Chemistry Chapter on Enthalpy

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Questions and Answers

What does a negative value of ΔH indicate about the heat flow during a chemical reaction?

The reaction is endothermic.

The reaction occurs under constant volume.

The enthalpy is constant.

The reaction is exothermic. (correct)

What is the relationship between ΔHvap and ΔHcond for a specific substance?

ΔHcond = ΔHvap

ΔHcond > ΔHvap

ΔHcond = -ΔHvap (correct)

ΔHcond < ΔHvap

Under which condition is enthalpy change, ΔH, measured?

At constant pressure (correct)

At variable pressure

At constant temperature

At constant volume

Which of the following correctly describes the heat flow when a liquid converts to a gas?

It absorbs heat and is endothermic. Signup and view all the answers

What represents the enthalpy change for melting ice in terms of its sign?

Positive value Signup and view all the answers

Hess's law can be summarized by which of the following statements?

The total enthalpy change is independent of the pathway taken. Signup and view all the answers

Which of the following factors directly influences the calculation of heat flow for a phase change?

The number of moles of substance Signup and view all the answers

How does the internal energy for a reaction relate to heat flow and work during a constant volume process?

ΔE = qv Signup and view all the answers

What is meant by energy density in relation to fuels?

The amount of energy that can be released per gram of fuel burned Signup and view all the answers

Which factor is NOT typically considered when evaluating the economic merits of a fuel?

The aesthetic appeal of the fuel’s packaging Signup and view all the answers

What role does a transmission substation play in the electrical grid?

Increases voltage to reduce loss over long distances Signup and view all the answers

Which of the following factors does NOT influence electricity demand in the grid system?

Size of individual appliances Signup and view all the answers

In the context of the electrical grid, what is the purpose of a distribution substation?

To step down voltage and distribute electricity to consumers Signup and view all the answers

What does a negative value of ΔH indicate about a reaction?

The reaction releases heat to the surroundings. Signup and view all the answers

In the combustion of methane, how much heat is released when 1 mole reacts with oxygen?

890.4 kJ Signup and view all the answers

What is the heat of formation, ΔHf?

The heat of reaction for formation from elements. Signup and view all the answers

What happens when stoichiometric coefficients in a thermochemical equation are multiplied?

The heat of reaction must also be multiplied. Signup and view all the answers

Which statement best describes Hess's law?

The total enthalpy change is the sum of the enthalpy changes for individual steps. Signup and view all the answers

What is a characteristic of state functions?

Their values depend only on the state of the system. Signup and view all the answers

When manipulating a reaction under Hess's Law, which of the following is true?

The reaction can be reversed, and its enthalpy must change sign. Signup and view all the answers

Which of the following is NOT a form of heat of reaction?

Heat of fusion, ΔHfus Signup and view all the answers

What is the purpose of a heat curve in thermodynamics?

To break calculation into specific heat and phase change enthalpy calculations. Signup and view all the answers

What happens to the value of ΔHf when a reaction is multiplied by a constant?

ΔHf is multiplied by the constant. Signup and view all the answers

What type of reaction is classified as exothermic?

A reaction where making product bonds releases more energy than the energy required to break reactant bonds. Signup and view all the answers

Which statement regarding Hess's law is true?

Any convenient path can be used to calculate enthalpy change. Signup and view all the answers

How is the heat of vaporization utilized in power plants?

By allowing heat energy released in combustion to vaporize water for electricity generation. Signup and view all the answers

How is energy treated in an exothermic reaction according to thermochemical equations?

As a product. Signup and view all the answers

What is the heat of combustion of propane (C3H8) given in the examples?

-2,219.2 kJ Signup and view all the answers

What does the heat of reaction refer to?

The net change in enthalpy for a chemical reaction. Signup and view all the answers

In which scenario is Hess’s law particularly useful?

Determining heats of reactions with multiple step processes. Signup and view all the answers

What is a significant limitation of using average bond energies to estimate enthalpy changes?

Bond energies vary between similar compounds, affecting accuracy. Signup and view all the answers

During which process does water convert from liquid to gas?

Vaporization. Signup and view all the answers

What must be true for the ΔH value in a thermochemical equation?

