Chemistry Unit 2

Energetics

Endothermic and Exothermic Reactions

Thermochemistry

• If at the end of a reaction energy has been given out, the reaction is exothermic
  • an example of this is neutralisation
• If at the end of a reaction energy has been taken in, the reaction is endothermic
  • an example of this is the breakdown of limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide

Enthalpy

Enthalpy change (ΔH)

• heat energy change at constant pressure
• standard condition (100KPa, 298K)

Physical States

• the physical states of the reactants and products affects the enthalpy change of a reaction

Measuring enthalpy changes

Standard enthalpies

• Standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all in their standard states
• Standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from it's elements under standard conditions, all in their standard states

Measuring the enthalpy change

q = m.c.ΔT

q = measured enthalpy change

m = mass of substance

c = specific heat capacity

ΔT = temperature change

The Simple Calorimeter

• can use a simple calorimeter to find the enthalpy change when a fuel burns
• burn the fuel in a known mass of water and then measure the temperature rise

Hess's Law

• the enthalpy change for a chemical reaction is the same, independent of the route taken
• energy can't be created or destroyed

Enthalpy of formation

• FOPOMIR (formation = products  - reactants)

Enthalpy of combustion

• CRAMP (combustion = reactants - products)

Kinetics

Collision Theory

• most collisions between molecules do not lead to reaction because they don't have enough energy or are the wrong orientation 

Factors that affect the rate of chemical reactions

• Increasing the temperature - the speed and therefore kinetic energy
• Increasing the concentration of a solution - more particles means more chance of collisions, however then all particles are used up which slows the rate of reaction
• Increasing the pressure of a gas reaction - more particles so bigger chance of collisions
• Increasing the surface area of solid reactants - more sites for reaction

Activation Energy

• for a collision to result in a reaction, the molecules must have a certain minimum energy to start breaking bonds

The Maxwell-Boltzmann distribution

The effect of temperature on reaction rate

• at higher temperatures the peak of the curve is lower, and it moves to the right
• the number of particles with very high energy increases
• the graphs show that at higher temperatures more of the molecules have energy greater than Ea (activation energy) so more reactions will take place
• a small increase in temperature produces a large increase in the number of particles with energy greater than Ea

Catalysts

• substances that affect the rate of chemical reactions without being chemically changed themselves
• used in industry to speed up reactions, which is cheaper than increasing the temperature or pressure

How catalysts work

• provide a different pathway for the reaction with a lower activation energy
• don't affect the enthalpy change or…