Thermochemistry Flashcards
Thermochemistry
heat changes of chemical reactions
helps to determine if a particular reaction will occur and if it will release or absorb energy as it occurs
two principles of thermodynamics
Laplace law
Hess’ Law
The Laplace Law
the heat change (or enthalpy change) of a chemical reaction is exactly equal but opposite in sign for the reverse reaction
∆Hforward reaction = ∆Hbackword reaction
(a) CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) [here, ∆H0 = -890.3 kJ mol-1]
(b) CO2 (g) + 2H2O (l) → CH4 (g) + 2O2 (g) [here, ∆H0 = +890.3 kJ mol-1]
Enthalpy Notations
enthalpy change 𝚫H(delta H) is the heat energy transferred in a reaction at constant pressure. The units are Kj mol⁻1
Standard conditions (𝚫Hᶿ)
100 kPa pressure and a stated temperature
Energy
stored in the bonds of chemical compounds.
Chemical energy may be released during a chemical reaction, often in the form of heat; such reactions are called exothermic.
Work
transfer of mechanical energy between two systems
Heat
transfer of thermal energy between systems
Exothermic reactions
have a negative enthalpy change value, because heat is given out (the chemicals lose energy)
Endothermic reactions
have a positive enthalpy change value, because heat energy is absorbed (the chemicals lose energy)
Lattice formation enthalpy
enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions
Lattice dissociation enthalpy
enthalpy change when 1 mole of a compound is formed from its elements in their standard states
Bond dissociation enthalpy
enthalpy change when all the bonds of the same type in 1 mole of gaseous atoms are broken
First ionisation energy
enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms
First electron affinity
enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous ions
three types of thermodynamics systems
open
closed
isolated
isolated system
thermodynamic system that cannot exchange either energy or matter outside the boundaries of the system
1. The system may be so distant from another system that it cannot interact with them.
2. The system may be enclosed such that neither energy nor mass may enter or exit.
Closed systems
only closed to matter, energy can be exchanged across the system’s boundaries
Open system
exchange both energy and matter with its surroundings
Hess’ Law
The enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction occurs.
1st law of thermodynamics
a systems energy content remains constant as well as not being able to be created or destroyed
Activation energy
minimum energy required
Increase temp effect on Maxwell-Boltzmann diagram
move right
Reaction Rate for a given chemical reaction
measure of the change in concentration of the reactants or the change in concentration of the products per unit time
speed of a chemical reaction
change in concentration of a substance divided by the time interval during which this change is observed
Rate formula
change in concentration (of products or reactants)/ time
Reaction rate and concentration
increases
more of the reacting molecules or ions are present to form the reaction products
limit is often reached where increasing the concentration has little effect on the rate of reaction.
When several reactants are involved, increasing the concentration of one of them may not affect the rate of reaction if not enough of the other reactants is available
Reaction rate and Temperature
increases
Particles can only react when they collide
heat a substance, the particles move faster and so collide more frequently
disproportionately large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reaction.
Reaction rate and Surface Area + on graph
cutting the substance into small pieces, or by grinding it into a powder
more particles are exposed to the other reactant
there are more collisions
the rate of reaction increases
has a steeper gradient at the start
becomes horizontal sooner
half-life of a reaction
amount of time needed for a reactant concentration to decrease by half compared to its initial concentration
used in chemistry and medicine to predict the concentration of a substance over time
concepts of half life plays a key role in
administration of drugs into the target, especially in the elimination phase, where half life is used to determine how quickly a drug decrease in the target after it has been absorbed in the unit of time (sec, minute, day,etc.) or elimination rate constant ke (minute-1, hour-1, day-1,etc.).
Zero order kinetics
rate of a reaction does not depend on the substrate concentration
slope of this plot is a straight line with negative slope
the half-life of zero order reaction decreases as the concentration decreases
First Order Kinetics
slope continually decreases as time progresses until it reaches zero
the length of half-life will be constant, independent of concentration
takes the same amount of time for the concentration to decrease from one point to another point
Second Order Reactions
decrease at a much faster rate
length of half-life increases while the concentration of substrate constantly decreases, unlike zero and first order reaction
rate equation
rate = k [A]a [B]b
shows the effect of changing the concentrations of the reactants on the rate of the reaction
rate constant
only changes if conc changes
The Arrhenius equation
k = Ae ^Ea/RT
gas constant
constant which comes from an equation, pV=nRT, which relates the pressure, volume and temperature of a particular number of moles of gas
Activation energy, EA units
joules per mole
