Chapter 16 - Chemical Energetics Flashcards
What are the 2 processes
involving energy changes?
Endothermic - Absorbs thermal energy from surroundings causing a decrease in temperature
Exothermic - Releases thermal energy to the surroundings causing an increase in temperature
What are the 3 examples of endothermic reactions that are chemical reactions?
1) Thermal decomposition of carbonate compounds
2) Photosynthesis
3) Electrolysis of water
What are the 5 examples of endothermic reactions that are physical reactions?
1) Boiling
2) Melting
3) Sublimation
4) Dissolution of some ionic compounds
5) Evaporation
What are the 3 examples of exothermic reactions that are chemical reactions?
1) Respiration
2) Neutralisation
3) Combustion
What are the 3 examples of exothermic reactions that are physical reactions?
1) Freezing
2) Condensation
3) Dissolution of acids and alkalis
What are the 2 phases in a chemical reaction?
1) Bond-breaking phase
2) Bond-making phase
What do the atoms in the reactants break? What do the atoms do after breaking? What is this process called and what does it require?
- Atoms in the reactants break their existing chemical bonds
- Atoms then rearrange to form new bonds of the products
- The breaking of bonds requires thermal energy
- Since heat is absorbed from the surroundings, bond breaking is endothermic
After atoms break their existing chemical bonds, what do they do to form products? What process is this called?
- After atoms break their existing chemical bonds, they rearrange to form new chemical bonds to form products
- Since atoms release energy into the surroundings, this bond making is exothermic
What does the positive and negative sign mean in terms of direction if movement of energy to and from suroundings?
(+) positive = energy absorbed from surroundings
(-) negative = energy released into surroundings
What is bond energy?
Bond energy is the amount of energy absorbed to break one mole of a chemical bond. It is also the amount of energy released when one mole of that bond is formed.
What does the bond energy tell us?
Tells us the amount of energy released to the surroundings when one mole of the specific chemical bond is formed (e.g. H-H releases 435kJ/mol)
The ____ the bond energy, the ____ the chemical bond. Why?
The greater the bond energy, the stronger the chemical bond. This is because it indicates that more energy is required to break one mole of the bond (e.g. 1 mole of H-H bonds requires 435kJ of energy to break while 1 mole of Cl-Cl bonds requires 243kJ of energy to break. Hence H-H bond is stronger than Cl-Cl bond)
Which is stronger, singular bonds, double bonds or multiple bonds?
Multiple bonds are stronger than single bonds. However, double bonds are not twice as strong as single bonds
What is activation energy?
Activation energy is the minimum energy that the colliding reacting particles must possess to react with each other in order for a chemical reaction to take place
Describe the timeline of the bond phases
1) Collision & Activation Energy: Reactant molecules collide with each other. If they have at least activation energy (Ea), the reaction starts. If not, they just bounce off without reacting
2) Bond breaking: Atoms in the reactants break their existing chemical bonds. Since heat is absorbed from the surroundings, bond breaking is endothermic
3) Bond forming: After atoms break their existing chemical bonds, they rearrange to form new chemical bonds to form products. Since atoms release energy into the surroundings, this bond making is exothermic
When a reaction has low activation energy, how does the reaction take place?
Easily and spontaneously as less energy is required to break the bonds
When a reaction has high activation energy, how does the reaction take place?
Not as easy, more energy is required to break bonds
What does enthalpy change, ΔH, refer to?
The net difference in energy content between the reactants during bond breaking and products during bond forming
(Total energy absorbed during BB - Total energy released during BF)
If the net energy absorbed is higher than the net energy released, what reaction is occurring?
Endothermic
If the net energy absorbed is lower than the net energy released, what reaction is occurring?
Exothermic
