Midterm #3: Chapter 9
What is the ∆H of breaking ionic bonds equal to?
the lattice energy of a binary ionic compounds
When you can't determine the ∆Hrxn for a reaction by actually performing the reaction, use Hess's Law. What is Hess's Law?
the principle that enthalpy of reaction for a process that is the same of two or more reactions is equal to the sum of the ∆Hrxn values of the constituent reactions
Why is a coffee cup calorimeter a good model for calculating heat absorption/release?
they don't absorb much heat from the system and are useful in determining ∆Hrxn for reactions in aqueous solutions
Why do exothermic processes tend to be spontaneous?
they lead to a decrease in energy and an increase in the stability of the system
True or False: The energy lost by the reaction mixture in a bomb calorimeter equals the energy gained by the calorimeter.
true
True or False: Whatever energy the system loses is gained by the surroundings.
true
What is work? What is positive work/negative work and how does it apply to ∆Esys increasing/decreasing?
work - force applied over a distance (w=Fd) ∆Esys decreases if - heat flows out of the system OR if work is being done by the system (negative work) ∆Esys increases if - heat flows into the system OR if work is being done on the system (positive work)
What is internal energy in terms of q and w (not p-v work)?
∆E = q + w
What is a bomb calorimeter and what does it include?
- a constant-volume device used to measure the energy released in a combustion reaction - includes: bomb, water, insulated container, and other components
How can standard enthalpies of reaction be used to determine/predict the standard enthalpy of formation of a substance?
- break down the given reaction into a series of formation reactions of the reactants and products - use Hess's Law to combine the ∆Hf˚ values into an overall ∆Hrxn˚ value ∆Hrxn˚ = ∑(n products)(∆Hf˚ products) - ∑(n reactants)(∆Hf˚ reactants)
What is ∆Hrxn dependent on?
- difference in enthalpy between reactants and products
What is the first law of thermodynamics?
- energy cannot be created or destroyed, only transformed - total energy in the universe is constant
What does it mean to say that internal energy is a state function?
- it is a quantity that depends only on initial and final values; ∆E is independent of how it was acquired ∆E = ∆E final - ∆E initial
What are the requirements for a formation reaction to be considered a standard formation reaction?
- must produce 1 mole of product - reactants must be in their standard states
In terms of the universe, what are the system and the surroundings? What is energy of the universe in terms of the system and the surroundings?
- system: part of interest (like a chemical reaction) - surroundings: everything else not included in the system - universe: system + surroundings ∆Euniv = ∆Esys + ∆Esurr = 0 ∆Esys = -∆Esurr
What is the magnitude of energy released/absorbed during a chemical reaction proportional to?
- the quantity (moles) of reactants consumed - the enthalpy (heat) of reaction (∆Hrxn)
What is important to note when calculating the energy required for a solid to warm up to a temperature, change phase, and then warm up to a higher temperature?
- the substance requires a certain amount of energy to change temp (q=mc∆T) and another separate quantity of energy to change phase (q=(mass/moles)(∆Hfus/vap) once it reaches phase-change temps - the substance will have a DIFFERENT value for specific heat after changing phases
_________ _______ is a technique used to measure the quantity of energy produced in a combustion reaction. In this methods, reactants are placed in a ________ submerged in water. Combustion occurs and energy produced changes the temperature of the water.
BOMB CALORIMETRY is a technique used to measure the quantity of energy produced in a combustion reaction. In this methods, reactants are placed in a VESSEL (BOMB) submerged in water. Combustion occurs and energy produced changes the temperature of the water.
_______ _______ is the energy change that occurs when one mole of bonds in the gas phase is broken. The energy needed to break a particular bond is equal in ______ but opposite in ______ to energy released when the same bond forms from free atoms. What is the formula for heat of reaction based on bond breaking/forming?
BOND ENERGY is the energy change that occurs when one mole of bonds in the gas phase is broken. The energy needed to break a particular bond is equal in MAGNITUDE but opposite in SIGN to energy released when the same bond forms from free atoms. What is the formula for heat of reaction based on bond breaking/forming? ∆Hrxn = ∑∆H bond breaking - ∑∆H bond forming
Breaking bonds is (endothermic/exothermic), while forming bonds is (endothermic/exothermic).
Breaking bonds is ENDOTHERMIC, while forming bonds is EXOTHERMIC.
