54 terms

IB Chemistry SL Topic 5: Energetics and Thermochemistry

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energy
the measure of the ability to do work
work
to move an object against an opposing force
forms of energy
heat, light, sound, electricity, chemical
heat
mode of energy transfer; result of a temperature difference; produces an increase in disorder in how the particles behave
contrast between heat and work
heat increases disorder, work increases order
system vs surroundings
system is the area of interest, surroundings is everything else in the universe
closed system
can exchange energy but not matter with the surroundings
enthalpy
heat content of a system
a system is a reservoir of _____
heat
what happens to a system when heat is added to it from its surroundings?
enthalpy increases
ΔH
enthalpy
ΔH is positive when
heat is added to system from surroundings
ΔH is negative when
heat is released from system to surroundings
ΔH is stored in
chemical bonds and intermolecular forces (potential energy)
endothermic reaction
results in an energy transfer from the surroundings to the system
ΔH of an endothermic reaction
positive
endothermic reaction ΔH of products vs reactants
ΔH products > ΔH reactants
exothermic reaction
result in an energy transfer from system to surroundings
ΔH of an exothermic reaction
negative
exothermic reaction ΔH of products vs reactants
ΔH products < ΔH reactants
standard enthalpy of formation
enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under standard conditions
standard enthalpy change pressure
100kPa
standard enthalpy change concentration
1moldm^-3
standard enthalpy change temperature
298K
temperature
measure of average kinetic energy
absolute zero
lowest possible temperature attainable as this is the temperature at which all movement has stopped
Kelvin scale
Celsius +273; emphasizes this relationship between average kinetic energy and temperature
3 factors in temperature change
1. mass of object
2. heat added
3. nature of the substance
specific heat capacity
property of a substance which gives the heat needed to increase the temperature of unit mass by 1 K
specific heat capacity of common materials
calculating specific heat capacity
q=mcΔt
direction of change
in the direction of lower stored energy
type of reaction which causes increase in disorder of system
endothermic
standard enthalpy change of combustion
enthalpy change for complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions
calculating enthalpy changes
by carrying out the reaction in an insulated system; heat released or absorbed by the reaction can be measured from the temperature change of the water
enthalpy cycle
enthalpy change for a particular reaction is calculated from the known change of other reactions
Hess's Law
the enthalpy change for any chemical reaction is independent of the route provided the starting conditions and final conditions, and reactants and products, are the same
Hess's Law allows you to
calculate the enthalpy changes of reactions that we cannot measure directly in the lab
ΔHreaction =
total ΔH of products - total ΔH of reactants
total ΔH of reactants + ΔHreaction =
total ΔH of products
effect of reversing a reaction on ΔH
negates ΔH (reverses sign)
bond enthalpy
energy needed to break one mole of bonds in gaseous molecules under standard conditions (100 kPa and 298K)
bond enthalpies of multiples bonds vs. single bonds
multiple bonds generally have higher bond enthalpies and shorter bond lengths than single bonds
bond formation is an (endo/exo)thermic process
exothermic
state of bonds when a reaction is exothermic
bonds broken are weaker than the bonds formed
bond breaking is an (endo/exo)thermic process
endothermic
state of bonds when a reaction is endothermic
bonds broken are stronger than the bonds which are formed
role of the ozone
normal oxygen and ozone form a protective screen which ensures that radiation that reaches the surface of the Earth is different from that emitted by the sun
breaking bonds of ozone
by UV of different wavelengths
bond strength in oxygen vs ozone
double bond of oxygen stronger than 1.5 bond of ozone
formation of ozone
oxygen is broken by high-energy UV radiation to form reactive free radicals which reactant with another oxygen to form ozone; bonds are formed and the energy given out raises the temperature in stratosphere (exothermic)
significance of ozone depletion
dangerous UV light has been absorbed and the stratosphere has become warmer in order to make life on Earth possible
Separate solutions of HCl (aq) and H2SO4(aq) of the same concentration and same volume were completely neutralized by NaOH (aq). X kJ and Y kJ of heat were evolved respectively. Which statement is correct?

A. X=Y
B. Y=2X
C. X=2Y
D. Y=3X
B. Y=2X
compare the strength of bonds in reactants vs products of an endothermic reactions
bonds in the reactants are stronger than the bonds in the products
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