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Flashcards in General Chemistry-Thermochemistry Deck (190)
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How can you calculate the heat of combustion?

Since no heat is exchanged between the calorimeter and the rest of the universe, Q calorimeter is 0. So,
/\U system + /\U surroundings = /\U calorimeter = Q calorimeter- W calorimeter = 0
/\U system = -/\U surroundings
And since no work is done:
q system= -q surroundings
m steel c steel /\T + m oxygen c oxygen /\T = -m water c water /\T
Heat exchange between the system and the surroundings makes it possible for use to calculate the heat combustion.


What is an adiabotic process?

No heat is exchanged between the calorimeter and the rest of the universe, but it is exchanged between the steel decomposition vessel and the surrounding water.


What happens when a compound is heated?

The temperature rises until the melting or boiling point is reached. Then the temperature remains constant as the compound is converted to the next phase. Once the entire sample is converted then the temperature begins to rise again.


What do heat curves show?

Heating curves show that phase change reactions do not undergo changes in temperature.


Why can't the equation q=mc/\T be used for phase change reactions?

Phase changes do not undergo changes in temperature, so /\T would be 0.


Where does all the heat/energy go if the temperature is not going up but heat is still being added during a phase change reaction?

The solid absorbs energy, which allows particles to overcome the attractive forces that hold them in a rigid , three-dimensional arrangement. When melting an ice cube, all of the heat added during the process is used to overcome the intermolecular forces between water molecules in ice, forming liquid water.


What must be used during the transition between a solid-liquid boundry?

The enthalpy (or heat) of fusion (/\H fus) must be used to determine the heat transferred during the phase change.


When transitioning from a solid to liquid, change in enthalpy will be ____________. What about from a liquid to a solid.

positive because heat must be added.
From liquid to solid change in enthalpy will be negative because heat must be removed.


What must be used during the transition between the liquid-gas boundry?

The enthalpy (or heat) of vaporization (/\H vap)


What equation is used for enthalpy of vaporization?



What do the letters represent in the equation q=mL?

m: mass
L: latent heat, a general term for enthalpy of an isothermal process given in the units cal/g.


How do you calculate the total amount of heat needed to cross multiple phase boundaries?

A summation of the heats for changing the temperature of each of the respective phases and the heats associated with phase changes.


How do scientists express heat change at constant pressure?

Enthalpy (H)


How do you find the enthalpy change of a reaction?

/\H rxn, one must subtract the enthalpy of the reactants from the enthalpy of the products.
/\H rxn= H products - H reactants.


What corresponds to a endothermic process? What about exothermic process?

A positive /\H rxn is endothermic
Negative /\H rxn is exothermic


Is it possible to measure enthalpy directly?

No, only /\H can be measured and only for certain fast and spontaneous processes.


What is standard enthalpy of formation?

/\H degree f, the enthalpy required to produce one mole of a compound from its elements in their standard states.


What is standard state again?

The most stable physical state of an element or compound at 298 K and 1 atm


What is the "standard heat of a reaction?"

The enthalpy change accompanying a reaction being carried out under standard conditions.


How can the standard heat of a reaction be calculated?

Taking the difference between the sum of the standard heats of formation for the products and the sum of the standard heats of formation of the reactants.
/\H degree rxn= Sum /\H degree f products - sum /\H degree f reactants


What does Hess's law state?

Enthalpy changes of reactions are additive. When thermochemical equation (chemical equations for which energy changes are known) are added to give the net equation for a reaction, the corresponding heats of reaction are also added to give the net heat of reaction.


Hess's law is embodied in which equation?

Enthalpy equation.


What are a couple ways to write the equation for hess's law?

/\H rxn=(/\H reactants -> elements) + (/\H elements -> reactants)
/\H degree rxn= (sum /\H degree f products) - (sum /\H degree f reactants)


Hess's law applies to what?

Any state function, including entropy and gibbs free energy


How can Hess's law be expressed?

In terms of bond enthalpies


What is another name for bond enthalpies?

bond dissociation energies


What is bond dissociation energy?

The average energy that is required to break a particular type of bond between atoms in the gas phase- remember bond dissociation is an endothermic process.


What units are used for bond dissociation energy?

kJ/mol of bonds broken


What are bond enthalpies?

The averages of the bond energies for the same bond in many different compounds.


What is important to note about bond formation and bond breaking?

Bond formation is opposite of bond breaking and has the same magnitude of energy but with a negative rather than positive sign. That is the energy is released when bonds are formed. Bond formation is exothermic and bond dissociation is endothermic.