Chapter 5 - Thermochemistry Flashcards
define thermochemistry
the study of the energy changes that accompany physical or chemical changes in matter
define energy
the ability to do work
SI units for energy
Joules (J)
define work
the amount of energy transferred by a force over a distance
SI units for work
Joules (J)
how can energy be classified?
as potential energy or kinetic energy
define potential energy
the energy of a body of system due to its position or composition
define kinetic energy
the energy of an object due to its motion
what is the energy associated with chemical bonds?
potential energy
what are chemical bonds?
attractive forces between the nucleus of one atom or ion and the negative charges of the electrons in the other atom or ion
what is the amount of energy released/absorbed in a chemical reaction equal to?
the difference between the potential energy of the bonds in the reactants and the potential energy of the bonds in the products
define thermal energy
the total quantity of kinetic and potential energy in a substance
what does the quantity of thermal energy in a substance depend on?
how fast its entities are moving
what are entities?
atoms, molecules, ions, polyatomic ions
when a substance absorbs thermal energy, what happens?
its entities move at a greater speed, and the substance warms up
what happens when a substance releases thermal energy to its surroundings?
its entities move more slowly, and the substance cools
define heat
a verb that refers to the transfer of thermal energy from a warm object to a cooler object
define temperature
a measure of the average kinetic energy of entities in a substance
which has a higher temperature and why: iceberg vs hot cup of water
the hot cup of water has a higher temperature because the average water molecule in a cup of hot water has a higher temperature than the average water molecule in an iceberg
which has more thermal energy and why: iceberg vs hot cup of water
the iceberg has more thermal energy because the iceberg contains more water molecules
define law of conservation of energy
energy cannot be created or destroyed
what can energy only do?
be converted from one form to another
how do chemists divide the universe?
chemical system, its surroundings
define chemical system
a group of reactants and products being studied
define surroundings
all the matter that is not part of the system
what two ways can chemical systems be classified?
open, closed
define open system
a system in which both matter and energy are free to enter and leave the system
define closed system
a system in which energy can enter and leave the system, but matter cannot
what happens with the chemical bonds in a chemical reaction?
chemical bonds in the reactants are broken, new bonds are formed to produce the products
potential energy of entities in a stable structure vs potential energy of individual isolated atoms
potential energy of entities in a stable structure is lower
what does bond breaking and formation require from energy
bond breaking requires energy while bond formation releases energy
what do entities in a stable structure include?
atoms, ions, or molecules in an ionic compound
define exothermic
releasing energy to the surroundings
when is more energy released in an exothermic reaction?
more energy is released from the formation of new bonds of products than is required to break the bonds in the reactants
potential energy and bonds of products vs reactants in exothermic reaction?
products have lower potential energy and stronger bonds than reactants
what happens when an exothermic reaction releases energy?
the temperature of the surroundings increase
define endothermic
absorbing energy from the surroundings
what occurs in an endothermic reaction with the chemical system?
absorbs energy from its surroundings and increases its potential energy
potential energy and bonds of products vs reactants in endothermic reaction
products have higher potential energy and weaker bonds than the reactants
what type of reaction can a chemical reaction take?
endothermic or exothermic
what type of reaction can a nuclear reaction take?
exothermic
name two nuclear reactions involving large quantities of energy
fusion and fission
define fusion
the process of combining two or more nuclei of low atomic mass to form a heavier, more stable nucleus
define fission
the process of using a neutron to split a nucleus of high atomic mass into two nuclei with smaller masses
what types of changes can a potential energy change result from?
any physical, chemical, or nuclear change
magnitudes of potential energy changes from lowest to highest
phase change, chemical change, nuclear change
what is the symbol for specific heat capacity?
c
define specific heat capacity
the quantity of thermal energy required to raise the temperature of 1 g of a substance by 1 ℃
SI units for specific heat capacity
J/(g・℃)
what is the specific heat capacity of liquid water?
