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Flashcards in General Chemistry-Thermochemistry Deck (190)
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121

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
So,
/\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.

122

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.

123

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.

124

What do heat curves show?

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

125

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.

126

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.

127

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.

128

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.

129

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

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

130

What equation is used for enthalpy of vaporization?

q=mL

131

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.

132

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.

133

How do scientists express heat change at constant pressure?

Enthalpy (H)

134

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.

135

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

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

136

Is it possible to measure enthalpy directly?

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

137

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.

138

What is standard state again?

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

139

What is the "standard heat of a reaction?"

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

140

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

141

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.

142

Hess's law is embodied in which equation?

Enthalpy equation.

143

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)

144

Hess's law applies to what?

Any state function, including entropy and gibbs free energy

145

How can Hess's law be expressed?

In terms of bond enthalpies

146

What is another name for bond enthalpies?

bond dissociation energies

147

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.

148

What units are used for bond dissociation energy?

kJ/mol of bonds broken

149

What are bond enthalpies?

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

150

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.