enphalpy Flashcards
what is thermochemistry
thermochemistry is the study of heat changes during a chemical reaction
what happens when bonds are formed
energy is released when bonds are formed in the reactants.
what is the definition for an exothermic reaction
an exothermic reaction releases heat energy to the surroundings . the energy required to break the bonds in the reactants is less than the energy released when the bonds in the products are formed (temp will increase )(energy change is negative) eg combustion
what is the definition for an endothermic reaction
an endothermic reaction takes in heat energy from the surroundings . the energy required to break the bonds in the reactants is greater than the energy released when the bonds in the products are formed.(temp will decrease) (energy change is positive)
what are some examples of exo reactions
combustion , respiration , freezing , rusting , fission , neutralisation
what are some examples of endo reactions
evaporation , photosynthesis , ionisation , thermal composition
what is enthalpy change
enthalpy change is the heat energy change measured under conditions of constant pressure. it is usually measured in kilojoules per mol KJmol-1
what are the standard conditions of enphalpy changes
a pressure of 100kpa
a temperature of 298k
for chemicals in solution concentration of 1moldm-3
all chemicals must be in their standard states
how do you calculate the heat change
q = mc(delta t)
where q= the heat change measured in joules
m is the mass of the substance being heated (in grams)
c= the heat capactity of the substance being heated (JK-1g-1)
delta h= the temp change of the substance being heated ( temp change in kelvin )
how do you calculate the enphalpy change of a reaction
delta h = q/ n x 1000
where delta h is the enphalpy change in kilojoules per mol KJmol-1
n is the number of moles reacted of the limiting raegent in the reaction
what is the definition for the enphalpy change of combustion
the enphalpy change when one mol of substance is completely burnt in oxygen under standard conditions , all products and reactants being in their standard state
what is the definition for the enphalpy of formation
is the enphalpy change when one mol of compound is formed from its constituent elements under standard conditions , all products and reactants being in their standard state
what is the definition for the standard molar enphalpy of reaction
is the enphalpy change associated with a stated reaction under standard conditions , all products and reactants being in their standard state
what is the definition for the enphalpy of neutralisation
is the enphalpy change when 1 mol of water is formed from a neutralisation reaction under standard conditions , all products and reactants being in their standard state
what is the formula for chloroethane
c2h5cl
what is the formula for methyl propane
CH3CHCH3CH3
what is an enphalpy change of formation that is also an enphalpy change of combustion
H2 + 1/2 O2—>H20
what is the formula for propan - 2 - ol
CH3CHOHCH3
what is the formula for ethanol
C2H5OH
Why do we use polystyrene cups for enthalpy changes
Cheap , waterproof , lightweight ,and offer some insulation against heat loss to the surroundings
What state are average bond enpalpies in
Gaseous
What is the definition for average bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule
Is bond breaking endo or exothermic
Endothermic as energy is always put in to break a bond , energy change is positive
Is bond forming endo or exothermic
Exothermic so energy change is negative
Why is average bond enthalpy not a standard enthalpy change
Because they are determined for molecules in a gaseous state and under standard conditions not all molecules are gaseous
How do you calculate enthalpy change from average bond enthalpies
Bonds broken - bonds formed ( sum of all energy stored in bonds in reactants - sum of all energy stored in products )
What are the limitations of average bond enthalpies
As you are using average bond enthalpies , the actual energy involved in breaking and making individual bonds would be slightly different as the bonds may be in different environments
What is the enthalpy change
The overall effect of these two changes ( bonds formed and broken ) if you need more energy to break bonds then it’s endothermic and if more energy is released it’s exothermic
What is bond dissociation enthalpy
The energy needed to break the attraction between atoms and ions
What is the enthalpy change of a reaction dependent on
The enthalpy change for a reaction is independent of the route taken , providing start and finish conditions are the same
How do you do a Hess cycle for formation using formation data
Point arrrows up , bottom of Hess cycle is elements in standard states and enthalpy change = products - reactant
What do you do with the arrows during combustion
Point them down , enthalpy change = reactants - products
Is turning a liquid into a gas endo or exothermic
Endothermic
What bonds are broken when a liquid turns into a gas
Intermolecular bonds ( London forces) between molecules are broken
