3.2 physical chemistry Flashcards
What is chemical energy?
It is a special form of potential energy that lies within chemical bonds.
Outline what happens when chemicals react in terms of energy.
When chemicals react together to form new substances bonds break in the reactants and new bonds are formed as products are made.
This process changes the chemical energy of the substance.
Define enthalpy.
Thermal energy that is stored in a chemical system.
How can we measure enthalpy of reactants/products?
It is impossible to directly measure their enthalpy. However, we can measure the energy absorbed or released to the surroundings during a chemical change.
Define, exothermic reaction.
the enthalpy of the products is smaller than that of the reactants and there is heat loss to the surroundings.
Define exothermic reaction in terms of bonds broken.l
More energy is released when forming bonds than was needed to break bonds.
Define, endothermic reactions.
The enthalpy of the products is greater than that of reactant and there is a heat gain to the chemical system.
Define endothermic reactions in terms of bonds.
The energy needed to break existing bonds is greater than energy released when forming bonds
Define enthalpy change of reactions.
It is the enthalpy change associated with a given reaction under standard conditions.
Define enthalpy change of formation.
Energy change that takes place when 1 mole of a compound is formed from it’s constituent elements in their standard state at standard conditions.
Define enthalpy change of combustion.
Energy change that takes place when 1 mole of a substance is completely combusted in oxygen under standard conditions.
Define enthalpy change of neutralisation.
Energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions.
Define standard conditions.
pressure: 100KPa
temperature : 298k
Define bond enthalpy.
How much energy is needed to break bonds
Define average bond enthalpy.
Energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds.
Why are average bond enthalpies used in calculations
Because the energy required to break an individual bond can change depending on where it is.
How do you calculate enthalpy change?
Total energy absorbed - total energy released
(reactants - products)
Describe a bomb calorimeter.
It contains many compartments:
- The outer layer is an air jacket
- Next is a water bath with a thermometer and stirrer
- Lastly is the sample with electrically heated wires and pure oxygen
Why are bomb calorimeters so useful.
They are a sophisticated piece of equipment that minimises heat loss as much as possible.
What equation can be used to calculate enthalpy change?
Q = MCT
Enthalpy change = mass * specific heat capacity * temperature change
What is the specific heat capacity of water?
4.18Kj/mol
What are the steps to calculating enthalpy change of combustion?
1- Calculate the amount of heat given out by the fuel using q=mct.
2- Find out how many moles of fuel produced this heat.
3- Divide Q by the moles.
How does calculating enthalpy change of combustion differ to that of reaction?
For the enthalpy change of reaction you need to find the enthalpy change for the number of moles shown in the balanced chemical equation, instead of calculating per mole.
What does Hess’ law state?
Hess’ law states that:
‘The total enthalpy change of a reaction is always the same no matter, which route is taken’
how do you calculate rate of reaction?
rate of reaction = change in concentration / time
define activation energy
minimum amount of energy needed in order to react
How does temperature affect rate of reaction?
more kinetic energy so there are more collisions
More particles have the minimum activation energy so when particles collide they’re more succesful
What is the affect of concentration on the rate of reaction
What is the affect of concentration on the rate of reaction
What is the affect of concentration on the rate of reaction
there are more molecules in the same volume so they will be close and there will be a greater chance of them colliding with sufficient energy to overcome activation energy
What is the effect of pressure on the rate of reaction?
When the pressure of a gas is increases the molecules are closer together as the same number of molecules occupy a smaller volume and more collisions are likely to occur with sufficient energy to overcome the activation energy
What are the two types of catalysts
Homogenous
heterogenous
Define homogenous catalyst
catalyst in the same phase as reactants
e.g. enzymes
Define heterogenous catalyst
catalyst in a different phase to the reactants
e.g. iron in Haber process
What is the Boltzmann distribution?
A graph depicting the change in number of particles with minimum activation energy at a higher temperature in comparison
Define dynamic equilibrium
the concentration of reactants and products remains constant and the forward and backwards reactions are the same and there is no observable change
What factors affect the position equilibrium
concentrations of reactants and products
pressure in reactions involving gases
temperature
Define le Chateliers principle
when a system in dynamic equilibrium is subjected to a change the position of equilibrium will shift to minimise the change
What is the effect of concentration on the position of equilibrium
increasing the concentration of reactants will cause the system to oppose the change by decreasing its concentration so the position of equilibrium will move to the right forming more products
same goes for the products
What is the effect of pressure on the position of equilibrium
increasing the total pressure of the system causes the position of equilibrium to shift to the side with lower pressure i.e. fewer gas molecules
What is the effect of temperature on equilibrium
Increasing the temperature causes the equilibrium to shift to the endothermic direction in order to decrease the temperature and take in heat
decreasing the temperature will cause the equilibrium to shift to the exothermic direction to oppose the change and release heat to increase the temperature
But too low of a temperature can cause the rate of a reaction to slow
What is the effect of a catalyst on the position of equilibrium
It doesn’t alter the position of equilibrium but rather speed up the forward and reserve reactions equally and increases the rate at which equilibrium is reached
What are the conditions of the Haber process
400-450c
200 atm
iron catalyst
N2 and H2 1:3 by volume
What is the purpose of the equilibrium constant Kc
Chemists use it to consider the position of equilibrium and gives a measure of where the equilibrium lies
What is the equation for Kc
product/reactants
[D]d [C]c/[A]a[B]b
