Topic 3: Chemical Reactions - Subtopic 2: Energetics
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Why do chemical reactions occur?
Elements can achieve a more stable energy state by gaining a full outer shell of electrons
What does the process of a chemical reaction involve?
• Transfer of energy in and out of reaction mixtures • System and Surroundings • Energy comes from chemical bonds themselves • Energy most commonly comes in form of heat energy
What happens to energy and the temperature of surroundings and system in an exothermic reaction?
• Energy given out • Temp. of surroundings increase • Temp. of system decrease (energy transferred from chemical energy store of chemical system) • Overall transfer: System –> Surroundings
What are typical exothermic reactions?
• Combustion • Oxidation • Neutralisation
What are examples of practical uses of exothermic reactions?
• Hand-warmers • Self- heating cans use reaction in base of container
What happens to energy and the temperature of surroundings and system in an endothermic reaction?
• Energy taken in • Temp. of surroundings decrease • Temp. of system increase (energy transferred to chemical energy store of chemical system) • Overall transfer: Surroundings –> System
Which are more common: exothermic or endothermic?
Exothermic
What are typical endothermic reactions?
• Electrolysis • Thermal Decomposition • First Stages of Photosynthesis
What is a practical use of a endothermic reaction?
• Sport injury treatment cold packs • Take heat away from injured area to prevent swelling
How can the changes in heat content be determined?
• Thermometer • Temperature Probe
What is activation energy?
Minimum amount of energy required for the collision to be successful
What causes different activation energies in different reactions?
• Chemical identities involved
What are reaction profiles?
Graphical Representation of the relative energies of the reactant and products in chemical reactions
What does an exothermic reaction profile look like?
• Energy given out • Energy of products less than reactant • Downward arrow
What does an endothermic reaction profile look like?
• Energy taken in • Energy of products higher than reactant • Upward arrow
What process is bond breaking?
• Energy needed to break bonds • Absorbed from reaction surroundings • Endothermic
What process is bond forming?
• Energy released to form bonds • Releases back to surroundings • Exothermic
What is the energy change of an endothermic reaction?
Positive
What is the energy of an exothermic reaction?
Negative
What is the bond energy associated with the chemical bond known as?
Mean amount of energy required to break bond, or given out when bond is formed
How to calculate total energy change in reaction by using bond energies?
• Add together all bonds in reactants • Add together all bonds in products Total Change = Reactants - Products