Chapter 9 Enthalpy Flashcards
(25 cards)
What is enthalpy
- measure of heat energy in a chemical system
- chemical system -atoms ,molecules or ions making up the chemicals
- energy stored within bonds
How do you calculate enthalpy change
reactants - products
what is the law of conservation of energy
- energy cannot be created or destroyed
What happens with a chemical reaction involving an enthalpy change
- system is the chemicals - reactants and prodcts
- surrounding are apparatus and labatories whcih are not the chemical system
- universe is everything includes both system and surroundings
What is an exothermic reaction
- energy transferred from system to the surrounding
- enthalpy change in negative
- temperature of the surroundings increases as they gain energy
What is an endothermic reaction
- energy transferred from the surroundings to the system
- enthalpy change is potitive
- temperature of the surroundings decreases as they lose energy
What is the activation energy
- is the minimum amount of energy required for a reaction to take place
What are standard conditions
- pressure - 100Kpa
- standard temperature 298K
- concentration - 1moldm-3
- standard tate physical stat of the substance under standard conditions
What is standard enthalpy change of reaction
- enthalpy change that accompanies a reaction in molar quantities expressed in a chemical equation under standard conditions all reactants and products being in their standard states
What is standard enthalpy change of combustion
- the enthalpy change that takes place when one mole of a substance reacts completley with oxygen under standard condition al reactants and products being in their strandard states
What is standard enthalpy change of formation
- the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under their standard states
What is the standard enthalpy change of neutralisation
- the enthalpy change that accompanies the reaction of an acid by a base to form one mole fo H2O under standard conditions
What is the standard enthalpy change of formation for elements not in a compounds
0Kjmol-1
What is the specific heat capacity of a substance
- energy required to raie the temperature of 1g of a substance by 1K
What is the specific heat capacity of water
- 4.18Jg-1K-1
How do you calculate heat energy
- energy = mass x change in temperature x4.18
For enthalpy change of combustion why are values different to the data book
- incomplete combustion
- heat loss to surrounding
- evaporation of alcohol from spirit burner
- non standard conditions
Is combustion exothermic or endothermic
exothermic
What is average bond enthalpy
the average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species
Describe the requirements for average bond enthalpy
- energy required to break bonds
- bond enthalpies are always endothermic ]bond enthalpies always have a positive values
What are the limitations for average bond enthalpies
- there caluclated from the actual bond enthalpies from different chemical environments
Describe bond making and bond breaking
- energy required to break bonds in endothermic - positive
- energy released when bonds formed in bond making is exothermic - negative
What is Hess’s law
- if a reaction can take place by two routes and the starting and finishing conditions are the same the total enthalpy change is the same for each route
How do you calculate enthalpy change of combustion from Hess’s cycle
- 2-1
