3.2.1 +3.2.2 Enthalpy Changes + Rates Of Reaction Flashcards
What does system mean in a chemical reaction?
The atoms and bonds involved in the chemical reaction
Explain the law of conservation
The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, It can only be transferred from one form to another.
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 is an endothermic reaction?
A reaction with an overall positive enthalpy change - enthalpy of products > enthalpy of reaction
What is an exothermic reaction?
A reaction with an overall negative enthalpy change - enthalpy of products < enthalpy of reactants
What is the meaning of activation energy?
The minimum energy required for a reaction to take place
What are the standard conditions?
100 kPa
298K
What does in standard state mean?
The state an element/compound exists at in standard conditions
Define enthalpy change of formation
The energy change that takes place when one mole of a compound is formed from its constituent elements in their standard state under standard conditions
Give an example of an equation which represents standard enthalpy formation
H2 (g) + 1/2 O2 (g) -> H2O (l)
Define enthalpy change of combustion
The energy change that takes place when one mole of a substance is completely combusted
Give an example of an equation which represents standard enthalpy of combustion
C (s) + O2 (g) ->CO2 (g)
Define enthalpy change of neutralisation
The energy change that takes place when one mole of water is formed from a neutralisation reaction
How can you calculate enthalpy change from experimental data?
Use the equation Q=mcT where M is the mass of the substance being heated, c is the specific heat capacity of that substance and Delta T is the change in temperature
What are the advantages of using a bomb calorimeter?
Minimise heat loss
pure oxygen used so it ensures complete combustion
Why may experimental methods for enthalpy determination not be accurate?
Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion
What does average bond enthalpy mean?
The main energy required to break one mole of bonds in gaseous molecules
Why would using bond and 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 and formation apply to that molecule therefore they are more accurate
How to calculate enthalpy change of reaction using average bond enthalpies?
DeltaH = sum of (bond enthalpies of reaction)- sum of (bond enthalpies of products)
What is the equation used to calculate rate?
Rate equals change in concentration / time
What is the unit for rate of reaction?
Mol dm-3s-1
What most particles do in order to react?
Collide with sufficient energy (activation energy) and the correct orientation
Do most collisions result in a reaction
No
What are the factors that affect rate of reaction?
Temperature
Pressure
Concentration
Surface area
Catalyst
What is the effect of increasing temperature on rate of reaction? why?
Increasing temperature
increases the rate of reaction
Much higher proportion of particles have energy greater than the activation energy
Many more successful collisions per second
Increased rate
What is the effect of increasing concentration/pressure on rate of reaction and why?
Increased concentration/pressure
Increased rate of reaction
There are more particles in a given volume
More frequent successful collisions
Increased rate
What are the variables in an experiment that can be monitored to calculate the rate of reaction?
Concentration of reactant or product
Gas volume of products
Mass of substances formed
How to calculate rate from a concentration time graph
Draw a tangent
Work out the gradient of the tangent using the equation
Gradient equals change in Y / change in X
What is a catalyst?
A substance which increases the rate of reaction but is not used up in the reaction
How do catalysts work and how do they increase the rate of reaction?
They provide an alternate reaction pathway (with a lower activation energy)
Due to lower activation energy more particles have energy
so more frequent collisions
so increased reaction rate
What does homogeneous catalyst mean?
A catalyst that is in the same phase as the reactants, e.g. liquid catalyst mixed with liquid reactants
What does heterogeneous catalyst mean?
Catalyst used in the reaction is in different phases to the reactants, e.g. gaseous reactants passed over solid catalyst
What are catalytic converters?
They are present in vehicles to reduce toxic emissions and prevent photochemical smog
Define activation energy
The minimum energy that particles must collide with for a reaction to occur
Name some important features of boltzamann distribution
Area under the curve = total number of molecules
Area under the curve does not change when conditions alter
The curve starts at the origin
Curve does not touch or cross the energy access
Only the molecules with energy greater than activation energy can react
What are the axis in Boltmann distribution?
X axis is energy
Y axis is number of molecules with a given energy
