Energetics I Flashcards
What is the enthalpy change in a reaction?
The heat change in a reaction at a constant pressure.
What are the units for enthalpy change?
Kj mol^-1
What is an example of an endothermic reaction?
A thermal decomposition reaction
In an endothermic reaction, do the products have higher energy than the reactants?
Yes
In an exothermic reaction do the products have lower energy than the reactants?
Yes
What is an example of an exothermic reaction?
A combustion reaction
To break a bond what needs to happen?
Energy needs to be absorbed
In an endothermic reaction, are bonds broken in the reactants?
Yes
In an exothermic reaction, when bonds are formed, is energy released?
Yes
What is the bond enthalpy?
The amount of energy required to break 1 mole of a bond type in a molecule in its gaseous state.
Enthalpy change =
Total energy to break bonds- total energy released when bonds are formed
When products are being produced in an exothermic reaction, are bonds made?
Yes
What is calorimetry used to work out?
Thr enthalpy change of combustion
Describe the steps involved in calorimetery
1) A fuel is burned to raise the temperature of the water by a specific amount. The fuel is weighed before and after to work out the mass of the fuel burned.
2) The energy from the fuel is transferred into the water, although some is lost to the surroundings too.
3) A lid is placed on top to prevent heat loss and wind shields. It is placed by the flame to prevent a draught moving the flame.
What equation can energy transferred be calculated using?
q = mc change in t
What is q?
The heat energy gained or lost
What is m?
The mass of water/the solution
What is t?
Temperature
What are the units for heat energy transfer?
Joules
What are the units for mass?
Grams
What is c?
The specific heat capacity of water
What is temperature measured in?
Kelvin
What is the specific heat capacity of water?
4.18 j/kg
Enthalpy =..
Enthalpy = energy / moles
How can the energy from a calorimetry experiment of solutions be calculated?
1) Add an acid to a polystyrene cup and measure the temperature.
2) Then, add alkali/solid, stir it, then measure the enthalpy change.
State Hess’ law
The total enthalpy change of a reaction is independent of the route taken
When you’re given formation data, do the arrows point up or down?
The arrows point up
If you’re given combustion data do the arrows point up or down?
Down
What are standard conditions?
100kPa (1atm) pressure
298k
Standard states
Does the temperature go up or down in exothermic reactions?
Up
Does the temperature go up or down in endothermic reactions?
Down
The less enthalpy a substance has…
The more stable it is
What is the activation energy, Ea?
The minimum amount of energy required to break the reactant bonds and start a chemical reaction.
Can you directly measure the actual enthalpy of a system?
No
How do you work out enthalpy changes?
- By experiment.
- Using data books
What is the standard enthalpy change of reaction?
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
What is the standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
What is the standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.
What is the standard enthalpy change if neutralisation ?
The enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water
What is the mean bond enthalpy?
The energy needed to break one mole of bonds in in the gas phase, averaged over many different compounds.
Why might the value obtained from carrying out enthalpy experiments be different from the data book value for enthalpy change?
- Conditions may not have been in standard states.
- Incomplete combustion may have occurred.
- Heat energy may have been lost to the surroundings.
- There may have been a wrong approximation of the Specific Heat capacity of the solution.
-
What is the standard lattice enthalpy?
The energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions
What is the standard lattice energy a measure of?
Ionic bond strength
The more negative thr lattice energy…
The stronger the ionic bonding
What needs to be assumed when working out the theoretical lattice energy?
- That all ions are spherical
- That the charge is evenly distributed
Is it true that the polarisation of ionic bonds leads to some covalent character in ionic lattices?
Yes
What is thr polarising power affected by?
- Ionic charge
- Ionic radius
Is it true that bonds are polar if the difference in Pauling electronegativity is greater than 0.4?
Yes
Is bond breaking endothermic or exothermic?
Endothermic
Is bond making endothermic or exothermic?
Exothermic
What is the enthalpy change of hydration?
Thr enthalpy change when 1 mole of gaseous ions dissolves in water.
What is the enthalpy change of solution?
The enthalpy change when 1 mole of solute dissolves in water
What are the steps in working out the enthalpy change of solution?
1) Ionic lattice and dissolved ions go on the top.
2) Connect ionic lattice to the gaseous ions by the lattice energy
3) Connect the gaseous ions to the dissolved ions by the hydration enthalpies of each ion.
What is the enthalpy change of hydration affected by?
Ionic charge and ionic radius
What is entropy?
The measure of the disorder of a system
The nore disordered a system is, the higher the what?
The higher the entropy
Continue from energetics 2 page and entropy page
