8.1 & 8.2 Flashcards
what is enthalpy
measure of the total energy of a system
what is the enthalpy change
the heat energy that is transferred between the system and the surroundings at constant pressure
define exothermic
where heat energy is transferred from the system to the surroundings
define endothermic
where heat energy is transferred from the surroundings to the system
give 3 examples of exothermic reactions
freezing water
combustion
dissolving NaOH in water
give 3 examples of endothermic reactions
melting ice
photosynthesis
dissolving ammonium nitrate in water
do exothermic reactions have a positive or negative enthalpy change
negative
do endothermic reactions have a positive or negative enthalpy change
positive
define standard enthalpy change
the enthalpy change measured at 100kPa and a stated temperature (298K) when the number of moles of substances in the equation as written react
define standard enthalpy change of combustion
the enthalpy change measured at 100kPa and a stated temperature (usually 298k) when one mole of a substance is completely burned in oxygen
summarise the experiment to determine the enthalpy change of combustion for a liquid
weigh a spirit burner with the liquid
add a known volume of water to a copper can
measure the initial temperature of the water
light the spirit burner
stir the water
once the temperature has risen by roughly 20 extinguish the burner and immediately weigh it
measure the final temperature
what is the equation to calculate heat energy (Q)
Q = mcT
m - mass of water
c - specific heat capacity (usually 4.18)
T - change in temp
what is the equation for the enthalpy change of combustion (#cH)
cH = Q/n
n - moles of ethanol burned
state some sources of error within the experiment for calculating total enthalpy of combustion
- some heat energy produced dissipated to the surroundings not the water
- incomplete combustion may have occurred
- some heat energy produced may have been transferred to the copper not the water
- conditions are not standard
define standard enthalpy change of neutralisation
the enthalpy change measured at 100kPa and a stated temperature (usually 298k) when one mole of water is produced by the neutralisation of an acid with an alkali
why do almost all strong acids and alkalis have the same standard enthalpy of neutralisation
because they fully ionise in aqueous solution so this reaction occurs,
H+(aq) + OH-(aq) -> H2O(l)
which is the same reaction for all strong acids and alkalis
summarize the experiment to determine the enthalpy change of neutralization
use a pipette fitted with a safety filter to add 25cm^3 of 1mol/dm^3 of acid into an expanded polystyrene cup
measure temp of the acid
carry out the same technique to place 25cm^3 of <1mol/dm^3 of alkali in a beaker
measure temp of the alkali
add the alkali and the acid
stir and measure the maximum temperature reached
state some sources of error within the experiment for calculating total enthalpy of neutralization
answers should be given to the same number of significant figures as T
uncertainty of measurements of pipette and thermometer
heat energy transferred to thermometer & polystyrene cup
define standard enthalpy of formation
the enthalpy change measured at 100kPa and a specified temperature (298k) when one mole of a substance is formed from its elements in their standard states
what is Hess’s law
states that the enthalpy change of a reaction is independent of the path taken in converting reactants into products, provided the initial and final conditions are the same
define bond enthalpy
energy required to break one mole of the stated bond in gaseous state
what is mean bond enthalpy
the enthalpy change when one mole of a bond averaged out over many different molecules is broken
how do you calculate the enthalpy change of a reaction
total bond enthalpy for bonds broken - total bond enthalpy for bonds made
