bond enthalpy Flashcards
what is enthalpy
H is a measure of the heat energy in a chemical system
what is a chemical system
refers to atoms molecules or ions making up a chemical reaction
what is enthalpy change
enthalpy change = Hproduct - H reactant
whats the law of conservation of energy
energy cannot be created or destroyed
what is the system
the chemicals,reactants,products
what is the surrounding
apparatus everything is not in the chemical system
what is the universe
everything contains both system and surrounding
what type of energy transfer is exothermic
system to surrounding
what type of energy transfer is endothermic
surrounding to system
what happens in exothermic reactions in terms of conservation of energy
chemical system releases heat energy to the surrounding
any energy loss by the chemical system is balanced by the energy gain by surroundings
enthalpy change is negative
the temp of the surroundings increases as they gain enrgy
what happens in endothermic reaction in terms of conservation of energy
chemical system takes in heat from surroundings
any energy gain by chemical system is balanced by the same energy loss by the surroundings
Enthalpy change is positive
the temperature of the surroundings decrease as they lose energy
What is activation energy
minimum energy required for a reaction to take place
what happens to reactions with small activation energy
reactions take place very rapidly because energy needed to break bonds are readily availablefrom the surroundings
what happens to reactions with large amount of activation energy
such a large energy barrier that reaction may take place extremly slowlyor even not at all
what are the standard conditions
standard pressure- 100 Kpa
standard temperature - 298K
standard concentration - 1 mol dm^-3
standard state -100 kpa and 298K
what is the standard enthalpy change of reaction
is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactant and products in their standard state
what is the standard enthalpy change of formation
is the enthalpy change that takes place when one ,mole of a compound is formed from its elements under standard conditions with all reactant and products in their standard state
what is the standard enthalpy change of combustion
is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactant and product in their standard states
what is the enthalpy change of neutralisation
is the energy change that accompanies a reaction of an acid by a base to form one mole of H2O(l) under standard conditions with all reactants and products in their standard conditions
what is the formula for calculating energy change
Q=MC(change in temp)
Q=heat energy (J)
M= mass (g)
c= specific heat capacity
temp = the value of temp is exactly the same in degrees and kelvin
why are reactions less exothermic than we expect it
heat loss
incomplete combustion
non standard conditions
what could minimise error from heat loss and incomplete combustion
use of draught screens and oxygen gas
how can we determine the enthalpy change of reaction
using plastic cups made of polystyrene foam . These are cheap lightweight and offer some insulation against heat loss to surroundings
what happens when carrying out a reaction between aqeous solutions
the solution itself is the immediate surroundings. The chemical particles within the solution may react when collide and any energy transfer is between the chemical particles and water molecules in the solution a thermometer in the solution will record any temperature change to be calculated using mcat
what is average bond enthalpy
is the energy required to break one mole of a specified type of bond in a gaseous molecule
what is always required is break bonds
energy
what type of reaction is bond enthalpy
endothermic so always have positve value
what are limitations of bond enthalpies
vary depending on the chemical environment of the bond
what is bond breaking
endothermic
what is bond making
exothermic
State Hess’ 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
What is the calculation for the enthalpy change in formation
Product - reactant
What is the calculation for enthalpy change in combustion
Reactant-product
