Energetics + Kinetics

Thermochemistry: This is the study of heat changes in a chemical reaction
1. During a chemical reaction:
  1. Bonds are broken:
    1. Endothermic
      1. Takes energy from the surroundings
  2. Bonds are reformed:
    1. Exothermic
      1. Gives energy to the surroundings
  3. Whether an reaction is endo or exo thermic overall, you need to decide whether more energy is given out, or more energy is taken in
    1. If more energy is given out i.e. reactants - products = -ve, then you have an exothermic reaction
    2. If more enrgy is taken in i.e. reactants-products = +ve, then you have an endothermic reaction
Enthalpy change is the heat energy change of a chemical reaction under constant pressure
1. An enthalpy diagram looks like so:
  1. The peak is called the intermediate stage or the transition state
    1. In an endothermic reaction, the products have more energy than the reactants
      1. In an exothermic reaction, the products have less energy than the reactants
Measuring energy change:
1. 1. Measuring heat energy change of 2 solutions
  1. We need to know the difference between temperature and heat
    1. Heat: The total kinetic energy of all the particles in a given substance. It is dependent on the no. of particles present
    2. Temperature: This is the AVERAGE kinetic energy for any particle at any given point. It is not dependent of the number of particles present
  2. What happens is that you have 2 solutions of known conc and known volume
    1. You bring them both down to room temperature and note it down
      1. Mix the 2 solutions
        Measure the temperature at the end
        Note the temperature change
        Put it into the formula:
        q = m x c x delta t
        q is the heat energy change
        Measured in J
        To find the heat energy change per mole you put your values into:
        Delta H = q/n
        Delta H is measure in kJ per mole so you have to make sure you convert q from J to kJ by dividing by 1000
        Remember, if the reaction is exothermic, then the energy change will be negative
        m is the mass of the substance being heated, in this case it is the solution, so it's the total volumes added together
        measured in grams where 1cm3 = 1g
        C is the specific heat capacity. Water has a SHC of 4.18
        measured in J/g-1/k-1
        Delta is the temp change
        measured in degrees or kelvin
2. 2. Calorimetry
  1. This is the process used to describe another way of measuring heat energy change
    1. This is usually used when you've got a substance that you dont want to waste
      1. e.g. some new type of fuel
        You put it in a spirit burner
        Light it up and heat a known volume of water
        Record initial temp
        Light for however long you want. When you've finished, weigh the final mass of spirit burner
        Record final temp of water
        Put it in the formula: q= m x c x deltaT
        m will be the mass of the water that has been heated
        then use the formula delta H = q/n
        use n=m/mr to find n
        Record initial mass
    2. Assumptions:
      1. We assume that all the heat is transferred to the water, but that's not true, some of it could be transferred to the beaker or the air
        This could be fixed using a polystyrene cup
        putting a lid on it
        A draught screen to stop flame from flickering
3. 3. Mean bond enthalpies
  1. This is defined as the amount of energy required to break one mole of a particular covalent bond averaged over several compund where the bond exists in a gaseuous state.
    1. It is averaged over several compounds because the particular bond might be easier to break when in one compound but difficult to break when in another compound
      1. The fact that an average is used means that the enthalpy change might not be as accurate
        You get given a table of values
        You add up the total number of bond and multiply it by its M.B.E
        Then you do reactants - products
4. 4. The use of Hess's Law
  1. This law states that the enthalpy change of a reaction is independent of the route taken
    1. In other words, the energy it takes to go from A to B is the same as the energy it takes to go from A to C + the energy is takes to go from C to B because you start at the same place and you finish at the same place
  2. a) The standard enthalpy of formation
    1. This is defined as the enthalpy change when one mole of a compound is formed from its constituent element under standard condition where the reactant and products are in their standard states
      1. Standard Conditions: 25 degrees, 1 atm
      2. This just means, how much energy do you need to form 1 mole of a compound from its elements?
        We can use thermodynamic cycles to help us
        With formation, the arround is always going from the elements, to the reaction e.g.:
        You the add the energies
  3. b) The standard enthalpy of combustion
    1. This is defined as the enthalpy change when one mole of a compund is completely burned in excess oxygen under standard conditions where the reactants and products are in their standard states
      1. Here we look at the enthalpy values when we combust the products and reactants
