energetics (physical) Flashcards
Define enthalpy change
Energy change at a system providing the pressure is constant.
State what happens when there is an enthalpy change
Energy is transferred between systems and surroundings. (Systems are the chemicals, surroundings is everything outside the chemicals)
Give 2 examples of an exothermic reaction
Conbustion and neutralisation
What happens in an exothermic reaction?
Products have less energy than the reactants because energy is transferred from the system to the surroundings.
What happens in an endothermic reaction?
Energy is taken in from the surroundings and the products have more energy than the reactants.
What are the standard conditions of enthalpy change?
100kPa pressure.
298K
solutions at 1moldm‐³
What is the standard enthalpy change of formation?
the energy transferred when 1 mole of the compound is formed from its element under standard conditions.
What is standard enthalpy change of combustion?
Enthalpy change that occurs when 1 mole of a substance is combusted completely under standard conditions.
Give an example of an endothermic reaction
thermal decomposition
What energy change is breaking bonds associated with?
energy is taken in to break bonds so it is an endothermic reaction
what energy change is making bonds associated with?
energy is released to make bonds so it is an exothermic reaction
What are some uses of thermochemistry?
•Measuring and comparing the energy values of fuels
•Calculating the energy requirements for industrial processes
•Working out the theoretical energy released/taken in during a reaction.
•Predicting if a reaction will take place or not
Give an example of an equation which represents standard enthalpy change of formation
H²(g) + ½O(g) -> H²O(l)
Give an example of an equation that represents standard enthalpy change of combustion
C(g) + O²(g) -> CO²(g)
What is the difference between heat and temperature?
Heat is the sum of all particles’ energy so it is affected by the amount of substance. However, temperature is related to the mean kinetic energy in a system so it is independent on the number of particles present.
How can you calculate enthalpy change from experimental data?
Use the equation Q=mc^T where m is mass of substance being used (usually water), c is the specific heat capacity of that substance (water is 4.18J‐¹K‐¹) and ^T is change of temperature.
How could you improve a standard calorimeter?
Add draught screens at the side and add a lid on to of the beaker to insulate and reduce heat loss to the surroundings.
What is a flame colorimeter and how does it differ to a normal calorimeter?
Reduces heat loss to the surroundings for more accuracy because it has a spiral chimney made out of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.
How would you measure the enthalpy change for a reaction occurring in aq?
Use an expanded polystyrene cup as a calorimeter (good insulator so reduces heat loss)
Heat is generated in the solution so measure the temperature change. Then take the heat capacity to be 4.18 and the density of the solution = 1gm‐³
What can you use to make experimental determination of enthalpy change more accurate?
cooling curves
What is Hess’s Law
States the enthalpy change for a reaction regardless the route taken
What is the enthalpy of an element?
The enthalpy of all elements in their standard states is 0
Define bond dissociation enthalpy
The enthalpy change required to break a covalent bond with all species in the gaseous state; differs for the same bond type in different molecules
Define mean bond enthalpy
Average value (across different chemical environments) for the bond dissociation enthalpy of a given bond
Why may experimental methods for enthalpy determination not to be very accurate?
Heat is lost to the surroundings.
Not in standard conditions.
Reaction may not go to completion
Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
Bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion/formation apply to just that molecule
