Questions and Answers
What does Hess's law of summation state?
When you write a reverse reaction, you must also change the _____ of ΔH.
sign
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 _____ _____
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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?
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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.
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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.
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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.
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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.
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The symbol used for the standard heat of formation is _____
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How is the standard heat of reaction determined?
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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.
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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.
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Define standard heat of formation.
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What does Hess's law of heat summation state?
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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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