_____________ is an analytic technique in which changes in temp of an object/device with known heat capacity is used to determine the energy released/absorbed by a process occurring inside the calorimeter. A ______________ is a device used to measure the absorption/release pf energy by a physical change or chemical process.
CALORIMETRY is an analytic technique in which changes in temp of an object/device with known heat capacity is used to determine the energy released/absorbed by a process occurring inside the calorimeter. A CALORIMETER is a device used to measure the absorption/release pf energy by a physical change or chemical process.
_________ _____ is the potential energy stored in chemical bonds. _______ ______ is the sum of all kinetic and potential energy. A change in energy (∆E) occurs when the collective internal energies of reactants are __________ the collective internal energies of the products.
CHEMICAL ENERGY is the potential energy stored in chemical bonds. INTERNAL ENERGY is the sum of all kinetic and potential energy. A change in energy (∆E) occurs when the collective internal energies of reactants are GREATER OR LESS THAN the collective internal energies of the products.
_________ __ _________ is the overall change in enthalpy that occurs when a solute is dissolved in a solute. It involves breaking of ionic bonds in the _____ ______, breaking of H-bonds in _______, and ______-______ interactions. This process can be ______ or ______ , depending on the substance. It can be determined using a ________. What is the formula for this quantity given the above information?
ENTHALPY OF SOLUTION is the overall change in enthalpy that occurs when a solute is dissolved in a solute. It involves breaking of ionic bonds in the IONIC SOLUTE, breaking of H-bonds in WATER, and ION-DIPOLE interactions. This process can be ENDOTHERMIC or EXOTHERMIC, depending on the substance. It can be determined using a CALORIMETER. What is the formula for this quantity given the above information? ∆Hsoln = [∆H ionic bonds + ∆H water-water] - (∆H ion-dipole) [in brackets] = endothermic (in parentheses) = exothermic
_______ is the sum of the internal energy and pressure-volume work in a system. ____________ ________ is the heat change at constant pressure; a quantity of heat transferred into/out of a system during a chemical reaction or physical change. At constant pressure, this _______ equals ______.
ENTHALPY is the sum of the internal energy and pressure-volume work in a system. ENTHALPY CHANGE (∆H) is the heat change at constant pressure; a quantity of heat transferred into/out of a system during a chemical reaction or physical change. At constant pressure, this ∆H = q.
True or False: Higher specific heat indicates less energy is required to increase the temp of an object.
FALSE - higher specific heat indicates that MORE ENERGY is required to increase the temp
True or False: Enthalpy is not a state function.
FALSE IT IS BC HESS LAW
______ _______ are quantities of energy released during the complete combustion of 1g of a substance. ______ _______ is a quantity of energy released during the complete combustion of a particular volume of a liquid fuel.
FUEL VALUES are quantities of energy released during the complete combustion of 1g of a substance. FUEL DENSITY is a quantity of energy released during the complete combustion of a particular volume of a liquid fuel.
If a reaction is endothermic, the energy of forming bonds is (greater than/less than) the energy of breaking bonds. If a reaction is exothermic, the energy of forming bonds is (greater than/less than) the energy of breaking bonds.
If a reaction is endothermic, the energy of forming bonds is LESS THAN the energy of breaking bonds. If a reaction is exothermic, the energy of forming bonds is GREATER THAN the energy of breaking bonds.
It is (impossible/possible) to determine the exact enthalpy of a substance; instead, we can measure the enthalpy of _____ of a substance. The _______ ________ ____ ________ is the enthalpy change that takes place at constant pressure (1 atm) when one mole of a substance is formed from its constituent elements in their standard states.
It is IMPOSSIBLE to determine the exact enthalpy of a substance; instead, we can measure the enthalpy of FORMATION of a substance. The STANDARD ENTHALPY OF FORMATION is the enthalpy change that takes place at constant pressure (1 atm) when one mole of a substance is formed from its constituent elements in their standard states.
How can you use ∆Hsoln to calculate the lattice energy of binary ionic compounds?
