Heat
qsys=10 J
qsurr= ? J
-10
How do the calculations differ between heat and enthalpy?
Heat is in J or kJ, usually using q=mCdeltaT.
Enthalpy is in kJ/molrxn, usually using q/molrxn.
The enthalpy of fusion of water is 6 kJ/mol. What is the enthalpy associated with freezing water?
-6 kJ/mol (freezing is exothermic, since heat is released. Make the enthalpy value negative.)
In general, does more energy get released from condensing or freezing? Why?
Condensing. Much more energy is lost going from gas to liquid than from liquid to solid. Gas forms a lot more intermolecular forces to become a liquid, lowering the potential energy significantly. Liquid to solid, there are already many IMFs present.
A container of water has a mass of 10 g, specific heat of 4.2 J/gC, and gains 5 kJ of heat. Find the temperature change.
5 = (10)(4.2)(deltaT)
What is the mistake?
5 kJ should be 5000 J
5000 = (10)(4.2)(deltaT)
2 NH3 → 3 H2 + N2 ΔH=84 kJ/mol
What is the enthalpy of formation of NH3?
-42 kJ/mol
Formation of NH3: 1.5 H2 + 1/2 N2 → NH3
(reverse reaction, halved)
If you are making a bond, is bond energy negative or positive? Why?
Negative (making bonds is exothermic due to a decrease in potential energy)
The enthalpy of a reaction is positive. How do the bond energies of the reactants and products compare?
The energy needed to break the reactant bonds is greater than the energy released to make the product bonds.
Two objects at different temperatures are put in contact with each other and reach thermal equilibrium. Object A has higher specific heat than object B. How do their final temperatures compare? How do their temperature changes compare?
Their final temperatures are equal. Object B changed temperature more than object A due to the specific heat difference (object A requires more heat change to change temperature than object B).
A forward reaction's enthalpy is -50 kJ/mol. A student conducts the reverse reaction. Will the temperature of the surroundings increase or decrease?
Decrease. Forward reaction is exothermic, so reverse reaction is endothermic. The endothermic reverse reaction will cause the temperature of the surroundings to decrease since heat is entering the system.
1/2 H2(g) + 1/2 O2(l) → H2O ΔH = 30 kJ/mol
1/2 H2(g) + 1/2 O2(g) → H2O(g) ΔH = -10 kJ/mol
O2 (g) --> O2 (l) ΔH = ? kJ/mol
ΔH = -80 kJ/mol
Flip 1st reaction, add enthalpies.
1/2 O2 (g) --> 1/2 O2 (l) ΔH = -40 kJ/mol
Multiply by 2 --> -80 kJ/mol
Higher bond energy means a bond is more or less stable?
More stable (more energy is required to break it, more energy is released when it is made)
An endothermic reaction takes place in a carbon polymer gel. Then you repeat the experiment with double the mass of the limiting reactant. Will the final temperature of the carbon polymer gel be equal, greater, or lesser than the original experiment?
Less. Double the reaction moles means double the q. Since the reaction is endothermic, double the heat would be absorbed into the system from the surroundings, resulting in a lower final temperature.
You react magnesium with oxygen in an open container of uncooked rice. You measure the temperature change of the rice. What other information do you need to figure out the enthalpy of the reaction?
The mass of rice and specific heat of rice will give you the q of the rice, which is opposite of the q of the reaction.
The mass of magnesium will give you the moles of reaction. You then divide the q of the reaction by the moles of reaction to find the enthalpy.
C + O2 → CO2 ΔH= ?
O=O BE=495 kJ/mol
C=O BE=745 kJ/mol
ΔH= (1*495)-(2*745)= -995 kJ/mol
Two different metals with different specific heats are heated from 5 to 10 degrees celsius. How do their average kinetic energies compare at the molecular level?
They are the same since they reached the same temperature. The metal with the higher specific heat just required more heat input to reach that temperature.
You dissolve KOH in water, and it feels warm. What can you infer about the potential energy and heat energy of the system and surroundings?
Potential energy in the system was released as heat energy to the surroundings.
The potential energy contained in the products of a reaction is 40 J. The reaction was exothermic. Is the potential energy of the reactants greater than, less than, or equal to 40 J?
Greater than. Potential energy decreased from reactants to products and was released as heat.
How can you find the enthalpy of a reaction using heats of formation?
ΔHrxn=Hf, products-Hf,reactants
The temperature of a solution decreases when a solute is dissolved. What sign should be used for qsys?
+
Heat left the surroundings, which means it was gained by the system.