4.18 J/(g・℃)
specific heat capacity of water vs sand and what this causes
specific heat capacity of water is higher than sand so it takes less to raise the temperature of sand and it will become hotter than the water
define calorimetry
the experimental process of measuring the thermal energy change in a chemical or physical change
define calorimeter
a device that is used to measure thermal energy changes in a chemical or physical change
what does a calorimeter consist of?
a well insulated reaction chamber, a tight fitting cover with insulated holes for a thermometer, and some mechanism to stir the contents of the calorimeter
what does using an insulated chamber minimize?
energy loss to the surroundings
what is a coffee cup calorimeter composed of?
polystyrene cover and two cups, inner cup holds chemical system and a liquid, usually water, outer cup provides additional insulation
how are changes in the thermal energy of the chemical system detected in a coffee cup calorimeter?
through temperature changes in the water
2 assumptions of insignificance made when using a coffee cup calorimeter
any thermal energy transferred from the calorimeter to the outside environment is negligible, any thermal energy absorbed by the calorimeter itself is negligible
assumption of calculations made when using a coffee cup calorimeter
all dilute, aqueous solutions have the same density and specific heat capacity as water
density of water
1.00 g/mL
when are these assumptions valid?
for chemical or physical changes that take place in water or a dilute, aqueous solution
what is q
the total amount of thermal energy released or absorbed by a chemical system
what does the magnitude of q depend on?
the mass of the substance, the specific heat capacity of the substance, the temperature change experienced by the substance as it warms or cools
formula to calculate q
q = mcΔT
what does the symbol m stand for
mass of the substance
formula to calculate ΔT
ΔT = Tfinal - Tinitial
what happens when q has a negative value to thermal energy, reaction change, and water
system transfers thermal energy to the surroundings, reaction change is exothermic, temperature of water increases
what happens when q has a positive value to thermal energy, reaction change, and water
system absorbs thermal energy from its surroundings, reaction change is endothermic, temperature of the water decreases
in a coffee cup calorimeter, what is the surroundings?
the water
what substances contain a certain amount of thermal energy?
any substance undergoing a physical or chemical change
what is the symbol for enthalpy?
H
define enthalpy
the total amount of thermal energy in a substance
how is enthalpy measured?
cannot be measured
what is the symbol for enthalpy change?
ΔH
define enthalpy change
energy released to or absorbed from the surroundings during a chemical or physical change
how is enthalpy change measured?
using calorimetry data
formula for enthalpy change equal to thermal energy in and out of the system
ΔHsystem = |qsystem|
what condition needs to be in place to use the formula ΔHsystem = |qsystem|
pressure remains constant
for a chemical reaction, what is the formula for enthalpy change?
ΔH = Hproducts - Hreactants
when is the enthalpy change positive, and what kind of reaction is this?
products have a greater enthalpy than reactants, endothermic reaction
when is the enthalpy change negative and what kind of reaction is this
products have a lower enthalpy than reactants, exothermic reaction
in an exothermic reaction, explain what happens with the kinetic and potential energy of the chemical system and the surroundings
chemical system goes from high potential energy to low potential energy, surroundings goes from low kinetic energy to high kinetic energy
what is the symbol for molar enthalpy change?
ΔHr
define molar enthalpy change
the enthalpy change associated with a physical, chemical, or nuclear change involving 1 mol of a substance
SI units for molar enthalpy change
J/mol
what does the subscript r represent in the molar enthalpy change
reaction type
how to balance equations for thermochemical reaction
relevant substance has coefficient of 1, other substances may have fractional coefficients
4 types of thermochemical reaction
solution, combustion, vaporization, formation, neutralization
subscript symbol for solution thermochemical reaction
sol
subscript symbol for combustion thermochemical reaction
c
subscript symbol for vaporization thermochemical reaction
vap
subscript symbol for neutralization thermochemical reaction
neut
explain solution reaction
solid reactant to aqueous products with appropriate charge on products
explain combustion reaction
gas and oxygen gas forms carbon dioxide gas and liquid water
explain vaporization reaction
liquid turns to gas
explain formation reaction
solid reactants unless they are gas forms liquid product
explain vaporization reaction
aqueous base and acid forms aqueous product and liquid water
relevant substance of vaporization reaction
per mole of acid or base
which reactions don’t have relevant substance in reactant
formation
what does the enthalpy change depend on
quantity of matter than undergoes the change
how much more energy is needed to convert 1 L of liquid water to water vapour than 500 mL
2 times
formula to calculate enthalpy change for substances other than 1 mol
ΔH=nΔHr
which substances undergo a change in enthalpy
all those in chemical reactions
what is a chemical equation that describes the enthalpy change of a reaction
thermochemical reaction
what are the two ways to write a thermochemical reaction
adding energy term as product/reactant, writing enthalpy change after balanced equation where ΔH = +/- enthalpy change
what is indicated by an energy term added as a product and what type of reaction is this
indicates energy released by the chemical system, exothermic reaction
what is indicated by an energy term added as a reactant and what type of reaction is this
indicate energy absorbed by the chemical system, endothermic reaction
2 ways to represent thermochemical enthalpy changes
energy terms, potential energy diagrams
define potential energy diagram
a graphical representation of the energy transferred during a physical or chemical reaction
in a chemical reaction, what has potential energy?
