chapter 5 chemical energetics

about exothermic reactions

- heat energy is given out to the surroundings -so temperature of surroundings more than the temperature of the system -energy of the product is LESS than the energy of the reactant/ reactant energy MORE(top) - DELTA H = (-ve)

endothermic reaction-

- heat energy taken in by the reaction from the surroundings - energy of reactants LESS than product / product more (above) -DELTA H- +VE -temperature of system more than temperature of surroundings

which enthalpy change reaction has a higher activation energy

-endothermic reaction as reactants are further away in energy to the transition state/ has low energy hence requires more energy to reach activation energy to start reaction and break bonds.

the standard enthalpy change of formation of an element under standard state is e.g standard enthalpy change of formation of O2 = 0kj/mol

0 kj/mol

what are the standard conditions of a reactions

1- pressure -101KPA 2- temperature- 298K/ 25C 3- the substance in the reaction are in their physical states

Q - heat transfered =

MASS X C X CHANGE IN TEMP C= specific heat capacity

energy transfer (enthalpy changes - changes in chemical energy) occur during a reaction due to bond breaking and bond formation

bond breaking---> endothermic bond formation---> exothermic

bond breaking value ( DELTA H +ve) -endothermic

bond formation value ( DELTA H -ve) exothermic

which enthalpy change reaction has a lower activation energy? (exo/endo) and explain why

exothermic reaction because in an energy level diagram energy is already high hence it requires a small amount of energy to start the reaction / and is close in energy to transition state

define the term activation energy

is the minimum energy required for the reactant molecules to have a successful collision in order to start the reaction

define standard enthalpy change of formation

the enthalpy change when one mole of a compound is formed from its elements under standard conditions

define standard enthalpy change of combustion

the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions

define standard enthalpy change of neutralisation

the enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions

define standard enthalpy change of reaction

the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

enthalpy change/ energy change formulas ( you have to use bond energies )

total energy to break bonds - total energy released forming bonds

for enthalpy change of combustion/formation/neutralization must always simplify reactant and products to ONE MOLE

unit of enthalpy change- KJ/MOL

ENTHALPY CHANGE FORMULA-

1 1) -vemass x specific heat capacity x (change in temp) / n

specific heat capacity of water-

4.184 J/gC

terms:- bond energy / bond dissociation energy / exact bond energy

amount of energy needed to break one mole of a specific covalent bond in the gas phase

chemical reactions undergo changes in the chemical energy this is known as enthalpy changes

and the enthalpy changes can either be: - exothermic - endothermic