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What does Hess's law of summation state?
What does Hess's law of summation state?
When you write a reverse reaction, you must also change the _____ of ΔH.
When you write a reverse reaction, you must also change the _____ of ΔH.
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Why may it be difficult to measure the enthalpy change?
Why may it be difficult to measure the enthalpy change?
The reaction may be slow, might have intermediate steps, or materials might be destroyed.
A common set of conditions reference point is known as _____ _____
A common set of conditions reference point is known as _____ _____
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What are the stable forms of a substance at standard state?
What are the stable forms of a substance at standard state?
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What is the symbol for the standard heat of formation?
What is the symbol for the standard heat of formation?
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What does the standard heat formation of a compound represent?
What does the standard heat formation of a compound represent?
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The ΔH degree of a free element in its standard state is set to _____ for diatomic molecules.
The ΔH degree of a free element in its standard state is set to _____ for diatomic molecules.
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How can you calculate the heat of reaction under standard conditions?
How can you calculate the heat of reaction under standard conditions?
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Hess's law of heat summation states that for a chemical equation, the ____ change for the final equation equals the sum of the enthalpy changes for the individual steps.
Hess's law of heat summation states that for a chemical equation, the ____ change for the final equation equals the sum of the enthalpy changes for the individual steps.
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Hess's law makes it possible to measure the heat of reaction _____
Hess's law makes it possible to measure the heat of reaction _____
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In cases where measuring the heat for a reaction is hard, the _____ is used to calculate heats of reaction at standard conditions.
In cases where measuring the heat for a reaction is hard, the _____ is used to calculate heats of reaction at standard conditions.
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The standard heat of formation of a compound is the ____ in enthalpy that accompanies the formation of _____ mole of a compound from its elements.
The standard heat of formation of a compound is the ____ in enthalpy that accompanies the formation of _____ mole of a compound from its elements.
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The symbol used for the standard heat of formation is _____
The symbol used for the standard heat of formation is _____
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How is the standard heat of reaction determined?
How is the standard heat of reaction determined?
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The standard heat of formation is determined at 100 degrees Celsius.
The standard heat of formation is determined at 100 degrees Celsius.
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Hess's law of heat summation is not related to the law of conservation of energy.
Hess's law of heat summation is not related to the law of conservation of energy.
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When using Hess's law of heat summation, intermediate reactions are summed and terms are canceled to arrive at a final equation.
When using Hess's law of heat summation, intermediate reactions are summed and terms are canceled to arrive at a final equation.
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Define standard heat of formation.
Define standard heat of formation.
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What does Hess's law of heat summation state?
What does Hess's law of heat summation state?
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What is the symbol for the standard heat of formation?
What is the symbol for the standard heat of formation?
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Study Notes
Hess's Law
- Hess's law of summation allows for the calculation of the heat of reaction by combining thermochemical equations, yielding a final heat of reaction.
- For a reverse reaction, the sign of ΔH must be changed accordingly, from negative to positive or vice versa.
- Hess's law enables the indirect measurement of heat changes for reactions.
Enthalpy and Standard State
- A common reference point for standard conditions is termed the standard state, which is defined at 25°C and 101.3 kPa.
- The standard heat of formation (ΔH°_f) is the change in enthalpy associated with forming one mole of a compound from its elements in their stable forms.
Standard Heat of Formation
- The standard heat of formation for any free element in its standard state, including diatomic molecules, is assigned a value of zero.
- The standard heat of formation is particularly useful for calculating reaction heats at standard conditions.
Calculation of Heat of Reaction
- To find the standard heat of reaction, subtract the ΔH°_f of the reactants from the ΔH°_f of the products.
- Hess's law also states that if a chemical equation can be represented as the sum of several steps, the total enthalpy change is the sum of changes from each step.
Common Misconceptions
- The standard heat of formation is not determined at 100°C, but rather at standard state conditions (25°C).
- Hess's law of heat summation is indeed aligned with the law of conservation of energy, indicating energy conservation during chemical reactions.
Summary Notes
- The standard heat of formation symbolizes the energy change for forming one mole of a substance from its elements.
- The summation approach in Hess's law allows for effective calculation of reaction enthalpies by using intermediate reactions, similar to algebraic summation and cancellation of terms.
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