3.2 Flashcards
define enthalpy
the thermal energy stored in a chemical system. we can measure this by seeing if the temp of a reaction changes
how you know a Reaction has taken place
color change, effervescence, precipitate formed, temp/energy change
exothermic reactions
give out energy to their surroundings, make things feel hot, the energy of the products is less than that of the reactants
endothermic reactions
take in energy form their surroundings, make things feel cold, the energy of the products is greater than that of the reactants
activation energy
the energy needed to start breaking the bonds in the reactants. the minimum energy needed for a reaction to take place
how to calculate enthalpy change
enthalpy to break bonds - enthalpy to make bonds
negative enthalpy change means the reaction is
exothermic
positive enthalpy change mean the reaction is
endothermic
enthalpy change of neutralisation
(delta H neut) enthalpy change when one mole of water is formed from a neutralisation reaction can be exothermic or endothermic
enthalpy change of formation
(delta H f) enthalpy change when one mole of a compound is formed from its elements in their standard state usually exothermic
enthalpy change of combustion
(delta H c) enthalpy when one mole of a substance reacts completely with O2 always exothermic
standard conditions for enthalpy changes of reactions
pressure- 100 kPa (1 atm)
room temp- 289k (25`c)
measuring enthalpy change experimentally
Q=mcT T=temp change c=specific heat capacity of substance being heated m= mass of object being heated Q=heat lost or gained from reaction in J
Hess’s law
The total enthalpy change of a reaction is independent of the route by which the reaction takes place provided the initial and final condition are the same
to increase the rate of reaction
increase pressure, increase surface area, increase temp, add a catalyst, increase concentration, increase surface area
why increase surface area
after rate of reaction as more particles are exposed and able to react
why use a catalyst to increase rate of reaction
lowers the activation energy
why increase the concentration
more particles per unit volume increase the chances go a successful collision
rate of reaction
change in concentration of x over time
two types of catalysts
heterogeneous catalyst, homogenous catalyst
heterogenous catalyst
catalyst is in a different phase to the reactants i.e. solid catalyst vs gaseous reactants
homogeneous catalyst
catalyst is in the same phase as the reactants i.e. all liquid or gaseous
benefits of catalysts in general
increase sustainability, reduce temp needed -> cheaper
Botzmann distribution
shows the distribution of molecules of certain energies, area under the curve= no. pf molecules in the system, if the temp is increased the curve becomes broader and flatter
