Energetics 4 Flashcards
the amount of energy involved when a chemical reaction takes place is important for many reasons
- energy values of fuels
- energy require for industrial processes
- the theoretical amount of energy to break bonds and amount of energy released when bonds are made
- predict whether a reaction will take place
exothermic
energy has been given out
neutralising acids
endothermic
energy is taken in
breakdown of limestone
standard conditions for measuring enthalpy changes
pressure 100kPa
temperature 298K
physical states of the reactants and products
physical states affect the enthalpy change like energy is needed to change a liquid into a gas
standard molar enthalpy of formation
enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.
An element formation energy is zero
standard molar enthalpy of combustion
enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states
temperature
is related to the average kinetic energy of the particles in a system.
heat
a measure of the total energy of all the particles present in a given amount of substance.
flows to a high temperature to low temperature
measuring an enthalpy change of a reaction
- mass of a substance that is being heated up or cooled down
- temperature change
- specific heat capacity of the substance
specific heat capacity
the amount of heat needed to raise the temperature of 1g of substance by 1K
simple calorimeter
burn the fuel to heat a known mass of water and then measure the temperture rise of the water.
flame calorimeter
reduce heat loss
- the spiral chimney is made of copper
- flame is enclosed
- fuel burns in pure oxygen rather than air
displacement reactions
metal is more reactive than another will displace a less reactive one from the compound
Hess’s law
Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
enthalpy changes of formation
H1 = H2+H3
🔺H = breaking bonds - forming bonds
Standard conditions
Pressure 100kPa ( normal atmospheric pressure) Temperature 298k (normal room temperature)
Specific heat capacity equations
Q = mc🔺T
Enthalpy of neutralisation
The enthalpy change during neutralisation with an acid and an alkali to form one mole of water
Enthalpy change of displacement
A metal that is more reactive than another will displace the less reactive one from a compound. If the compound will dissolve in water, this reaction can be investigated using a polystyrene breaker as before
Formation arrows
Elements to reactants
Combustion arrows
Reactants to elements
Mean bond enthalpy
Average value of the bond dissociated enthalpy for a given type of bond taken from a range of different compounds
Bond dissociation enthalpy
Enthalpy change required to break a covalent bond with all species in the gases
Bond enthalpies methane
If the bonds in methane are broken one at a time, the energy required is not the same fro each bond
Enthalpy change (∆H)
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
