas into a liquid
  • Energy required to convert a solid directly into a gas
  • Energy required to convert a solid at its melting point to a liquid (correct)
  • Energy required to convert a liquid into a solid

During a phase change, what happens to the temperature of the substance?

  • It decreases rapidly
  • It remains constant (correct)
  • It constantly rises
  • It fluctuates significantly

Which of the following is needed to calculate the heat during a phase?

  • Concentration of the substance
  • Pressure and volume of the substance
  • Phase identity of the substance
  • Specific heat, sample mass, and temperature change (correct)

What is the enthalpy change when converting 1.00 mol of ice at -25 °C to steam at 125 °C?

<p>56.0 kJ (B)</p> Signup and view all the answers

Which statement is true about the heat of vaporization?

<p>It's the energy required to turn a liquid at boiling point into a gas. (B)</p> Signup and view all the answers

What is the specific heat of liquid water?

<p>4.18 J/g.K (C)</p> Signup and view all the answers

What is the formula to calculate the total heat during a phase change?

<p>Mass x Specific Heat x Temperature Change (D)</p> Signup and view all the answers

What does the specific heat refer to?

<p>The amount of heat required to raise the temperature of a substance per unit mass by 1 degree K (D)</p> Signup and view all the answers

What happens to the system's energy when a gas expands in a sealed vessel against atmospheric pressure?

<p>The system loses energy as work is done. (A)</p> Signup and view all the answers

What sign indicates that energy is leaving the system as work?

<p>Negative work indicates exit of energy. (D)</p> Signup and view all the answers

In the equation for work done by the system, which variable represents the change in volume?

<p>ΔV (B)</p> Signup and view all the answers

If a system absorbs heat from the surroundings, what type of process is occurring?

<p>Endothermic process (A)</p> Signup and view all the answers

How much work is done when the volume changes from 0.250 L to 0.980 L at a constant pressure of 1.35 atm?

<p>29.83 J (A)</p> Signup and view all the answers

What is the defining feature of kinetic energy?

<p>Energy that causes an object to move. (C)</p> Signup and view all the answers

What does the symbol ΔE represent in thermochemistry?

<p>Change in internal energy. (A)</p> Signup and view all the answers

Which unit is used for measuring energy in the SI system?

<p>Joule (B)</p> Signup and view all the answers

What does the term 'system' refer to in thermodynamics?

<p>Molecules under study. (D)</p> Signup and view all the answers

How does heat transfer occur according to thermochemistry principles?

<p>From warmer objects to cooler objects. (B)</p> Signup and view all the answers

Which of the following best describes potential energy?

<p>Energy due to an object's position. (D)</p> Signup and view all the answers

What does thermochemistry specifically study?

<p>Chemical reactions and their energy changes. (B)</p> Signup and view all the answers

What is the primary role of work in a chemical reaction?

<p>To accompany heat changes. (B)</p> Signup and view all the answers

What is the calculated value of ΔH for the combustion of benzene (C6H6)?

<p>-3267 kJ (A), -41.8 kJ/g (C)</p> Signup and view all the answers

Which of the following enthalpy values corresponds to the combustion of propane (C3H8)?

<p>-227.0 kJ/mol (B), -50.3 kJ/g (C), -2220 kJ/mol (D)</p> Signup and view all the answers

Using Hess's Law, what is the enthalpy of reaction for converting carbon to carbon monoxide (C to CO)?

<p>-110.5 kJ (A)</p> Signup and view all the answers

What is the sign of the ΔH value for the combustion of carbon to CO2?

<p>Negative (B)</p> Signup and view all the answers

What is the reaction equation for the combustion of carbon to yield CO2?

<p>C(s) + O2(g) → CO2(g) (B)</p> Signup and view all the answers

How is the enthalpy for the combustion of CO adjusted using Hess's Law?

<p>It is reversed and subtracted from that of C to CO2. (A), It is added directly to that of C to CO2. (B)</p> Signup and view all the answers

What is the molar mass of benzene (C6H6)?

<p>78.1 g/mol (D)</p> Signup and view all the answers

What value represents the enthalpy of formation (ΔHf) for carbon dioxide (CO2)?

<p>-393.5 kJ/mol (D)</p> Signup and view all the answers

What is the relationship between enthalpy change and heat at constant pressure?

<p>Enthalpy change is equivalent to the heat gained or lost. (A), Enthalpy change is a state function but heat is not. (D)</p> Signup and view all the answers

Which of the following definitions applies to specific heat capacity?

<p>Energy required to raise 1 g of a substance by 1 K. (D)</p> Signup and view all the answers

How is the enthalpy of reaction, ΔH, calculated?

<p>ΔH = Hproducts - Hreactants (C)</p> Signup and view all the answers

What instrument is used to measure heat flow in calorimetry?

<p>Calorimeter (C)</p> Signup and view all the answers

In bomb calorimetry, what is primarily measured?

<p>Internal energy, ΔE (B)</p> Signup and view all the answers

What does calorimetry allow us to measure directly?

<p>The heat flow during a reaction (C)</p> Signup and view all the answers

What is the specific heat capacity value for water?

<p>4.184 J/g∙K (B)</p> Signup and view all the answers

What signifies that a property is a state function?

<p>It only depends on the current state, not the path. (D)</p> Signup and view all the answers

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Study Notes

### Energy & Phase Change

  • Energy is either added or released during a phase change.
  • Phase changes include melting/freezing, vaporizing/condensing, and subliming/depositing.

Heat of Fusion, Vaporization, & Sublimation

  • Heat of fusion is the energy required to change a solid at its melting point to a liquid.
  • Heat of vaporization is the energy required to change a liquid at its boiling point to a gas.
  • Heat of sublimation is the energy required to change a solid directly to a gas.

Heating Curves

  • Temperature and heat added are plotted on a heating curve.
  • Within a phase, heat is the product of specific heat, sample mass, and temperature change.
  • During a phase change, the temperature of the substance does not rise.
  • For phase changes, heat is the product of mass and the heat of fusion or vaporization.

Calculating Enthalpy Change (ΔH)

  • Can be calculated for converting 1.00 mol of ice at -25 °C to steam at 125 °C under constant pressure of 1 atm.
  • Steps involved:
    • Calculate enthalpy change for each individual phase
    • Sum up the individual enthalpy changes to get the total enthalpy change (ΔHA→F)
  • Specific heats of ice, liquid water, and steam are 2.03, 4.18, and 1.84 J/g.K, respectively.
  • ΔHfus = 6.01 kJ/mol and ΔHvap = 40.67 kJ/mol for H2O.

Hess's Law

  • Definition: Enthalpy change for a reaction is independent of the pathway.
  • Can be used to calculate enthalpy of reaction for a reaction not directly measured if the enthalpy changes of other reactions are known.
  • Reactions can be reversed or multiplied to obtain the desired reaction.

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