Basic Chemical Kinetics
Mr. Lee says: In general, chemical reactions can be sped up in these ways:
- Increase the temperature: An increase in the average kinetic energy of particles leads to more collisions (and thus, more reactions)
- Increase the Concentration (or Pressure, if it's a gas reaction): It's simple -- more particles in a confined space means more collisions are likely to occur.
- Decrease the Particle Size: This leads to higher surface area exposure. In big clumps, interior particles are "shielded" from the reaction. Example: A cube of sugar will dissolve less quickly than powdered or granulated sugar particles.
- Add a Catalyst: A catalyst lowers the activation energy of a reaction, thus speeding the reaction up. It does NOT alter the heat of the reaction overall. See the example picture. In biology, enzymes are catalysts. Catalysts are not consumed in the reaction.
- Increase the temperature: An increase in the average kinetic energy of particles leads to more collisions (and thus, more reactions)
- Increase the Concentration (or Pressure, if it's a gas reaction): It's simple -- more particles in a confined space means more collisions are likely to occur.
- Decrease the Particle Size: This leads to higher surface area exposure. In big clumps, interior particles are "shielded" from the reaction. Example: A cube of sugar will dissolve less quickly than powdered or granulated sugar particles.
- Add a Catalyst: A catalyst lowers the activation energy of a reaction, thus speeding the reaction up. It does NOT alter the heat of the reaction overall. See the example picture. In biology, enzymes are catalysts. Catalysts are not consumed in the reaction.
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Function of Enzymes
Biological catalysts are called enzymes. Enzymes are proteins (meaning they are made of amino acids) and they can sometimes be easily spotted because they may end in "-ase". For example, the enzyme that breaks down lactose (a sugar) in the body is called lactase.
Biological catalysts are called enzymes. Enzymes are proteins (meaning they are made of amino acids) and they can sometimes be easily spotted because they may end in "-ase". For example, the enzyme that breaks down lactose (a sugar) in the body is called lactase.
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On the image, you can see that the potential energies (delta H) of reactants and products are not changed with a catalyst.
The activation energy does decrease with a catalyst. It's easier to get over the "hill" of energy required to make a reaction go forward when you're using a catalyst... so the reaction speeds up (both forwards and backwards). |
Reaction Energy Diagrams and Reaction Mechanisms (Professor Dave)
The second half of the video is AP-level.
The second half of the video is AP-level.
Thermochemistry: Heating & Cooling Curves
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A heating curve shows the change in temperature as heat is added, starting with the solid form, then through to liquid and gas.
The first "plateau" is the solid-to-liquid phase change/transition, which happens at the melting point temperature. The second "plateau" is the liquid-to-gas phase change/transition, which happens at the boiling point temperature. |
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The cooling curve for a substance is just the opposite of a heating curve.
Heating curves show an increase in energy of the system (because heat is being added). Cooling is a loss of heat energy. |
Thermochemistry: Specific Heat Capacity >>> q=mcΔT
Below, Professor Dave offers an introduction to thermochemistry.
Mr. Lee says: While heating a substance that is not changing phase--like heating up water from 20 °C to 80 °C--the amount of the substance matters! For example, heating up 15 g of water by 60° C takes more joules of energy than heating up just 2 g of water by 60 °C.
The amount of heat energy that is required to raise the temperature of 1 gram of the substance by 1 degree C (or K) is called the specific heat capacity (denoted by the letter c). The specific heat capacity of water is 4.184 J/g·°C, which means that 4.184 joules of heat energy (heat energy = "q") is needed to increase the temperature of 1 g (mass = "m") of water by 1 °C (change in temperature = "ΔT"). A calorie is defined as the amount of energy needed to increase the temperature of 1 g (mass = "m") of water by 1 °C (change in temperature = "ΔT").
1 calorie = 4.184 J
The amount of heat energy that is required to raise the temperature of 1 gram of the substance by 1 degree C (or K) is called the specific heat capacity (denoted by the letter c). The specific heat capacity of water is 4.184 J/g·°C, which means that 4.184 joules of heat energy (heat energy = "q") is needed to increase the temperature of 1 g (mass = "m") of water by 1 °C (change in temperature = "ΔT"). A calorie is defined as the amount of energy needed to increase the temperature of 1 g (mass = "m") of water by 1 °C (change in temperature = "ΔT").
1 calorie = 4.184 J
Endothermic vs. Exothermic (Fuse School)
This video includes an introduction to reaction energy diagrams
This video includes an introduction to reaction energy diagrams
Thermochemistry: Latent Heat during Phase Changes >>> q=nΔHx
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Mr. Lee says: The temperature during a phase change does NOT change, so you can not use q=mcΔT. During a heating phase change, the energy you add (in J or kJ, and less commonly: cal) goes into breaking the intermolecular forces between molecules.
Phase Change Terminology Heating (Endothermic) S → L: Fusion (aka, melting) L → G: Vaporization S → G: Sublimation Cooling (Exothermic) G → L: Condensation L → S: Solidification G → S: Deposition |
Thermochemistry: Enthalpy (ΔH) Stoichiometry (Thermochemical Equations)
Chemical Equilibrium and Le'Chatlier's Principle
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