        We can also use the thermodynamic cycle here
        Here, the arrows go from the products and reactants, down to the oxides that they combust to
        If anything is going in the opposite direction than what we want, then we just reverse the sign on the value
Kinetics is the study of the rate at which chemical reactions proceed
1. It is also the study of factors that speed up the rate of reactions
2. Rate of reaction: This is the change in the concentration of reactants/products per unit time
Collision theory
1. This the idea that in order for a reaction to take place, a collision is required
  1. Reactants need to collide with a big enough force so that bonds are broken.
    1. This is done by achieving the activation energy
      1. This is the minimum amount of energy required to start off a reaction/have a succesful collision
        The reactants also need to be in the correct orientation, meaning the molecules that play an active part need to be facing the same way
Factors that affect the rate of a reaction
1. Surface Area
2. Temperature
3. Conc/Pressure
4. Catalyst
  1. A catalyst is a substance that speeds up a reaction whilst remaining chemically unchanged
  2. It works by providing an alternative route with a lower activation energy
    1. The enthalpy diagram looks like this:
  3. There are 2 types of catalysis
    1. Heterogenous
      1. This is where the catalyst and the reactants are in different phases
        There is a distinct boundary between them
      2. Examples:
        Catalytic Converters in cars
        Catalyst Used: Platinum, Iridium, Rhodium
        Reactants: NOx's, CO, C, unburned hydrocarbons
        Products: N2, CO2, H2O
        Use: Removing harmful gases and pollutants
        Poison: Lead
        Catalytic cracking
        Catalyst: Zeolite (Aluminium Silicate) (s)
        Reactants: Long chain Alkanes (l)
        Products: Cycloalkanes, branched alkanes
        Use: More useful fractions
        The Haber Process
        Catalyst: Iron (Fe) (s)
        Reactants: N2, H2 (g)
        Products: NH3
        Uses: Fertiliser, Drugs, Dyes
        Hydration of Ethene
        Catalyst: H3PO4 on an inert silica support
        Reactancts: C2H4, H2O
        Products: C2H5OH
        Uses: chemical feedstock, solvents, detergents, fuel
    2. Homogenous
      1. This is where the catalyst and the reactants are in the same phase
        Meaning there is no distinct boundary between them
      2. Free Radical Substitution:
        Catalyst: Chlorine free radical
        Reactants: Methane, chlorine
        Products: Chloromethane
        Uses: Refrigerants
      3. Hydrogenation of alkenes
        Catalyst used: Nickel
        Reactants: C2H4 and H2
        Products: C2H6
        Uses: Making margerine
  4. Catalysis takes place in 3 stages:
    1. 1. Adsorption
      1. The formation of weak temporary bonds of the catalyst and the reactants
        The strength of adsorption needs to be monitered, we can't have bonds that are too strong
    2. 2. Reaction takes place
    3. 3. Desorption: Where the products and now seperate from the catalyst
5. Increasing any of these will increase the rate of reaction. This can be shown on a diagram
  1. A steeper curve shows a greater rate of reaction
  2. The diagram shows that at the start of a reaction, there's a faster rate because there are more reactants to have a succesful collision with because there are more reactants around
    1. As time goes on, the rate slows down becayse there's not as many reactants left and the producst get in the way of a succesful collision
Maxwell Boltzman Distrbution Diagram
1. This is a diagram that shows the distribution of energy or spread of energy amongst particles in a sample
  1. The peak of the graph shows the most probable energy
    1. The average energy will always be to the right of the curve
      1. The area under the curve represents the total number of particles
        The curve never meets the axis because you can never predict the maximum amount of energy that a particle will have
        If you lower the temp, you get a sharper peak near the lower end: More particles have a lower energy
        If you increase the temp you get a broader peak because the energy is spread over a range of higher energies
        More particles have an energy that is greater than or equal to the activation energy
        This is why there is a higher rate of reaction
        The area under the curve will remain the same
        Both conc and pressure will have the same affect on the graph
        An increase will shift the graph up
        A decrease will shift the graph down
        The use of a catalyst will shift the Ea
        There will be more particles with an energy that is greater than or equal to the activation energy
        There will be more or less particles with an energy that is greater than or equal to the activation energy
        Fewer particles have an energy that is greater than or equal to the activation energy
        This is why there is a lower rate of reaction

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Energetics + Kinetics

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