both products and reactants
potential energy symbol for potential energy diagram
Ep (KJ)
x-axis, y-axis and title of potential energy diagram
x-axis: reaction progress, y-axis: potential energy, potential energy symbol, title: potential energy diagram during a(n) __ reaction
how do you show change from the arrow of potential energy diagrams
use molar enthalpy change symbol = +/- change
in a potential energy diagram, what reaction is it when arrow goes down?
exothermic
in a potential energy diagram, what reaction is it when arrow goes up?
endothermic
what does the bond energy of a single covalent bond breaking mean?
the energy required to break one mol of the covalent bond into 1 mol of one atom and 1 mol of another
what does the bond energy mean when a covalent bond forms?
the amount of energy released
how do we use bond energies
can calculate enthalpy change of an chemical reaction f we know the bond energies of the chemical bonds of the reactants and products
define bond dissociation energy
the energy required to break a given chemical bond
why are bond dissociation energies given as average bond energies?
the energy depends on the types of atoms and bonds in the same molecule
which has larger bond energies: multiple bonds or single bonds
multiple bonds
what is suggested by multiple bonds having larger bond energies?
multiple bonds are stronger
what is the relationship between number of bonds between atoms and bond length
as the number of bonds increase, the bond length shortens
what is meant by number of bonds
number of electrons shared
what is meant by bond length
distance between the nuclei
what must happen first in a chemical reaction
bonds must break
what must happen for bonds to break, what is the reaction type, and what is the sign of the energy term
energy must be added, an endothermic process, energy terms are positive
what must happen for bonds to form, what is the reaction type, and what is the sign of the energy term?
energy is released, an exothermic process, energy terms are negative
what is the middle step of the enthalpy change of a reaction using bond energies
ΔH = sum of energies required to break old bonds (positive) - sum of energies released in the formation of new bonds (negative)
what is the equation for enthalpy change using bond energies
ΔH = Σn x D (bonds broken) - Σn x D (bonds formed)
what does the symbol D mean and what is the sign?
bond energy, always positive
what are the units of D
KJ/mol
why can we use bond energies?
because bond energies of similar bonds are nearly the same in different molecules and they provide a good approximation of the actual bond energy value
what is the change in enthalpy of a chemical process independent of?
the path taken
what does the change in enthalpy being independent of the path taken mean?
going from an initial set of reactants to a final set of products in one step or a series of steps has the same enthalpy change
what happens if you add the thermochemical equations used in a series of steps, including the sum of enthalpy changes?
you get the overall, net reaction and the enthalpy change for the net reaction
define hess’s law
the enthalpy change for the conversion of reactants to products is the same whether the conversion occurs in one step or several steps
how is hess’s law useful?
useful to study energy changes in chemical reactions that cannot be analyzed using calorimetry
what 2 rules must you follow when using Hess’s rule to calculate enthalpy change?
if you reverse the chemical reaction, you must also reverse the sign of ΔH. if the coefficients in a balanced equation are multiplied by a factor, the value of ΔH is multiplied by the same factor
what is proportional to the magnitude of ΔH
the number of moles of reactants and products in a reaction
define formation equations
represent the formation of a compound from its elements
define standard enthalpy of formation
the change in enthalpy that accompanies the formation of 1 mol of a compound from its elements in the standard state
what is the symbol for standard enthalpy of formation
ΔHf°
define standard state
the most stable form of a substance under stable conditions
what is the standard ambient temperature and pressure
25 degrees celsius and 100 kPa
what is the standard state of most elements
solid
which elements are gases in their standard state?
diatomic elements, and noble gases
list the diatomic elements
hydrogen, oxygen, nitrogen, fluorine, chlorine
which elements are liquids in their standard state?
mercury, bromine
what is the standard state for a substance in a solution?
1 mol/L
what does the degree sign for standard enthalpy of formation mean?
the process takes place under standard conditions
what is the standard enthalpy of formation of an element in its standard state?
0
why is the standard enthalpy of formation of an element in its standard state 0?
an element is in its most fundamental form at standard conditions
what must you do to balance the standard enthalpy of formation equation?
1 mol of the product
type of reaction of enthalpies of formation
exothermic or endothermic
how do we calculate standard enthalpies of reaction from standard enthalpies of formation formula?
ΔHr° = ΣnproductsΔH°products - ΣnreactantsΔH°reactants
