energetics 1 p1 Flashcards
what does system mean in a chemical reaction?
the atoms and bonds involved in the chemical reaction
explain the law of conservation
the amount of energy in an isolated system remains the same. energy cannot be destroyed or created, it can only be transferred from one form to another
what energy change is breaking bonds associated with?
energy is taken in to break bonds –> endothermic reaction
what energy change is making bonds associated with?
energy is released to make bonds –> exothermic reaction
what is an endothermic reaction?
a reaction with an overall positive enthalpy change (+ΔH) –> enthalpy of products > enthalpy of reactants
what is an exothermic reaction?
a reaction with an overall negative enthalpy change (-ΔH) –> enthalpy of products < enthalpy of reactants
explain an enthalpy change diagram for an endothermic reaction
reactants lower
products higher
activation energy points upwards
enthalpy on y axis and reaction progress on x axis
explain an enthalpy change diagram for an exothermic reaction
reactants higher
products lower
what is meant by activation energy?
the minimum energy required for a reaction to take place
what are the standard conditions?
100kPa
298 K
what does ‘in standard state’ mean?
the state an element/compound exists at in standard conditions
enthalpy change of formation
the energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions
H2(g) + 1/2O2(g) –> H2O(l)
enthalpy change of combustion
the energy change that takes place when 1 mole of a substance is completely combusted
C(s) + O2(g) –> CO2(g)
enthalpy of neutralisation
the energy change that takes place when 1 mole of water is formed from a neutralisation reaction
enthalpy change of reaction
the energy change associated with a given reaction
enthalpy change formula for experimental data
Q = mcΔT
m is mass of substance (usually water)
c is specific heat capacity
ΔT is change in temperature
why might experimental methods for enthalpy determination not be accurate?
- heat lost to the surroundings
- not in standard conditions
- reaction may not go to completion
average bond enthalpy
mean energy required to break 1 mole of bonds in gaseous molecules
why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate
enthalpy change of reaction using bond enthalpies formula
ΔH = ∑(bond enthalpies of reactants/bonds broken) - ∑(bond enthalpies in products/bond made)
what is Hess’ Law?
states that the enthalpy change for a reaction is the same regardless of the route taken
which way do the arrows go on a Hess cycle for formation and for combustion?
formation - upwards
reactants and products
elements
combustion - downwards
reactants & oxygen and products & oxygen
combustion products
