Energetics Flashcards
Enthalpy Change
the change in heat energy at constant pressure
enthalpy change units
kJ mol-1
What sign is enthalpy change for endothermic processes?
positive
What sign is enthalpy change for exothermic processes?
negative
What are the standard conditions for enthalpy change measurements?
100kPa pressure
298k temperature
1.0 moldm-3 concentration for all solutions
Is breaking bonds endothermic or exothermic?
endothermic
is making bonds endothermic or exothermic?
exothermic
What is 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 endothermic or exothermic?
endothermic
are mean bond enthalpies positive or negative?
positive
the more positive the bond enthalpy the ___________ the amount of energy needed to break the bond and so the ___________ the bond
the more positive the bond enthalpy the LARGER the amount of energy needed to break the bond and so the STRONGER the bond
define standard enthalpy of reaction
any chemical reaction under standard conditions
define standard enthalpy of formation
enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products in their standard states
why is the enthalpy of Na(s) zero?
Na is an element in its standard state
Why is the enthalpy of formation of liquid Na not zero?
Na(l) is not the standard state of Na
calculation for enthalpy of formation
FPR
formation = products - reactants
what is the standard enthalpy of formation of all elements in their standard state?
zero
define standard enthalpy of combustion
enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions, all reactants and products are in their standard states
which substances cannot be combusted?
water, carbon dioxide, and most other oxides
what is the enthalpy of combustion of water and carbon dioxide?
zero
why may the enthalpy of combustion of a compound be difficult to measure?
incomplete combustion may occur
calculation for enthalpy of combustion
CRP
combustion = reactants - products
define Hess’ Law
states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
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 is the equation for calculating energy released or taken in by reactions during calorimetry?
Q = mc∆T
Q = Energy (J)
m = mass of water (surroundings) (g)
c = Specific Heat Capacity (usually 4.18 J K−1 g−1)
∆T = Change in temp (in Kelvin or ⁰C)
How do you calculate enthalpy change from Q (energy released/taken in)?
∆H = Q / n
∆H = Enthalpy change (kJ mol−1)
Q = Energy taken in or released by the reaction (kJ)
n = number of moles reacted or formed
how do you decide the sign of ∆H in calorimetry calculations?
if temp increases = exothermic = -∆H
if temp decreases - endothermic = +∆H
what is the major assumption made in calorimetry experiments?
energy transferred to the water from the reacting chemicals (or vice versa for endothermic reactions) is equal to the energy released (taken in) by the reaction
sources of error for combustion calorimetry
heat loss to the surroundings
incomplete combustion of the fuel
heat energy transferred to metal calorimeter
some fuel evaporates
sources of error for solution calorimetry
heat loss to the surroundings
improvements to minimize sources of error for combustion calorimetry
add a lid - reduces heat loss
insulate sides of calorimeter - reduces heat loss
reduce distance between flame and beaker - reduces heat loss
put sleeve around flame to protect it from draughts
improvements to minimize sources of error for solution calorimetry
add a lid - reduces heat loss
insulate calorimeter - reduces heat loss
Steps to measure an enthalpy change using a cooling curve
record the temp for a suitable time (3 mins) before adding reactants together
to establish an accurate initial temperature
mix reactants the record temperature every minute until a trend is seen
plot a graph of temperature against time
