Enthalpy Flashcards
what is enthalpy?
the heat content that is stored in a chemical system (units kJmol-1)
what is the law of conservation of energy?
no heat energy lost in chemical reactions, it transfers from one place to another
draw an exothermic energy profile diagram
temp increases and negative energy change- heat is transferred from system to surroundings.
enthalpy of products lower than enthalpy of reactants
draw an endothermic energy profile diagram
temp decreases and positive energy change- heat is transferred from system to surroundings. enthalpy of products is higher than enthalpy of reactants
what is activation energy?
the minimum energy required to start a reaction (Ea)
what are the standard conditions for standard enthalpy changes?
100 kPa pressure
298 K (25 degrees)
1 moldm-3
what is the standard enthalpy change of combustion?
the enthalpy change when one mole of substance reacts completely with oxygen under standard conditions- always exothermic
what is the standard enthalpy change of formation?
the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states- always exothermic
what is the standard enthalpy change of formation for all elements eg H2?
0 kJmol-1
what is the standard enthalpy change of neutralisation?
the enthalpy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mole of water under standard conditions- always 57.5 kJmol-1
what is the standard enthalpy change of reaction?
the standard enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions- depends on moles in the equation
what are the 2 methods of experimentally determining enthalpy change?
1) spirit burner for combustion
2) solutions in polystyrene cup for neut and reaction
describe the method for using a spirit burner to determine enthalpy change?
you need to know:
1) water temp change
2) mass of water
3) mass of fuel burnt
describe the method for using a polystyrene cup to determine enthalpy change?
for neut- mixing 2 solutions, acid + base
for reaction- mixing 2 solutions or a solid placed into a solution
we need to know:
1) temp change of solution
2) mass of solution
3) mass of stuff, depends lol
what are the 2 equations to calculate enthalpy change from experimental data?
1) q= mc(delta)T
energy transferred= mass of surroundings x 4.18 x temp change
2) (delta) H= q / n
enthalpy change = energy transferred / moles
ALL ENTHALPY CHANGE VALUES NEED A + OR -
what are the errors that could happen in calorimetry?
1) heat loss to surroundings
2) not done under standard conditions
what are the errors that could happen in a combustion experiment specifically?
1) liquid fuel may evaporate
2) heat capacity of beaker not considered
3) incomplete combustion
how do cooling curves work?
issue: we often get an incorrect max temp as solution starts to cool immediately.
1) initial temp readings are taken for a temp start
2) reactants mixed in a cup
3) temperature recorded at regular intervals
is energy required or released to break bonds?
required
is energy required or released to make bonds?
released
what does mexo bendo mean?
making bonds= exothermic
breaking bonds= endothermic
why are reactions exothermic (in terms of making and breaking bonds)?
more energy released when making bonds than required to break bonds
what are reactions endothermic (in terms of making and breaking bonds)?
more energy is required when breaking bonds than released when making bonds
what is bond enthalpy?
when one mole of bonds are broken in a gaseous molecule at 298K.
what is the equation for enthalpy change (in terms of bond enthalpy)?
(delta)H= sum of bond enthalpies of bonds broken- sum of bond enthalpies of bonds made
what are errors associated with bond enthalpy calculations?
1) using average bond enthalpies- not specific to the reaction
2) always in gaseous states- not standard for many substances
what are the 3 reasons that reactions can’t experimentally be set up?
1) high activation energy
2) slow reaction rate
3) more than one reaction taking place
what is hess’ law?
if a reaction can take place by more than one route and the initial + final conditions are the same, the total enthalpy change is the same