It corresponds to the reaction as written. Signup and view all the answers

What happens during the combustion of methane?

Specific bonds are broken and different bonds are formed, resulting in energy release. Signup and view all the answers

What do you calculate in order to find the heat absorbed by an engine producing nitric oxide gas?

The heat change associated with the formation of nitric oxide. Signup and view all the answers

What is the value of ΔHf° for water stated in the examples?

-285.8 kJ/mol Signup and view all the answers

What is the significance of bond breaking and making in a chemical reaction?

Energy dynamics determine whether the reaction is exothermic or endothermic. Signup and view all the answers

Study Notes

Defining Enthalpy

Enthalpy is the heat flow under constant pressure conditions
Enthalpy change (ΔH) is represented by: ΔH = qp
Exothermic reactions release heat, ΔH is negative.
Endothermic reactions absorb heat, ΔH is positive.

ΔH of Phase Changes

The heat required to convert a liquid to a gas is the heat of vaporization (ΔHvap), which is endothermic.
The heat released to convert a gas to a liquid is the heat of condensation (ΔHcond), which is exothermic.
ΔHcond = –ΔHvap
The magnitude of enthalpy change depends on the substance involved.

Heat of Reaction

The enthalpy change for a reaction is called the heat of reaction.
It can be estimated using bond energies.
Breaking bonds requires energy, making bonds releases energy.
Bond energy method is used to estimate enthalpy changes for reactions where thermochemical data is unavailable.

Thermochemical Equations

A thermochemical equation summarizes the overall energetics for a chemical reaction.
The sign on the ΔH indicates whether the reaction is endothermic or exothermic.
If stoichiometric coefficients are multiplied by a factor, the heat of reaction must also be multiplied by the same factor.

Heats of Reaction for Specific Reactions

Heat of combustion (ΔHcomb) is the heat released during a combustion reaction.
Heat of neutralization (ΔHneut) is the heat released during a neutralization reaction.
Heat of formation (ΔHf) is the heat change for the formation of one mole of a compound from its elements in their standard states.
The standard state is the most stable form of an element at 25°C and 1 atm, indicated with a superscript °.
ΔHf° = 0 for an element in its standard state.

Hess’s Law

Hess's law states that the enthalpy change for any process is independent of the particular way the process is carried out.
Enthalpy is a state function, its value relies solely on the state of the system, independent of its history.
Hess's Law utilizes known enthalpy values for individual reaction steps to calculate the enthalpy change for an overall reaction.

Hess’s Law Steps

To calculate enthalpy changes using Hess's Law, manipulate given chemical reactions so their overall effect matches the desired reaction.
Rules for manipulating reactions:
- Reactions can be reversed, changing the sign of ΔHf.
- Reactions can be multiplied by a constant, which multiplies the value of ΔHf by the same constant.

Formation Reactions and Hess’s Law

The enthalpy change for a reaction can be calculated using Hess's law and heats of formation.

Energy and Stoichiometry

A thermochemical equation allows for the stoichiometric treatment of energy.
Energy is treated as a product in exothermic reactions and as a reactant in endothermic reactions.
The thermochemical equation converts between the number of moles of a reactant/product and the amount of energy released/absorbed.

Energy Density and Fuels

Factors considered when evaluating a fuel's economic merits include:
- Technology for extracting the fuel
- Pollution released during combustion
- Relative safety
- Ease of transport
- Energy density
Energy density is the amount of energy that can be released per gram of fuel burned.
Fuels with higher energy densities require less fuel transport.

Power Distribution and the Electrical Grid

The electrical grid balances electricity demand with production.
Demand varies based on factors like season, time of day, and power plant operating conditions.
Key components of the electrical grid include:
- Generating station: produces electricity (e.g., burning fossil fuels).
- Transmission substation: increases voltage to reduce loss over long distances.
- Distribution substation: reduces voltage and distributes electricity to consumers.

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Description

Test your understanding of enthalpy concepts, including the definition of enthalpy, ΔH of phase changes, and heat of reaction. This quiz will challenge your knowledge of exothermic and endothermic reactions, as well as thermochemical equations. Prepare to dive deep into the energetics of chemistry!