energetics Flashcards
reaction system
products and reactants part of reaction
surroundings
other things like water beaker
air bunsen burner
anything surrounding reaction
factors that effect pressure exerted by surroundings
molecules in air
solution in container
lid on beaker or not
constant pressure
pressure of reaction system and surroundings and pressure on system dont change
what causes a reaction to give off heat
as reactants turn into products energy flows between 2.
heat energy
into is taking in energy
out of is give heat
exothermic
gives out energy from reaction
endothermic
taking in energy from reaction
size of arrow in energy level diagrams
shows how much heat energy flows between system and surroundings
what do energy level diagrams show
if reaction is endothermic or exothermic and size of heat energy flow
what to include in energy level diagrams
x axis energy
y axis progress of reaction
reactants and products curve
reactions under constant pressure
heat energy into or out of system and changes temp of surroundings
reactions not under constant pressure
changes surrounding temps and how quickly products form and how much product forms
enthalpy change
change in amount of heat energy either given it or taken out from the surroundings during a reaction at constant pressure
value of enthalpy change for exothermic
negative
value of enthalpy change for endothermic
positive
symbol of enthalpy
H
change in symbol
delta
units of enthalpy change
J joules
what are standard conditions
atmospheric pressure 100000pa
298K
standard enthalpy changes
delta H with circle on top right with line through circle
formation reaction
when one mole of a substance is produced from its constituent elements under standard conditions with both reactants and products in their standard states
enthalpy change of formation sign
delta small f and H
combustion
substance plus oxygen is C02 plus water
standard enthalpy of combustion
enthalpy change when 1 mole of a substance is completely burned in oxygen in standard conditions with all products and reactants in their standard states
calorimetry
isolate objects surroundings and measure temp change
how does mass effect temp
more mass is less decrease or increase of temp
specific heat capacity
amount of energy it takes to increase 1g of a substance by one kelvin
SI unit for specific capacity
J K-1 g-1
q=
m c delta T
calorimetry when measuring combustion reactions
flame calorimetry
how to decrease heat energy loss to air
increase volume of water
bomb calorimeter
calorimetry of reactions in solution
constant pressure calorimetry
first type of constant pressure
zinc mixed with acid
thermochemical cycle
shows the different possible routes that a reaction can tale from starting reactants to final products
enthalpy change of alternative route
enthalpy 1 plus enthalpy two
hess law direct quote
the enthalpy change in a reaction at constant pressure is independent of the route taken
another name for alternative route reactions
data book reactions
data book reactions design
alternative route and find total enthalpy change using data book reactions
term enthalpy change as applied to a chemical reaction
heat energy change at constant pressure
improvement to mixing two acids together for enthalpy change
use pipette instead of measuring cylinder
insulate beaker/ use cup
reweigh watch glass after adding solid
measure initial and final temp
time intervals
plot temp against time
extrapolate point of adding
find temp change for calculating enthalpy
calculating final temp of enthalpy change question
when same find moles of chemical
take enthalpy change and times by moles
answer times 1000
100 times 4.18
divide both answers
add answer to initial temp change to get final temp
how to improve percentage uncertainty
increase concentration
equation for temp change
q/mc
finding temp rise of reaction
find moles
times by enthalpy change
temp change is heat energy divided by total mass times specific heat capacity
disadvantage of using glass beaker on tripod and gauze
glass poorer conductor than copper
tripod and gauze reduce heat transfer
why enthalpy of combustions lower than text books
incomplete combustion
heat loss to surroundings
exam questions on enthalpy of formation to enthalpy change
write out direct route
calculate enthalpy of formation of each elements
add formation values to diagram
add up sum of values
if reaction balanced in combustion
add oxygen balanced for combustion reaction
how to get one mole of bonds
times the bond enthalpy or long given number by avagadros constant
bond dissociation enthalpy
energy required to break one mole of a bond in its own atoms in gaseous state
how to find bond disscociation enthalpy
-take 1 mole of atom
record how many energy it takes for bond to break
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mean bond enthalpies
measure the enthalpy change when 1 mole of bonds are broken taken as an mean from a range of different compounds
how to calculate atomisation enthalpy
-draw out molecules
- calculate how many bonds
-calculate enthalpy from individual energy changes
calculating heat capacity exam question
moles of substance
moles times amount of energy released
answer divided by temp change
2 step reactions arrows
facing in a thermochemical cycle way
formation cycle arrows
both facing up
arrows in combustion cycle
arrows facing down
hydrated salts arrows
both facing down
how to answer hess law questions
write direct route equation given
-identify which type of reaction and draw thermochemical cycle according to type of reaction
-use values given to find enthalpy change for alternative routes
-label values onto thermochemical cycle
-add values together
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