ENZALPHY CHANGE

• Chemical reactions involve changes in energy
• Fireworks, for example, give out heat, light, and sound energy when the chemicals are combusted
• Many chemical reactions involve the release of energy
• For other chemical reactions to occur, energy must be supplied
• It is often useful to determine if energy is released or taken in during a chemical reaction by considering the surroundings
• Exothermic reactions
• An exothermic reaction gives out (transfers) energy to the surroundings
• Energy (in Joules, J) is released
• ΔH - the change in heat (energy) is negative
• Endothermic reactions
• An endothermic reaction takes in energy to the surroundings
• In an endothermic reaction energy (in Joules, J) is absorbed
• ΔH - the change in heat (energy) is positive
• Making and breaking bonds
• Energy must be supplied to break the bonds between atoms or ions
• Breaking bonds is an endothermic process
• Different bonds have different strengths and will require different amounts of energy to break
• Bond making is an exothermic process
• Overall energy change
• In an exothermic reaction, the energy released from the new bonds forming is greater than the energy required to break all of the initial bonds
• Energy is released to the surroundings
• In an endothermic reaction, the energy released from the new bonds forming is less than the energy required to break all of the initial bonds
• Energy is absorbed from the surroundings
• Bond energy
• Bond energy is the amount of energy required to break apart a mole of molecules into its component atoms
• In polyatomic molecules, bond energies quoted are averages
• Bond length tells us the distance between the two nuclei involved in the bond
• Double and triple bonds are shorter and stronger than single bonds
• Enthalpy change definitions
• There are 2 types of chemical reactions with energy changes of interest: formation and combustion
• Standard conditions for enthalpy changes are 298 K temperature, 1 atm pressure, and 1 mol dm^-3 concentration
• Standard state is the most stable form of the element under standard conditions
• The standard molar enthalpy change of formation is the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions and with standard states
• The standard molar enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions and with standard states