M3 - Enthalpy changes Flashcards
What is the temperature change in an endothermic reaction?
A decrease in temperature
what’s the enthalpy change equation using enthalpy change of combustion values ?
= sum of reactants - sum of products
How do you find enthalpy changes using enthalpy change of formation values?
= sum of products - sum of reactants
how do you calculate the enthalpy changes from average bond enthalpies?
ΔH= sum of bonds broken - sum of bonds formed
What is Hess’ law?
If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.
What kind of enthalpy change is bond formation?
Exothermic as it releases energy
What are some properties of bond enthalpies?
Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value
Define average bond enthalpy
The energy required to break one mole of covalent bonds in a gaseous molecule.
How can heat loss be accounted for using a graph of temperature against time?
Extrapolate the cooling curve back to when it was added.
What are the causes for less energy being transferred than expected when working out ΔHc?
Heat loss to the surroundings
Incomplete combustion
Evaporation
Non-standard conditions
what’s the enthalpy change equation using ‘average bond enthalpies’ ?
ΔrH=Σ(bond enthalpies of reactants) - Σ (bond enthalpies products)
Define standard enthalpy change of neutralisation
The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O, under standard conditions, with all reactants and products in their standard states.
How is enthalpy change worked out from the energy change and change in temperature?
Q = mcΔT and then
ΔH = -Q/n
ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)
What is the equation used to measure an energy change?
Q=mcΔT
Q - energy change with surroundings (J)
m - mass (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)
Define standard enthalpy change of combustion
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of formation
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.
What are the standard conditions?
Standard pressure - 101kPa / 1atm
Standard temperature - 298K / 25 degrees celsius
Standard concentration - 1mol/dm^3
Standard state - The physical state of a substance under standard conditions
Define activation energy
The minimum energy required for a reaction to take place by breaking chemical bonds.
Define an endothermic reaction
A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat beaing taken out of the surroundings.
Define an exothermic reaction
A reaction in which the enthalpy of the products is lower than the enthalpy of the reactants, resulting in heat loss to the surroundings.
Define enthalpy change
The heat exchanged with the surroundings during a chemical reaction.
The difference between the enthalpy of the products and the reactants.
Define enthalpy
The heat content that is stored in a chemical system.
What is the law of the conservation of energy?
Energy cannot be created or destroyed, just converted from one form to another.
Define ‘standard enthalpy change of formation’, ΔfHθ (1)
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (1)
What is Hess’ Law?
The enthalpy change of a reaction depends only on the initial and final states and is independent of the route taken
What are the units of enthalpy change?
Units = kJ mol-1
B
What are the standard conditions for temperature, pressure and concentration?
Temperature = 298K or 25 degrees celsius
Pressure = 1atm or 101kPa
Concentration = 1 moldm-3
B
Define ‘standard enthalpy change of neutralisation’, ΔneutH (1)
The enthalpy change that accompanies the formation of one mole of H2O(l) from neutralisation, under standard conditions (1)
What is meant by ‘activation energy’? (1)
Energy: The minimum energy required to start a reaction by breaking bonds (1)
Define ‘standard enthalpy change of combustion’, ΔcHθ (1)
The enthalpy change for complete combustion of one mole of a substance under standard conditions, all reactants and products being in their standard states (1)
B
Define ‘Standard enthalpy change of reaction’, ΔrHθ
The 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
Define average bond enthalpy, ΔEHθ
The average enthalpy change for the breaking of 1 mole of bonds in gaseous
molecules (by homolytic fission) (1)
Define Enthalpy change, ΔH
The amount of heat released (or absorbed) by a chemical reaction, carried out at constant pressure
