3.2 Physical Chemistry Flashcards
what is enthalpy change
All chemical reactions involve a change in energy.
The exchange of energy between the reaction mixture and the surroundings is enthalpy change ΔH in joules.
what is an exothermic reaction
Exothermic- energy is transferred from reaction to surroundings. Chemical energy is released and temperature of surroundings increase. ΔH is negative as energy is lost.
what is an endothermic reaction
Endothermic- ΔH is positive as products have higher energy than the reactants and heat is taken in from surroundings.
how to test for enthalpy change experimentally
Reactions in solution Result obtained is an approximate as heat is lost Energy released or absorbed when the reaction takes place, q, can be calculated using q=mcΔT m-mass of reaction mixture C- specific heat capacity ΔT- change in temperature. Solvent used mostly is water. ΔH= q/1000 or (mcΔT)/1000n
link between enthalpy change and temperature
If ΔT increases, the reaction is exothermic so ΔH is negative
If ΔT decreases the reaction is endothermic, so ΔH is positive
what is bond enthalpy
The enthalpy change required to break and separate 1 mol of bonds in the molecule of gas so the resulting gaseous particles exert no forces upon each other.
X-Y(g) → X(g) + Y(g)
ΔH= sum of reactant bond enthalpy – sum of product bond enthalpy
standard enthalpy change of combustion
the enthalpy change which occurs when 1 mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state
standard enthalpy change of reaction
the enthalpy change which occurs when equation quantities of materials react under standard conditions
standard enthalpy change of formation
the enthalpy change which occurs when 1 mole of the compound is formed from its elements under standard conditions and with everything in it’s standard state
standard enthalpy change of neutralisation
the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
what is Hess’ law
States that if a reaction can take place by more than 1 route, the enthalpy change for the reaction is the same irrespective of the route taken, provided the initial and final conditions are the same
[for mechanism see physical flashcard]
how to measure the rate of reaction
Change in mass Change in volume Change in concentration Change in acidity Change in color
factors affecting the rate of reaction
The frequency of collisions can be increased by:
- Increasing the concentration
- Increasing the surface area of a solid
- Increasing the temperature
- Increasing the pressure in a gaseous reaction
what is collision theory
A reaction cannot take place unless a collision occurs between the reacting particles.
For a reaction to occur:
Colliding particles must have the correct orientation
Energy needs to exceed activation energy
what is the effect of concentration on rate of reaction
increases the chance of a collision occurring between the reacting particles. The more collisions there are, the faster the rate of reaction.
Increasing the pressure has the same effect for gaseous molecules as it compresses the gas and increases the concentration
what is the effect of surface area on rate of reaction
If a reactant is solid, its surface area will affect the rate at which it is able to react.
Breaking a surface into smaller pieces will increase its surface area and thus more of its surface is exposed to a reagent and the reaction will proceed faster
what is the effect of a catalyst on rate of reaction
Alter the rate of reaction without getting used up by the reaction.
Provide an alternate route, or mechanism, for the reaction that has a lower activation energy.
The enthalpy change and distribution of energies remain the same
what is heterogenous catalysts
In a different phase to the reactants
what are homogenous catalysts
In the same phase as the reactants
what does a Boltzmann distribution curve show
The curve going through the origin shows there are no particles with 0 energy
At high energy the curve never touches the horizontal axis showing there is no theortetical maximum energy
Area under the curve represents the total number of particles
what must happen for a reaction to occur and how does temperature influence this
A reaction can only occur if the colliding particles have an energy greater than, or equal, to the activation energy. Increasing the temperature increases the energy of the particles and therefore lowers the activation energy.
what are reversible reactions
A reaction that can go both backwards and forwards
what is a dynamic equilibrium
When the forward and the backward reaction are equal.
the system is in equilibrium because the amount of each chemical stays constant
The equilibrium is dynamic because the reactants and products are both constantly interacting
Can only be achieved in a closed system
what is Le Chatelier’s principle
States that if a closed system at equilibrium is subject to a change, the system will move to minimise the effect of the change
how does Le Chatelier’s principle work
If the concentration is increased, the system will move to decrease the concentration of that component
If the pressure is increased, the system will move to decrease the pressure
If the temperature is increased, the system will move to decrease the temperature
what is the effect of changing pressure on equilibrium
The greater the number of gas molecules, the greater the pressure is. If pressure is increased in a system, the equilibrium position alters to resist the increased pressure by reducing the total number of gas molecules. This is achieved by moving to the side where there are less gas molecules
what is the effect of heat on equilibrium
If a temperature increases, the mixture will respond by attempting to lower the temperature again by moving the equilibrium in the exothermic direction
what is the effect of catalysts on equilibrium
A catalyst doesn’t alter the amount of product.
Speeds up the forward and backward reactions equally.
Reduce time taken to reach equilibrium
what is the rate of reaction equation
change in concentration/ time
what is the equilibrium constant equation
aA + bB ⇌ cC + dD, so Kc= [products]/[reactants]
is to the power of moles
what do Kc values tell us
=1 means equilibrium is halfway
>1 equilibrium is towards the products
<1 equilibrium is towards the reactants
