actual energetics Flashcards
define enthalpy
the total chemical energy inside a substance
define enthalpy change
the amount of heating energy taken in or given out provided the pressure is constant
positive or negative
symbol used to represent enthalpy changes
ΔH
change in enthalpy
explain what is meant by an exothermic reaction
the energy is being transferred from the system (chemicals) to the surroundings and heat is given off to the surroundings.
the products less energy compared to the reactants
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
it means that exothermic reactions have a lower activation energy compared to endothermic reactions
change in enthalpy for exothermic reactions?
ΔH negative
why are exothermic reactions thermodynamically possible?
the enthalpy of the reactants is higher than that of the products.
explain what is meant by an endothermic reaction
the energy is being transferred from the surroundings to the system (system chemicals) and the temperature of the environment decreases (measuring using thermometer), energy system increases.
require an input of the heat energy e.g. thermal decomposition of calcium carbonate
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
means that endothermic reactions have a higher activation energy compared to exothermic reactions
change in enthalpy for endothermic reactions?
ΔH positive therefore
define the Standard Enthalpy Change Of Reaction
the enthalpy change whenever a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.
how do you calculate the Standard Enthalpy Change Of Reaction?
products - reactants
define the Standard Enthalpy Change Of Formation
the enthalpy change but whenever 1 mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kPa) all reactants and products being in their standard states therefore.
the enthalpy of formation of an element and why?
it is zero kJ mol^-1 because by definition.
define the Standard Enthalpy Change Of Combustion
the enthalpy change occurring whenever one mole of a substance is combusted completely oxygen under standard conditions (298K in addition to 100kPa), all reactants and products being in standard statesi
why is incomplete combustion not as exothermic as complete?
it leads to the production of soot (carbon), carbon monoxide and water.
standard conditions are
pressure is 100kPa
temperature 298K, all substances’ normal should be at this state so
solutions 1moldm^-3 therefore
how can you measure the enthalpy change for a reaction experimentally using an insulation container?
substances are mixed in an insulated container and the rise in temperature is measured
could be solid dissolving or reacting in a solution or both solutions
method:
- washes the equipment (cup and pipettes) w solutions to be in use
- dry the cup after washing
- put polystyrene cup in a beaker for insulation and support
- measure out desired volumes of solutions w volumetric pipettes and transfer to an insulated cup
- clamp thermometer into place making sure the thermometer bulb is immersed in solution
measure the initial temperatures of the solution or both solutions if 2 are being therefore so as used. Do this every minute for around approximately about 2-3 minutes
- At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance.
- If using a solid reagent then use ‘before and after’ weighing method
- stirs mixture (ensures that all of the solution is at the same temperature)
- Record temperature every minute after addition for several minutes
errors that might potentially be occurring as a result?
- energy transferred to the surroundings as heat loss therefore
- approximation in the overall specific heat capacity of the solution. the method assumes all of the solutions have heat capacity of water
- neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus.
- reaction or dissolving may be incomplete or slow.
- density of solution is taken to be the same as water.
improving accuracy
● An electronic temperature sensor and data logging software could be used to plot the graph accurately.
● A flame calorimeter could be used to improve accuracy:
○ Spiral chimney is made of copper.
○ Flame is enclosed.
○ Fuel burns in pure oxygen rather than air.
If the reaction is slow then the exact temperature rise can be
difficult to obtain as cooling occurs simultaneously with the
reaction
To counteract this we instead therefore take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.
We also take the temperature of the reactants for a few
minutes before they are added together to get a better
average temperature. If the two reactants are solutions then
the temperature of both solutions need to be measured
before addition and an average temperature is used.
