energetics Flashcards
what is bond energy
During a reaction, enthalpy changes take place because bonds are being broken and formed
Energy (in the form of heat) is needed to overcome attractive forces between atoms
Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
what are exothermic reactions
Bond forming
If more energy is released when new bonds are formed than energy is required to break bonds
heat is given out to the surroundings
and system gets colder
what are endothermic reactions
Bond breaking
surroundings get colder as energy taken in and system gets hotter
what are enthaply profile diagrams
An enthalpy profile diagram is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction as the reaction proceeds ( called ‘extent of the reaction’ below)
The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
The transition state is very unstable – a molecule in the transition state cannot be isolated and is higher in energy than the reactants and products
The activation energy (Ea) is the energy needed to reach the transition state
what is activation energy
We can define the activation energy as ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
what is the enthalpy profile for an exothermic reaction
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to endothermic reactions
enthalpy change is negative
what is the enthalpy profile for an endothermic reaction
In an endothermic reaction, the reactants are lower in energy than the products
The reactants are therefore further away in energy to the transition state
This means that endothermic reactions have a higher activation energy compared to exothermic reactions
enthalpy change is positive
what is enthalpy
The total chemical energy inside a substance is called the enthalpy (or heat content)
When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)
An enthalpy change can be positive or negative
what is enthalpy change
a heat energy change that is measured at a constant pressure
total energy of surroundings remain constant
what are the values standard enthalpy changes
To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions
These standard conditions are:
A pressure of 100 kPa
A temperature of 298 K (25 oC)
Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)
To show that a reaction has been carried out under standard conditions
ΔH= the standard enthalpy change
what is calorimetry
process of measuring a energy change experimentally
Calorimetry is the measurement enthalpy changes in chemical reactions
A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal can
what is specific heat capactiy
The energy needed to increase the temperature of 1 g of a substance by 1 oC is called the specific heat capacity (c) of the liquid
The specific heat capacity of water is 4.18 J g-1 oC-1
The energy transferred as heat can be calculated by:
what equation is used to find the heat energy give out
q= m c deltaT
q=heat given out
m= mass of water
delta T = temp rise in kelvin
c = specific het capacity
how to find enthalpy in terms of J or kJmol
e change per mole = q/moles
what is apparatus for reactions taking place in a solution- experiment
-insulated beaker and temp changed measured
-fast so max temp can be reached before it starts to cool
-specific heat capacity must be taken as 4.18 and usually use mass of water
-and stir
what is required for flame calorimeters
for combustion of substances
fuel is in bottle with a wick and it is burned so the heat is passed to the water which it heats
sph is used but in more accurate ones the heat capacity of flame can be determined
what are some main sources of error for calorimetry experiments
-heat capacity of metals not included in calcs
-heat lost to surroundings - place lid on cup to improve
-reaction could be incomplete or slow
-density of solution is taken to be the same as the water
are combustion reactions endothermic or exothermic
exothermic so negative value
are neutralisation reactions exo or endo
exo
what is Hess’s law
overall enthalpy changeat constant pressure for a reaction is independent of the route the reaction takes
what is Hess’s law built upon
the law of thermodynamics- energy cannot be created or destroyed only transferred bwt stores/ change form
- in a closed system- so energy present is always constant
why is Hess’s law better than calorimetry
-reduce energy dissipation to surroundings
-reduce human error
what can Hess’ law be used to find
the standard enthalpy change of a reaction from known standard enthalpy changes
what is the theory of Hess’ law
Calculating ΔHr from ΔHf using Hess’s Law energy cycles
The products can be directly formed from the elements = ΔH2
OR
The products can be indirectly formed from the elements = ΔH1 + ΔHr
The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)
Equation
ΔH2 = ΔH1 + ΔHr
Therefore,
ΔHr = ΔH2 – ΔH1
