physical chem; energetics Flashcards
define the term enthalpy change
it’s the heat energy change measured under conditions of constant pressure
what are the standard conditions for enthalpy change?
100 kPa
298K
what is an endothermic reaction?
a reaction that absorbs energy from the surroundings
what is an exothermic reaction?
a reaction that releases energy from the surroundings
describe the energy level diagram for an endothermic reaction
reactants are higher than the products
reactants are closer in energy to the transition state
lower Ea
state whether the enthalpy change value for an endothermic reaction is +ve/-ve & why
+ve as reactants gain energy from te surroundings
describe the energy level diagram for an exothermic reaction
reactants are lower in energy than the products
reactants are further away from the transition state
higher Ea
state whether the enthalpy change value for an endothermic reaction is +ve/-ve
-ve as reactant lose energy to the surroundings
explain how bonds are broken in a reaction
to break a bond energy needs to be absorbed. bonds are broken in the reactants which is an endo process so the ∆H is +ve
explain how bonds are made in reaction
when bonds are formed energy is released. bonds are made when products are being produced which is an exo process so ∆H is
-ve
how can enthalpy change be calculated by using mean bond enthalpies
enthalpy change = total energy to break bonds - total energy released forming bonds
what is calorimetry used for?
used to work out the enthalpy change of combustion
explain the process of calorimetry
- a fuel is burned to raise the temp of the water by a specific amount
- weigh the fuel before & after burning to work out the mass of the fuel burned
- a lid is placed on top to prevent heat loss
state where energy is transferred to from the fuel in calorimetry
energy is transferred from the fuel into the water although some is lost to the surroundings
how is the energy transfer in calorimetry calculated?
q = mc∆T