Must follow the Born-Haber cycle: ex: Na(s) + 1/2Cl2(g) --> NaCl(s) 1. Sublimation of 1 mole Na (endo) - ∆Hsub 2. Breaking covalent bonds of 1/2 mole Cl2 (Endo) - BE 3. Ionize 1 mol Na(g) atoms to 1 mole Na+ ions (Endo) - IE 4. 1 mol Cl atoms acquire 1 mol electrons to form 1 mol Cl- (exo) - EA 5. Form 1 mol of NaCl(s) from 1 mol Na+ and 1 mol Cl- (exo) - U (U is what you're trying to find)
What is pressure-volume work? Positive/negative work? Equations and relation to ∆E?
P-V Work: work done by or on a system involving changes in volume (and therefore pressure) W = -P∆V ∆E = q - P∆V positive - work done on system by surroundings negative - work done by system on surroundings
_______________ _________ ____ ________ is the enthalpy change associated with a reaction that takes place under standard conditions, which are ____ C, ___ atm, and ______ M. The ________ ______ of an element is the most stable form of a substance under standard conditions. Any element in its standard state has ∆Hf˚ = ____.
STANDARD ENTHALPY OF REACTION is the enthalpy change associated with a reaction that takes place under standard conditions, which are 25 C, 1 atm, and 1 M. The STANDARD STATE of an element is the most stable form of a substance under standard conditions. Any element in its standard state has ∆Hf˚ = 0.
Specific heats are temp (independent/dependent) and objects will have a different specific heat based on what _______ it is in.
Specific heats are temp INDEPENDENT and objects will have a different specific heat based on what PHASE it is in.
__________ is a measure of the kinetic energy within an object. _____ is disordered energy that is in the process of being transferred from a high-temp object to a low-temp object. The thermal energy of an object is determined by its _____, _____ and __________.
TEMPERATURE is a measure of the kinetic energy within an object. HEAT is disordered energy that is in the process of being transferred from a high-temp object to a low-temp object. The thermal energy of an object is determined by its TEMP, MASS and COMPOSITION.
The energy of chemical reactions depends on how much energy is required to _____ the bonds in reactants and how much energy is released as ________________________.
The energy of chemical reactions depends on how much energy is required to BREAK the bonds in reactants and how much energy is released as REACTANT ATOMS RECOMBINE TO FORM PRODUCTS.
When polar solutes dissolve in H2O, a negative ∆Hsoln indicates that solute-solvent intermolecular interactions are (stronger/weaker) than solute-solute and solvent-solvent intermolecular interactions. A positive ∆Hsoln indicates that solute-solvent intermolecular interactions are (stronger/weaker) than solute-solute and solvent-solvent interactions.
When polar solutes dissolve in H2O, a negative ∆Hsoln indicates that solute-solvent intermolecular interactions are STRONGER than solute-solute and solvent-solvent intermolecular interactions. A positive ∆Hsoln indicates that solute-solvent intermolecular interactions are WEAKER than solute-solute and solvent-solvent interactions.
To determine the heat energy from a combustion reaction, you must know the the __________ _____, or the heat capacity of the bomb calorimeter.
calorimeter constant
Endothermic vs. exothermic processes? q and ∆H values?
exothermic: energy flow (usually heat) from system (reactants) to surroundings (products) - negative q and negative ∆H endothermic: energy flow (usually heat) from surroundings to system (heat is a reactant) - positive q and positive ∆H
What is the difference between heat capacity and specific heat?
heat capacity - the energy required to raise the temp of an OBJECT by 1 degree C at constant pressure specific heat - the energy required to raise the temp of ONE GRAM of a substance by 1 degree C at constant pressure heat capacity (Cp) = (mass)(specific heat cp)
What is enthalpy/heat of fusion and enthalpy/heat of vaporization? How do intermolecular forces affect these values?
heat of fusion (∆Hfus) = energy required to convert one mole of solid substance at its melting point into the liquid state (melting) - increasing ∆Hfus with increasing intermolecular forces heat of vaporization (∆Hvap) = energy required to convert one mole of liquid substance at its boiling point into the gaseous state - increasing ∆Hvap with increasing intermolecular forces
Isolated system vs. closed system vs. open system?
isolated: a system completely shut off from its surroundings so that neither matter nor energy can be transferred closed: system that allows energy to flow with surroundings but not the transfer of matter open: system that allows energy and matter transfer with surroundings
What is the formula for q in terms of the calorimeter constant?
q = Ccal∆T q rxn = - q cal mc∆T = - Ccal∆T
How do you calculate thermal energy using mass, temp change, and specific heat?
q = mc∆T
