Chemical Reactions
Physical and chemical changes
Properties
- The properties of physical changes are:
- same internal structure
- reversible
- involves lower amount of energy exchange comapred to chemical reactions
- involves change in texture, shape, temp. or a change in the state of matter.
- Examples of physical changes are an ice cube melting, boiling water, or shredding paper.
- The properties of chemical changes are:
- new molecular bonds formed/broken, creating a new chemical formula
- irreversible
- involves significant energy changes
- involves color changes, releasing scent, formation of a precipitate, or fizzing.
- Examples of chemical changes are iron rusting, baking a cake, or digesting food.
Differences
| Feature | Physical | Chemical |
|---|---|---|
| New substance | No | Yes |
| Atomic bonds | Not broken or formed | Broken and formed to create new molecules |
| Reversibility | Usually reversible | Irreversible or hard to reverse |
| Mass | Stays the same | Stays the same (law of conservation of mass) |
| Visual clues | Changes in size, shape, or state of matter | Fizzing, fire, color change, or significant temperature change |
Rate of reaction
- Rate is a measure of the change that happens in a single unit of time.
- In general, to find the rate of a reaction, you should measure:
- the amt. of a reactant used up per unit of time, or
- the amt. of a product produced per unit of time.
Ways to change the rate of a reaction
- Changing the concentration of solutions
- A reaction goes faster when the concentration of a reactant is increased. Reduce the concentration to slow down the reaction.
- EXPLANATION: There are more particles per unit volume, ∴ higher frequency of collisions ∴ higher frequency of SUCCESSFUL collisions.
- Changing the pressure of gases
- A reaction involving gases goes faster when the pressure is increased. Decrease the presure to slow down the reaction.
- EXPLANATION: Increasing gas pressure means more gas particles per unit volume, ∴ successful collisions are more frequent.
- Changing the surface area of solids
- A reaction goes faster when the surface area of a solid reactant is increased. Decrease the surface area of a solid reactant to slow down the reaction.
- EXPLANATION: More particles of the solid are exposed, ∴ successful collisions are more frequent.
- Changing the temperature
- A reaction goes faster when the temperature is raised. A rate generally doubles for an increase of 10°C. Decrease the temperature to slow down a reaction.
- EXPLANATTION: There are two reasons: there are more collisions, and more have sufficient energy to be successful.
- Adding or removing a catalyst, including enzymes
- A catalyst is a substance that increases the rate of a reaction, and is unchanged at the end of the reaction.
- Enzymes are proteins made by cells, to act as biological catalysts.
- Adding a catalyst increases the rate of a reaction.
- EXPLANATION: A catalyst lowers the activation energy of a reaction, now the particles need less energy to react, ∴ more successful collisions and the reaction is faster.
Collision theory
- For a chemical reaction to occur, reacting particles must collide with one another. Not every collision results in a reaction. A successful collision must meet these conditions:
- The particles must collide with sufficient energy. This is the activation energy.
- The particles must collide with the correct orientation.