PHYSICAL CHEMISTRY Flashcards
(32 cards)
what is enthalpy change
the change in heat energy of a reaction at constant pressure
what is standard enthalpy change
enthalpy change that occurs under the standard conditions of 100kPa, 298K and any solution at a concentration of 1 mol/dm3
what is standard enthalpy of formation
enthalpy change when 1 mol of substance is formed from its constituent elements under standard conditions
what is the standard enthalpy of comustion
the enthalpy change when 1 mole of substance undergoes complete combustion in excess of oxygen under standard conditions
what is the enthalpy of combustion of oxygen and why
zero because oxygen cannot be combusted
what is the enthalpy of combustion of oxides such as CO2 and why
zero because they have already undergone complete combustion
what are exothermic reaction
reactions that release energy to the surroundings.
are the ^H values for exothermic reactions positive or negative and why
negative because energy is released so the products have less energy than the reactnats
what are endothermic reactions
reactions that take in energy from the surroundings
are the △H values for endothermic reactions positive or negative and why
positive because energy is taken in so the products have more energy than the reactants
what is the 1st law of thermodynamics
energy cannot be created or destroyed
what is Hess’s law
the enthalpy change for a reaction is independent of the route taken
what is the equation for the △H of a formation reaction
△H = △H(products) - △H(reactants)
what is the equation for the △H of a combustion reaction
△H = △H(reactants) - △H(products)
what is the specific heat capacity of water
the energy needed to raise the temperature of water by 1 degree
what is the specific heat capacity of water
4.18 J/g/K
equation for heat energy change
Q=mc△T
how do you work out △H
+-Q/n (convert Q from J to kJ by dividing 1000)
what is mean bond enthalpy
the enthalpy needed to break one mol of gaseous covalent bonds averaged over breaking that bond in several different molecules
are bond enthalpies positive or negative
always positive because energy is required to break bonds
△H(reaction) using bond enthalpies
bond enthalpies of reactants - bond enthalpies of the products
what are the 3 main ideas behind collision theory
particles must collide to react
collisions must be equal to or greater than activation energy to react
collisions must be of the correct orientation
how an a reaction be made to go faster
increase frequency of collisions
increase energy of particles
lower activation energy
what is activation energy
the minimum amount of energy that a collision must have in order to have a reaction
