3.2 Physical Chemistry (completed) Flashcards
3.2.1 Enthalpy change 3.2.2 Reaction rates 3.2.3 Chemical equillibrium
What is an exothermic reaction?
Energy is given out for example combustion of fuels, respiration
-tve enthalpy change
What is an endothermic reaction?
Energy is absorbed for example photosynthesis, thermal decomposition
+ tve enthalpy change
What is the equation for enthalpy change?
Products - reactants
What is the definition of activation energy in terms of bonds?
The energy needed to start a reaction by breaking chemical bonds in the reactants
What is the bond dissociation energy?
The energy released when a bond is formed
What is the average bond enthalpy?
The breaking of one mole of bonds in gaseous molecules
What is the equation for enthalpy change in terms of bonds?
Bonds broken - bonds formed
What are the problems with bond enthalpy calculations?
They always assume substances are in the gaseous state
Mean bond energy values are used
What is calorimetry?
A way to determine the enthalpy change of a reaction
What is the equation for enthalpy change from calorimetry?
q = mcΔT
Then divide by moles and 1000
What is the method of spirit burner calorimetry?
A known volume of cold water is measured into the beaker
The starting temperature of the water is recorded
The water is heated using the flame from the burning fuel
The final temperature of the water is recorded
The spirit burner containing the fuel is weighed before and after the experiment 
What is the enthalpy of reaction?
The overall energy change
If the conditions are standard then what is the energy change?
Standard enthalpy of reaction
What are the standard conditions?
100kPa 1atm
298K
1.0 M
What is the standard enthalpy change of combustion?
The enthalpy change that takes place when one mole of a substance
reacts completely with oxygen
under standard conditions
with all reactants and products in their standard state
What is the standard enthalpy change of formation?
The enthalpy change that takes place when one mole of a compound
is formed from its constituent elements
in their standard states under standard conditions
What is Hess’ law?
Hess’ law states that if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same
What is enthalpy change of combustion and formation equations?
C=R-P
F=P-R
Always use equation of data given
What must happen for a reaction to occur?
Particles must collide
What are the five factors that affect rate of reaction?
Temperature Pressure Catalyst Concentration Surface Area
What is the activation energy?
The minimum amount of energy for particles to react when they collide
What happens if particles collide but don’t have activation energy?
They bounce off of each other
What does a Maxwell-Boltzmann distribution look like?
Number of molecules y axis
Kinetic Energy x axis
Bell shaped from origin that never reaches x axis
Area under graph represents the total number of molecules
How does a Maxwell Boltzmann distribution change when temperature is increased?
The peak gets lower and the curve gets broader
How does increasing temperature affect the rate of reaction?
Increases
Increasing temperature it gives particles more energy so more of them have enough energy to react when they collide
Is the definition of a catalyst?
Substances which increase the rate of a chemical reaction without being consumed themselves by providing an alternative route for the reaction with a lower activation energy
What are the benefits and negative effects of catalysts?
Advantages:
Increased sustainability
Lower operating temperature
Less CO2 emissions by burning fossil fuels
Disadvantages:
Toxicity
What are the two types of catalysis?
Homogenous Catalysis
Catalysts and reactants are in the same phase
Heterogenous Catalysis
Catalysts and reactants are in different phases
What is Le Chatelier’s principle?
When a system in dynamic equilibrium is subject to change the system will adjust to minimise change
What are the five characteristics of a dynamic equilibrium?
- Rate of forward reaction equals rate of reverse
- Occurs in a closed system
- Concentrations of reactants and products remain constant
- Reactions to continue to occur as the reaction is dynamic
- The equilibrium can be established from either side
What are the advantages and disadvantages for high pressure and high temperature?
Increase rate and possibly increased yield
BUT
Costs for energy and equipment needed and also possibly unsafe
What are the advantages and disadvantages for low pressure and low temperature?
Reduce costs and possibly increase yield
BUT
Decrease rate
Where will an equilibriums shift if you increase temperature and pressure?
Favours endothermic and reaction that produces less moles
How do catalysts affect equilibrium position?
No effect
