Unit 2.1 Thermochemistry Flashcards
(27 cards)
Exothermic?
more energy released when bonds are made than taken in when bonds are broken
Endothermic?
more energy taken in when bonds are broken in comparison to energy released when bonds are made
what is an example of exothermic reactions?
burning of a fuel to give us heat energy. we also know that fuel contains chemical energy
what is a chemical system?
When chemicals react with one another, bonds have to be broken in the reactants and new ones
made in the products reactants + products
what is enthalpy?
energy contained within a substance including chemical, heat, etc
what are the units?
KJ mol -1
what is the formula for enthalpy change?
ΔH = Hproducts - Hreactants
what does this apply to exothermic reactions?
heat is
given out to the surroundings, so the enthalpy of the reactants is greater than the enthalpy of the
products, so ΔH is negative.
what does this apply to endothermic reactions?
heat is absorbed from the surroundings, so the enthalpy of the
products is greater than the enthalpy of the reactants, so ΔH is positive. This can be represented
by energy profile diagrams
what are the standard conditions for enthalpy change?
all substances in their standard states
- a temperature of 298K (25°C)
- a pressure of 1 atmosphere (101,000 Pa)
The symbol for a standard enthalpy change is ΔHθ
what is the standard enthalpy change of formation?
enthalpy change when one mole of a substance is formed from its elements with all standard states under standard conditions.
what is the formula for the formation of sulfur dioxide?
S(s) + O2(g) → SO2(g) ΔfH = -296.4 kJ mol
what is the standard enthalpy change of combustion?
the enthalpy change when 1 mole of substance is burned completely in Oxygen to produce combustion to produce combustion products with all the substances at standard conditions
what is the enthalpy change of a reaction?
ΔrHθ
what is θ?
symbol representing standard conditions
298k
1 atm or 101kPa
Hess’s law?
The enthalpy change of a reaction is independent of the route of the reaction
Bond disassociation enthalpy
the enthalpy change when 1 mole of gaseous covalent bonds are broken
what is so special about bond disassociation?
is always given as a positive
what about bond formation?
it will be the exact opposite - negative
what is bond enthalpy?
the amount of energy needed to break a covalent bond during a chemical
reaction.
what is the average bond enthalpy?
he average value of the enthalpy required to break a given type
of covalent bond in the molecules of a gaseous species. ie, calculated using a wide range of
compounds. These are used in calculations and won’t be as accurate as using actual bond
enthalpies, but they give a good indication, so are extensively used.
what is breaking bonds?
requires energy
endthermic
always positive
what is breaking bonds?
requires energy
endthermic
always positive
what is making bonds?
releases energy
exothermic
always negative
