Energy Changes + Rate Of Reaction
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What is energy change?
Difference of energy release by reactant and product
Definition of exothermic reaction
A reaction that transfers thermal energy (heat) to the surroundings, leading to an increase in temperature of surroundings
Definition of endothermic reaction
A reaction that takes in thermal energy (heat) from the surroundings, leading to an decrease in temperature of surroundings
Characteristics of exothermic reactions
Heat is released Increases temperature Decrease energy level Bonds are formed
Characteristics of endothermic reactions
Heat is absorbed Decreases temperature Increase energy level Bonds are broken
Name 3 physical exothermic reactions
Reverse sublimation Condensation Freezing Dissolving of acid/alkali
Name 3 chemical exothermic reactions
Combustion Respiration Neutralisation Rusting
Name 3 physical endothermic reactions
Sublimation Boiling/evaporation Melting Dissolving ammonia salt in water
Name 3 chemical endothermic reactions
Photosynthesis Thermal decomposition Cooking/baking
What is enthalpy change (∆H)?
Difference in energy constant of reactants and products
What is bond energy?
Energy required to form and break bonds, represents 1 mol of covalent bonds
Formula for enthalpy change
∆H = total energy of products - total energy of reactant
Steps to calculate enthalpy change
• Identify reactants/products • Balance equation • Identify bond structure • Which bonds are broken/formed? • Energy in or energy out?
Do exothermic pathway diagrams for enthalpy change go upwards or downwards?
Downwards
Do endothermic pathway diagrams for enthalpy change go upwards or downwards?
Upwards
Is negative enthalpy change exothermic or endothermic?
Exothermic
Is positive enthalpy change exothermic or endothermic?
Endothermic
What is activation energy?
Minimum energy that colliding particles must have to react
Does activation energy have to be higher or lower than energy of reactants for reactions to occur?
Equal or lower
Why are bonds formed in an exothermic reaction?
Less energy is needed to maintain at the stage. Hence, formed bonds are stable.
What is rate of reaction?
A measure of speed of an occurring reaction
Why are settings of reactions controlled?
For safety purposes
Name two examples of slow reactions
Fermentation Rusting Weathering
Name two examples of moderately fast reactions
Metal + acid Carbonate + acid Browning of cut fruit