4 energetics Flashcards
what is an exothermic process?
where energy is released from the chemical to the surroundings
temp of surrounding increases
what is an endothermic process?
energy is taken in by the chemical from the surroundings during the process
temp of surroundings decreases
define enthalpy change
change in heat energy at constant pressure
is the enthalpy change for exothermic processes positive or negative?
negative
is the enthalpy change for endothermic processes positive or negative?
positive
what are standard conditions?
pressure: 100 kPa
temp: 298K
conc: 1.0 mol dm^-3
is breaking bonds an exo or endothermic process?
endothermic
is making bonds an exo or endothermic process?
exothermic
define activation energy
the minimum energy needed to start a reaction
define mean bond enthalpy
the energy required to break one mole of a covalent bond into gaseous atoms averaged over a range of different compounds
are mean bond enthalpies exo or endothermic?
endothermic
define standard enthalpy of formation
the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions with all reactants and products in standard states
why is the enthalpy of formation of Na zero?
Na is an element
why is the enthalpy of formation of liquid Na not zero?
Na is not liquid in its standard state
define standard enthalpy of combustion
the enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions all reactants and products in standard states
why may the enthalpy of combustion of a compound be difficult to measure?
incomplete combustion may occur
what is an enthalpy of reaction?
any reaction that doesn’t match one of the other definitions
what does hess’s law state?
the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products
which way do arrows always point in hess cycles for enthalpies of formation?
up
what is at the top of the hess cycle in all three bond enthalpies?
reactants to products
what is at the bottom of the hess cycle for all three bond enthalpies?
bond enthalpy: gaseous atoms of elements
combustion: combustion products
formation: elements in standard states
what is the equation for calorimetry?
Q = mc x change in temp
what do the letters in the equation for calorimetry stand for?
Q= energy (J)
m= mass of water (g)
c= specific heat capacity (4.18)
T= change in temp (K or °C)
how is enthalpy change calculated?
Q/n
example exam question:
Give one reason why the bond enthalpy that you calculated above is different from the mean bond enthalpy quoted in a data book
The data book value is averaged over a range of different compounds
what following equipment would be needed to measure an enthalpy change of combustion?
thermometer
calorimeter
water
combusting fuel
how do you calculate mean bond enthalpies?
energy to break bonds - energy to form bonds
what is an assumption made in calorimetry?
the energy transferred to the water from the chemicals is equal to the energy released by the reaction
what are some sources of error in calorimetry?
- heat loss to surroundings
- incomplete combustion
- heat energy transferred to metal calorimeter
- some fuel evaporates
how can you reduce heat loss in calorimetry?
- add a lid
- insulate sides of calorimeter
- reduce distance between flames and beaker
how do you measure enthalpy using a cooling curve?
- record temperature for 3 minutes before adding reactants to establish an accurate initial temperature
- mix reactants and record temperature every minute until a trend
- plot a graph
- extrapolate the cooling curve back to the point of addition
- to establish a theoretical temperature change accounting for heat loss
what do you use a styrofoam cup for?
measuring enthalpy change when:
- two solutions added together
- a solid is added to a solution
what is a source of error for the source calorimetry?
heat loss to surroundings
