Physical chemistry Module 3 Flashcards
Enthalpy change definition?
ΔH, the heat energy transferred in a reaction at constant pressure
Units are kj mol^-1
What are the standard conditions?
298 kelvin (25 degrees)
100kPa (1 atm)
Standard states
Exothermic reaction definition?
Reaction gives out energy
Temperature of system goes up
ΔH is negative
Endothermic reaction definition?
Reactions absorbs energy
Temperature of system goes down
ΔH is positive
Activation energy definition?
The minimum amount of energy required to begin breaking reactant bonds and start a chemical reaction
What’s the standard enthalpy change of reaction?
Δ(r)H, Enthalpy change when the reaction occurs in the molar quantities shown chemical equation under standard conditions
What’s the standard enthalpy change of formation
Δ(f)H, enthalpy change when 1 mole of compound is formed from it’s elements in their standard states under standard conditions
What’s the standard enthalpy change of combustion?
Δ(c)H, enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
What’s the standard enthalpy change of neutralisation?
Δ(neut)H, enthalpy change when an acid and an alkali react together under standard conditions to form 1 mole of water
What is bond breaking?
Endothermic as requires energy
What is bond forming?
Exothermic as releases energy
What is enthalpy change in terms of bond forming and breaking?
The overall affect of both of them occuring
Average bond enthalpy of breaking bonds - average bond enthalpy of forming bonds
How is enthalpy change found in the lab?
Calorimetry
How do you calculate enthalpy changes using a formula?
Heat losss or gained in joules = mass of water in grams x 4.18 x change in temperature
Put all values into equation
Convert answer from joules to kJ by dividing by 1000
Work out how many moles of fuel produced this heat
Divide answer by the amount of moles to find standard enthalpy change (kJmol^-1) also make it negative
How to work out enthalpy change using enthalpies of formation?
Write out the reactants equalling the products, then bellow write out all the elements in their natural states that you require
Draw arrows from the natural state elements to the reactants and products, then look how many moles are made and of which substance to work out the enthalpy changes of formation for reactants and products
(if element stay the same won’t be an enthalpy change, remember to multiply by the amount of moles)
Work out total distance from the reactants to products, bearing in mind the first arrow is going in the wrong direction
How can enthalpy changes be worked out using the enthalpy change of combustion?
Same method before, however the direction of arrows have changed and so has what’s on the bottom
On the bottom have water and CO2 and both the lines will be going downwards with an amount of oxygen next to them to balance the equation
Work out total distance taking into account second line is going to wrong way
What must occur in a particle collision for a reaction to happen?
Must collide in the right orientation
Must collide with at least the activation energy
How to draw a Boltzman distribution?
Number of particles on y axis
Kinetic energy on x axis
Draw a line with a gradient of 1 going up into a smoth curve which then goes down into an asmyptote
Draw a line near the end to show the activation energy
What happens to a boltzman distribution if you increase the temperature
Peak moves to the right and also a little bit downwards
Activation energy is in the same place but now there is a higher percent of particles past the activation energy
How does increasing concentration speed up reactions?
The particles will be closer together, so they’ll collide more frequently. If there are more collisions more chances to react
How does increasing pressure speed up reactions?
If any reactants are gases, the particles will be closer together, so will collide more frequently, so more chances to react
How do catalysts speed up reactions?
They lower the activation energy by providing an alternative reaction pathway, if activation energy is lower more particles will have enough to react
They are chemically unchanged at the end of the reactiom
How does temperature speed up the rate of reacriom?
Particles exposed to more thermal energy so have more kinetic energy, so will move faster meaning greater proportion of molecules will have more than activation energy, also there will be more frequent collisions
What happens to the boltzman distribution if you introduce a catalyst?
The activation is lowered
What’s a heterogenous catalyst?
In a different physical state to the reactants
What’s a homogenous catalyst?
Catalyst which is in the same physical state as the reactants
Rate of reaction formula?
Rate of reaction = amount of reactant used or product formed / time
2 different ways to measure rate of reaction?
Change in mass
Volume of gas given off
How do you work out reaction rate from the graph?
Gradient (change in y/change in x)
If it’s not a straight line find the gradient of the tangent at the certain points
How to draw an endothermic enthalpy profile diagram?
Enthalpy, kJmol^-1 on y axis
Progress of reaction on x axis
The more stable Reactants are going to start at low energy, then there will be a big curve which goes up to less stable products
The distance from the reactants to the top of the curve is the activation energy
How to dran an exothermic enthalpy diagram?
Enthalpy, kJmol^-1 on y axis
Progress of reaction on x axis
The less stable reactants are going to start at a high energy, and then will produce a small curve which goes all the way down to the bottom for the more stable products
The activation energy is the distance from the products to the top of the curve
