- Created by: theawkwardgrape
- Created on: 03-01-17 12:17
Is a special form of potential energy that lies within chemical bonds - forces of attraction that bind atoms together in a compound.
- During a reaction, bonds break in reactants and form in products.
- This changes the chemical energy.
- Bonds break = exothermic (-)
- Bonds form = endothermic (+)
Heat content stored in a chemical system.
Is the head energy transferred in a reaction.
Exothermic and Endothermic Reactions
Law of conservation of energy
"If heat is released, the amount of energy that leaves a chemical system is exactly the same as the amount that goes to the surroundings. No heat energy is lost."
- Enthalpy of products is smaller than the ethalpy of reactants.
- Heat loss to surroundings.
- Negative enthalpy change.
- Enthalpy of products is greater than the enthalpy of reactants.
- Heat gain to chemical system.
- Positive enthalpy change.
Enthalpy Change Of...
(standard conditions: 100kPa, 298K)
Standard enthalpy change of reaction ΔrHo
Enthalpy change when the reaction occurs in the molar quantities shown in the equation.
Standard enthalpy change of formation ΔfHo
Enthalpy change when 1 mole of a compound is formed directly from its elements.
Standard enthalpy of combustion ΔcHo
Enthalpy change when 1 mle of a substance is completely burned in oxygen.
Standard enthalpy change of neutralisation ΔneutHo
Enthalpy change when an acid and alkali react together.
Activation energy is the minimum energy required for a reaction to take place.
Total enthalpy change is independent of the route taken.
- Enthalpy of Formation
- F = products - reactants
- Enthalpy of Combustion
- C = reactants - products
- It is not always possible to measure the enthalpy change directly: high Ea, slow reaction and more than one reaction can take place.
- Breaking bonds = endothermic as energy is needed.
- Forming bonds = exothermic as energy is released.
Bond dissociation enthalpy is the amount of energy needed per mole (g)
Average bond enthalpy is the energy needed to break 1 mole of bonds in gaseous states over many different compounds.
Determining Enthalpy Changes
Determining enthalpy changes
- Q = heat lost/gained.
- m = mass of water (g)
- C = specific heat capacity - 4.18
- ΔT = change in temperature.
There may be different values with experimental data and text books...
- Incomplete combustion
- Heat loss to surroundings
- To get a more accurate result, use a bomb calorimeter.
Enthalpy change that takes place when breaking 1 mole of a given bond in the gasous states.
- Energy needed to break bond : bonds formed stronger than bonds broken.
- Energy needed to form bond : bonds broken are stroner than bonds formed.
ΔH = Σ(bond enthalpy of bonds broken) - Σ(bond enthalpy of bond formed)
Rate of reaction
The change in the concentration of products and reactants in given time.
Rate = change in concentration moldm-3 s-1
States that for a reaction to take place molecules must:
- Collide with the minimum amount of kinetic energy.
- Collide in the right direction (orientation).
Factors Affecting Rate of Reaction (i)
- Increase in conc = increase in ROR.
- More molecules in the same vol - more frequent and successful collisions.
- Increase in pressure of gaseous reactants = increase in conc = increase in ROR.
- Same number of molecules occupy a smaller volume - more frequent and successful collisions.
- Increase in temp = increase in ROR.
- More kinetic energy = more molecules overcoming activation energy - collide more successfully.
Factors Affecting Rate of Reaction (ii)
4) Surface Area
- Using smaller particles increases SA = increase in ROR.
- Allows more frequent and successful collisions.
Lowers the activation energy of a reaction by providing an alternative route. e.g. transition metals - Fe in haber proccess or enzymes.
- More particles overcome Ea.
- Less energy, less fossil fuels burnt, less CO2 released.
The Boltzmann Distribution
- Area under the curve = the number of molecules in the sample.
- No molecules in the system with 0 energy.
- No maximum energy for a molecule.
- Only molecules with energy under the Ea are able to react.
Effect of Factors on the Boltzmann Distribution
- Kinetic energy of molecules increases.
- Area under the curve remains the same.
- Flattens and shifts to the right.
- Ea is lowered.
- More molecules in the system overcome the Ea.
The rate of forwards and backwards reaction are the same - so the conc of products and reactants are the same. (reached in reversible reactions)
Le Chatelier's Principle
When a system in dynamic equilibrium changes, the equilibrium will shift to oppose the change.
The position of the equilibrium can be changed by...
- Concentration of reactants/products.
Changing the Position of the Equilibrium
- Increase in reactants = shifts to the right. (RR)
- To decrease conc of reactants.
- Increase in products = shifts to the left.
- To decrease conc of products.
- Increase in pressure = shifts to the side with fewer moles.
- In order to decrease the pressure.
- Increase in temp = adding heat = moves to the endothermic side.
- To absorb heat.
- Decrease in temp = removing head = moves to the exothermic side.
- To replaces heat.
- Catalysts help reach the equilibrium faster - NOT changing the position.
- You have to compromise...
- A low temp = slower rate.
- A high pressure = too expensive and dangerous.
- Temp = 400-500°C.
- Pressure = 200 atm.
- Catalyst = Iron.
- Temp = 300°C.
- Pressure = 60-70 atm.
- Catalyst = H3PO4
The Equilibrium Constant
The Equilibrium Law
Tells us the relative proportions of reactants and products present in an equilibrium.
aA + bB <--> cC + dD [ABCD] = concentration. [abcd] = balancing numbers.
Kc = ([D]^d) x ([C]^c)
([A]^a) x ([B]^b)
- If Kc is 1 = halfway between the reactants and products.
- If Kc is smaller than 1 = products favoured.
- If Kc is bigger than 1 = reactants favoured.
- Forward reaction = endothermic.
- As temp increases, Kc increases.
- Conc of products increase, conc of reactants decrease.
- Backward reaction = exothermic.
- As temperature increases, Kc decreases.
- Conc of products and reactants decrease.