Module 3 - Enthalpy Changes Flashcards
define enthalpy (H)
-the heat that is stored in a chemical system
(elements and compounds contain enthalpy)
define enthalpy change (△H)
-difference in enthalpy between the products and reactants in a reaction
define an exothermic reaction
-chemical reaction where heat energy is transferred from the system to the surroundings
describe exothermic reactions
-any energy loss by the system is balanced by the energy gain by the surroundings
-the temperature of the surroundings increases so the temperature increases
-the enthalpy change is negative
examples of exothermic reactions
-burning/combustion
-neutralisation
-oxidation
-handwarmers
-respiration
-condensation
define an endothermic reaction
-chemical reaction where heat energy is absorbed from the surroundings into the system
describe endothermic reactions
-any energy gain by the system is balanced by the energy loss of the surroundings
-the temperature of the surroundings decreases, so we see a temperature decrease
-the enthalpy change is positive
examples of endothermic reactions
-thermal decomposition
-sports injury pack
-photosynthesis
-evaporation
is the enthalpy change for the addition of magnesium to HCl endothermic or exothermic
exothermic
is the enthalpy change for the addition of sodium thiosulfate crystals to water endothermic or exothermic
endothermic
describe the enthalpy of bonds in exothermic reactions
-the products have less enthalpy than the reactants
-more energy is released from making bonds than breaking bonds
-the enthalpy change is a negative value
define activation energy
-the minimum energy required to start a reaction
describe the enthalpy of bonds in endothermic reactions
-the products have more enthalpy than the reactants
-more energy is required for breaking bonds than making bonds
-the enthalpy change is a positive value
how to calculate activation energy of a backwards reaction for a forwards reaction enthalpy profile or vice versa
-use values already on the graph
-switch sign if arrow points a different way in this reaction
-keep sign the same if arrow points the same way
define enthalpy change of reaction
-the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation
define enthalpy change of formation
-the enthalpy change when 1 mole of compound is formed from its element under standard conditions
define enthalpy change of combustion
-the enthalpy change when 1 mole of substance completely combusts under standard conditions
define enthalpy change of neutralisation
-the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
symbol for enthalpy change of reaction
▲Hr
symbol for enthalpy change of formation
▲Hf
symbol for enthalpy change of combustion
▲Hc
symbol for enthalpy change of neutralisation
▲Hneut
symbol for standard conditions
⦵
describe standard conditions
standard pressure = 100kPa
standard temperature = 298K (25c)
standard concentration = 1mol dm-3 (solutions only)
describe how to tell when standard conditions are not used
-if the enthalpy change values are different to what is expected
method for calculating enthalpy change
1- work out heat energy (q)
2- work out the moles (of the limiting reactant)
3- work out the ▲Hr
3- convert to kJs and add sign
symbol equation for working out heat in enthalpy change
q= mc▲t
word equation for working out heat in enthalpy change
heat energy = mass of liquid you measure the temp of x specific heat capacity x change in temp (in degrees or k)
g to cm3 conversion
1g= 1cm3
