module 3.2 physical chemistry
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a chemical reaction occurs when there is a…
• colour change • effervescence • precipitate formed • pH change • temp change/ energy change/ enthalpy change
what is enthalpy
the thermal energy stored in a chemical system. we can measure the change in this by seeing if the temp of a reaction changes
what is enthalpy change and what is it measured in
• the difference in enthalpies is the energy change - measured using triangle H
triangle H can be positive or negative; what does it mean when there is a positive enthalpy
an endothermic reaction has occurred
triangle H can be positive or negative; what does it mean when there is a negative enthalpy
an exothermic reaction has occurred
how can enthalpy change be determined
by measuring the energy transfer between the system and surroundings
____ change is from the system to surroundings
exothermic
____ change is from the surrounding to system
endothermic
what happens during an exothermic reaction
-chemical system loses energy -surroundings gain energy -temp of surroundings increase - triangle H is negative (bonds lose energy= energy is released from bonds in reactants)-energy stored in the bonds in the products is less than the energy stored in the reactants
what happens during an endothermic reaction
-chemical system gains energy -surroundings lose energy -temp of surroundings decrease - triangle H is positive (energy is absorbed from surrounding particles
reactants ahve a certain amount of stored energy this means…
reacting molecules only react if they collide with enough energy for bonds between reactants to break- this is activation energy
examples of exothermic reactions
combustion, respiration, neutralisation, condensing, freezing
examples of endothermic reactions
photosynthesis, melting, boiling, thermal decomposition
what happens when a catalyst is added
activation energy is lowered
for any reaction to happen…
• bonds must be broken in reactants - new bonds must be made in products
breaking bonds require energy which is an ____ process
endothermic
making bonds release energy which is an ____ process
exothermic
if more energy is required to break bonds than is released making bonds, the reaction is…
endothermic
if more energy is released in making bonds than it is needed to break bonds, the reaction is…
exothermic
what is standard pressure at
100kPa (one atmosphere)
what is the value for standard concentration
1 mol dm^-1
what is the definition of standard state
the physical state of a substance under standard conditions
what is standard temperature
298K (25 degrees C)
What is standard enthalpy change of a reaction
standard enthalpy change of a reaction is the enthalpy change that accompanies a reaction in molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states- when molar quantity doubles so does the standard enthalpy change.