Kinetics and energetics

• Collision theory is that for a reaction to occur successfully, particles must collide with energy greater than or equal to the activation energy and in the correct orientation.
• The energy of particles in a substance are distributed in a Maxwell-Boltzmann distribution where zero molecules have zero energy, there is no theoretical maximum energy of a particle (asymptotic), the most probable energy is the peak of the curve, the mean energy is to the right of the the most probable energy, and the activation energy is way over to the right.
• The total area under a Maxwell-Boltzmann curve represents the total number of molecules and so is always constant even when conditions change.
• When a substance is heated, the particles move faster and collide more frequently. As such, the peak of the Maxwell-Boltzmann curve moves lower and to the right when the temperature is increased, and higher and to the left when the temperature is decreased.
• When the concentration or pressure of a substance is increased, collisions between the molecules become more likely as they are more closely packed together, and so the Maxwell-Boltzmann curve moves to the right.
• A catalyst increases the rate of reaction without being chemically changed by providing an alternate reaction pathway that requires a lower activation energy.
• When a catalyst is added to a reaction, the Maxwell-Boltzmann curve is unaffected, but the activation energy is moved to the left.
• Bond making is exothermic and bond breaking is endothermic.
• The enthalpy change of a reaction is the energy require to break bonds minus the energy required to make bonds.
• Enthalpy of formation is the enthalpy change when one mole of a substance is produced from its elements under standard conditions.
• Enthalpy of combustion is the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
• Mean bond enthalpies are the average energy to break a bond across all molecules.
• Calorimetry is an experimental method for finding enthalpy change by measuring temperature change over time, them extrapolating the data on a graph.
• Enthalpy change equals mass multiplied by specific heat capacity multiplied by temperature change.
• Specific heat capacity is the energy require to raise the temperature of one kilogram of a substance by one kelvin, without a change of state.
• Hess' law states that since energy in a reaction system must be conserved, the overall enthalpy change for a reaction is the same, regardless of the route taken.
• Bond enthalpy is the energy required to break one mole of a bond in a gaseous state under standard conditions.
• Hess' law can be used to form triangular diagrams to work out the enthalpy change of a reaction by using known enthalpy of combustion or formation values.
• In a Hess diagram for combustion, draw an arrow downwards, and for formation, draw an arrow upwards. If arrows are in the direction of reaction progress (anticlockwise), add the enthalpies together and if any arrows are against the direction of the reaction progress, then that enthalpy is subtracted to produce an overall enthalpy change.