Topic 3 Flashcards
define standard enthalpy change of reaction
the enthalpy change that accompanies a reaction under standard conditions and all reactants and products are in standard state
define standard enthalpy change of formation
the enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions when all reactants and products are in standard state
define standard enthalpy change of combustion
the enthalpy change when 1 mol of a substance completely reacts with oxygen under standard conditions and all reactants and products are in standard state
define exothermic
heat energy is lost to the surroundings. the energy released from bond formation is greater than the energy absorbed breaking the original bonds
define endothermic
heat energy is absorbed
the energy absorbed breaking the original bonds is greater than energy released forming new bonds
define activation energy and what way does the arrow face on an energy profile
the minimum energy required for a chemical reaction to take place
always up
what are standard conditions and standard state
298K and 1 atm and 1 moldm^-3
the physical state of elements under standard conditions
define enthalpy
heat content stored in a chemical system
define enthalpy change
the heat exchanged with the surroundings during a chemical reaction
the difference between the enthalpy of the products and the reactants
what is the equation for energy change
Q = MCΔT
how do you work out enthalpy change from energy change
ΔH = Q/n
define enthalpy change of neutralisation
the enthalpy change when 1 mol of water is formed from a neutralisation reaction under standard conditions and all reactants and products are in standard state
what are the causes for less energy bring transferred than expected when working out enthalpy change of combustion
heat loss to surroundings
incomplete combustion
evaporation
non-standard conditions
what is the value of enthalpy change of neutralisation
-57.5kJmol
define average bond enthalpy
energy required to break 1 mol of a specified type of bond in a gaseous molecule
what are the properties of bond enthalpy
energy is always required to break bonds]always positive and always have a positive enthalpy value
what kind of energy and enthalpy change are bond formation
exothermic and releases energy
what is Hess’s Law
If a reaction can take place by more than one route, the initial and final concentrations are the same and the total energy change is the same for each route
what is reaction rate
the change in concentration of a reactant/product in a given time
what makes an effective collision
sufficient speed and correct orientation
whta factors can change rate of a chemical reaction
concentration
temperature
pressure
use of a catalyst
surface area of solid reactants
what effect does increasing pressure have on reactants and why
increases rate
particles have more energy so there are more frequent successful collisions and therefore an increased rate
what is the rule of thumb for reaction rate and temperature
a 10 degree increase in temperature doubles the rate
what effect does increasing concentration/pressure have on rate of reaction
increases rate as there are more particles per unit of volume which leads to more successful and frequent collisions and therefore an increased rate
how can progress of a chemical reaction be followed
monitoring removal of a reactant/formation of a product
what 2 ways are there to determine rate of reaction when gas is produced
monitoring volume of gas produced or loss of mass of reactants
what does a catalyst do
increase rate of reaction by lowering activation energy by offering a lower reaction pathway leading to more particles exceeding activation energy
is not used up in the reaction/is regenerated at the end
what is a homogenous catalyst
catalyst that is in the same state as the reactants
how does a homogenous catalyst interact with reactants
forms an intermediate which breaks down to give the product and regenerates the catalyst
give 2 examples of homogenous catalysts
sulfuric acid - production of esters
chlorine radicals in zone repletion
what is different about the equilibrium constant to regular constants
only constant at a given temperature
how does a Boltzmann distribution model show the effects of a catalyst
Ea is lower than normal so a greater proportion of particles exceed the lower Ea
what is Le Chatelier’s principle
when a system in dynamic equilibrium experiences an external change, equilibrium will shift to minimise the effect
does a catalyst have an effect on equilibrium
no
How is Kc worked out in aA + bB -> cC + dD
[C]c x [D]d / [A]a x [B]b
what is an autocatalysis
a reaction product that acts as a catalyst for that reaction
what are the main features of a Boltzmann Distribution model
no molecules have 0 energy
no maximum energy
area under curved is equal to the number of molecules/particles
how does a Boltzmann Distribution curve change when temperature increases
peak is lower and shifted to the right, a greater proportion of molecules can exceed Ea
what are the main properties of a catalyst
not used up in a reaction
regenerated at the end of a reaction
may react with reactant to form an intermediate and provide a surface for the reaction to take place on
what is dynamic equilibrium
when the rate of the forwards reaction equals the rate of the backwards reaction at the same time in a closed system where the concentration of reactants and products remain constants
what are the benefits of using a catalyst
lowers Ea
less electricity is used
product is made faster and cuts costs and increases profits
what is a heterogenous catalyst
a catalyst that is in a different state to the reactants
how does a heterogenous catalyst interact with reactants
usually solids in contact with gaseous reactants / reactants in solution
reactants are adsorbed onto the surface where the reaction takes place
after the reaction, products leave via desorption
give examples of a heterogenous catalyst
iron in the haber process
nickel - hydrogenation of alkenes
