3.2 - Physical Chemistry Flashcards
Define the term enthalpy change.
The energy exchange that takes place with the surroundings at constant pressure during a reaction. Exothermic reactions will have a negative ΔH. Endothermic reactions require an input of thermal energy and will have a positive ΔH.
What does exothermic mean?
A reaction in which energy is given out from the system to the surroundings.
What does endothermic mean?
A reaction in which energy is taken in to a system from the surroundings.
Give examples of endothermic and exothermic enthalpy changes.
Endothermic - photosynthesis, thermal decomposition of calcium carbonate.
Exothermic - oxidation. It is used in the combustion of fuels. Carbohydrates such as glucose are oxidised in respiration.
Describe the term activation energy.
The minimum amount of energy required for a reaction to take place.
What are the standard conditions for an enthalpy change?
Temperature - 298K.
Pressure - 100KPa.
Solutions - conccentration of 1.0 mol dm-3
Reactants and products in normal physical state for these conditions.
Define enthalpy change of a reaction (ΔrH).
The enthalpy change when amounts of reactants react together under standard conditions of 298K and 100KPa, to give products in their standard states.
Define standard enthalpy change of formation (ΔfH).
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions of 298K and 100KPa.
Define standard enthalpy change of combustion (ΔcH).
The enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions of 298K and 100KPa in their standard states.
Give the formula needed to calculate enthalpy changes from experimental results.
ΔH = mcΔt. ΔH = enthalpy change in Joules. m = the mass of water that surrounded the reaction in grams. Δt = the temperature change.
How do you make sure you have the correct sign for the enthalpy change?
When you have calculated the enthalpy change you must give it a negative sign if the reaction was exothermic, but positive if the reaction was endothermic.
If the temperature increased - it’s exothermic.
If the temperature dropped - it’s endothermic.
How do you calculate the enthalpy change for the reaction?
Calculate the number of moles used.
Divide the energy change (in kJ) by the number of moles used to find the energy change per mole.
kJ/mol.
Why might the value calculated for an enthalpy change via experiment be different to the real value?
Energy loss to the surroundings during the experiment.
Not standard conditions.
If a combustion reaction is involved then there might have been some incomplete combustion.
Define average bond enthalpy.
The enthalpy change when 1 mole of gaseous covalent bonds is broken.
Explain, using bond forming and bond breaking, why a reaction can be exothermic.
Bond breaking absorbs energy while bond forming releases energy. More energy is released as bonds are formed than taken in when bonds are broken.
Explain, using bond forming and bond breaking, why a reaction can be endothermic.
Bond breaking absorbs energy while bond forming releases energy. More energy is absorbed as bonds are broken than released when bonds are formed.
How can bond enthalpies be used to calculate enthalpy changes?
ΔH = bonds broken - bonds formed.
Why will the enthalpy change calculated from average bond enthalpies be different from the actual enthalpy change value?
Average bond enthalpies are calculated based on the bond in a variety of different environments.
This will be different to the bond enthalpies for the actual environment.
What is Hess’ Law?
The total enthalpy change for a reaction is independent of the route by which that reaction takes place provided the initial and final conditions are the same. I.e. whatever route you look at, the enthalpy change is the same.
Describe the techniques and procedures used to determine enthalpy changes directly and indirectly.
Directly: Set up the reaction to take place in an insulated container (e.g. polystyrene cup). Add a thermometer to record the temperature change. This process works well for reactions that take place in solutions.
Indirectly: Indirect methods are often needed for measuring the energy given out by a fuel. A known mass of water is heated by the fuel and the temperature change of the water is recorded. A copper or bomb calorimeter is often used.
Describe the key features of collision theory.
Particles must collide in the correct orientation with enough energy in order to react.
Describe and explain the effect of concentration on the rate of reaction.
Increasing the concentration of a substance can increase the rate of reaction. It increases the number of particles in the same volume. This increases the number of collisions per second. This results in more reactions per second.
Describe and explain the effect of pressure on the rate of reaction.
Increasing the pressure can increase the rate of a reaction. It reduces the space between the molecules. As the molecules are closer together there will be more collisions per second. This results in more reactions per second.
How do you calculate the rate of reaction from the gradient of a graph?
Y axis should be the change in mass or concentration. X axis should be time in seconds. Take a tangent for the time or concentration you are trying to find the rate for. Calculate the gradient for the tangent.