Endothermic and Exothermic Reactions

What is enthalpy change (delta H) for an exothermic reaction?

Negative

What is enthalpy change (delta H) for an endothermic reaction?

Positive

Which side is energy added to in an exothermic reaction?

Right side

Which side is energy added to in an endothermic reaction?

Left side

What is a catalyst?

A substance which provides an alternative reaction pathway, with a lower activation energy required.

What are the physical properties of exothermic reactions?

• System loses energy

• System becomes cooler

• Surroundings become hotter

What are the physical properties of endothermic reactions?

• System gains energy

• System becomes warmer

• Surrounds become cooler

Does bond breaking release or require energy?

It requires energy, as energy is needed to break the strong bonds. This is the opposite for bond forming.

What does it mean if energy absorbed to break bonds is LESS than the energy released?

The net release is exothermic. Graph goes below the initial value.

What does it mean if the energy absorbed to break bonds is MORE than the energy released?

The net release is endothermic. Graph goes above initial value.

What is enthalpy?

The total energy within the system.

State the three principles of collision theory

1. Reacting particles must collide

2. Particles must collide with a specific minimum amount of kinetic energy (activation energy)

3. Must collide with a favourable orientation

What is the transition state or activated complex?

The point where enough activation energy has been absorbed, and all bonds have been broken. From this point on, energy is reabsorbed into the new bonds that begin to form.

What does Ea represent?

Energy difference between reactants and transition state which represents activation energy

What are the two main ways that allow for a reaction to be sped up?

• Increase frequency of collisions per second

• Increase amount of energy they have

What are the factors affecting reaction rate?

1. Nature of reactants

2. State of subdivision

3. Concentration of reactants

4. Gas pressure

5. Temperature

Describe the way that the nature of reactants affects reaction rate

• Covalent bonds are harder to break than ionic bonds; therefore they are slower

• If only weak/no bonds are broken, needs a low activation energy, and large numbers of particles will have enough energy for a successful collision

Describe the way that the state of subdivision affects reaction rate

• Smaller particles size = faster reaction rate (more exposed surface area)

• More particle collisions per second if more surface area is exposed

• Same percentage of successful collisions

• Eg: Aerosols and mists increase surface area

Describe the way that the concentration of reactants affects reaction rate

• As concentration increases, there are more reactant particles in a given volume

• More particle collisions per second

• More successful collisions per second (not greater percent of successful collisions)

Describe the way that gas pressure affects reaction rate

• Raising pressure can be done by:

  • Decreasing volume

  • Adding more gas particles (increasing concentration)

• Results in greater frequency of particle collisions

• Leads to an increase in frequency of successful collisions

Describe the way that temperature affects reaction rate

Only factor that has a two-fold effect on reaction as it:

• Increases number of collision per second

• Increases amount of energy particles have (more significant!)

Describe the increase in number of collisions per second

• Particles at a higher temp have more Ek than those at lower temp

• Particles are thus moving faster at higher temps

• Means they will collide more frequently, increase in frequency of particle collisions and thus an increase in frequency of successful collisions

Describe the increase in the amount of energy the particles have

• Effect is more significant

• When particles with more Ek collide, a greater percentage of collisions will have sufficient energy to be successful (collision theory)

• Results in an increase in reaction rate