1.4 Energetics Flashcards
What energy change is breaking bonds associated with?
Energy is taken in to break bonds —> endothermic reaction
What energy change is making bonds associated with?
Energy is released to make bonds —> 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 amount of energy released/ taken in in a reaction
Predicting if a reaction will take place or not
What is an endothermic reaction?
One with an overall positive enthalpy change —> energy in breaking bonds> energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change —> energy in breaking bonds < energy out making bonds
If a reversible reaction is endothermic one way, what type of the reaction is the other way?
Exothermic
Give 2 examples of exothermic reactions
Combustion of fuels
Neutralisation
Give an example of an endothermic reaction
Thermal decomposition
Define enthalpy change; what symbol is used to represent it?
Heat energy change of a system at a constant pressure
What are the standard conditions?
100kPa/ 1atm pressure
289K / 25 degree Celsius temperture
What does in standard state mean?
The state an element/compound exists at in standard conditions (100kPa, 298K)
Draw an enthalpy change diagram for an endothermic reaction and one for an exothermic reaction
Slide 26
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions (100kPa, 298K) with reactants and products in their standard states
Give an example of an equation which represents standard enthalpy of formation
There are many H2 (g) +1/2O2(g) —> H2O(l)
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa,298K) with reactants and products in their standard states
Give an example of an equation which represents standard enthalpy of combustion
C(s) + O2(g) —> CO2 (g)
What is the difference between heat and temperature?
Heat is the sum of all particles’ energy therefore it is affected by the ammount of substance
Temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present
How can you calculate enthalpy change from experimental data?
Use the equation Q=MCΔT
M is mass of substance being heated
C is specific heat capacity of that substance being heated
ΔT change in temperature
Draw a simple calorimeter
Slide 40
How could this calorimeter be made more accurate?
Add draught screens at the sides, add a lid on the top of the beaker, add mineral wool around the beaker —> all to insulate and reduce heat lost to the surroundings
What is a flame calorimeter
How does it differ to a simple calorimeter?
Reduces heat lost to the surrounding to give more accurate results:has spiral chimney made of copper, enclosed flame and 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 therefore reduces heat loss)
Heat is generated in the solution; measure this temperature change. Take heat capacity I’d solution to be 4.18 and density of solution = 1gcm^-3
What can you use to make experimental determination of enthalpy change of reaction more accurate?
Cooling curves
What is hess’s law?
Enthalpy change in a reaction is independent of the route taken.
What is the enthalpy of an element?
The enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 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 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; standard enthalpy of combustion and formation apply to that molecule therefore it is more accurate
