Chemical Reactions and Energy

Energy Exchanges

Some chemical reactions, like the ones observed in class, are rapid.

They occur as soon as the reactants are mixed, and product formation is quick.

However, there are some reactions that are slow.

To speed them up, something has to be added or a variable must be changed.

Collision Theory

The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen.

If there are a higher number of collisions in a system, more combinations of molecules are possible. Therefore, the reaction will go faster, and the rate of that reaction will be higher.

Collision Theory Principles

1) Particles must collide! You cannot make products if the particles do not collide.

2) Particles must collide with ENOUGH ENERGY! What is this energy called?

3) Particles must collide with proper orientation! (molecules have shapes!)

Variables that Affect Reaction Rate

1) Temperature How does increasing the temperature of a

reaction decrease the time it takes to react? You might try adding heat to increase the

energy of the particles and increase the number of collisions.

What other factors might affect how fast a reaction occurs?

Concentration

2) Concentration refers to the amount of a substance in a solution.

If you add more reactants, the reaction will proceed faster. Why?

Surface Area

3) Reactions happen on the surface of a solid (if present).

If you have a solid reactant increasing the surface area will increase reaction rate.

Example: Which will dissolve in water faster,1 gram of small particles of salt or a 1 gram block of salt? Why?

Catalysts

4) Catalysts are substances that increase the rate of reaction and are not consumed in the reaction.

They are not part of the reactants or the products! They lower the activation energy, which allows more particles to collide with enough energy to form products.

Platinum is a catalyst in reactions that synthesize cancer treatment drugs.

Since platinum is expensive, it is good that the platinum can be reused.

Inhibitors

Sometimes scientists want to prevent some reactions from occurring, if the reaction is not the intended one.

To prevent an unwanted reaction, inhibitors are added.

An inhibitor will combine with one of the reactants so that it cannot react with anything else.

General Trend

More Energy Out

Exergonic reactions are those that release energy.

Remember that energy can take many forms: heat and light are examples

When a reaction gives off heat, it is said to be exothermic. To us, the beaker will feel hot as a result.

When would the heat given off in a reaction be beneficial?

Energy Diagrams

In an exothermic reaction, the reactants have more energy than the products.

More Energy In

When more energy is required to break chemical bonds than to form them, the reaction is called endergonic.

When a reaction absorbs heat, it is said to be endothermic. To us, the beaker will feel cold as a result.

Where might this type of reaction be useful?

Energy Diagrams

In an endothermic reaction, the products have more energy than the reactants.