Rates and Equilibrium

Question 1

What are the 4 key steps of chemical reactions?

Answer 1

• Collision of particles
• Breaking of existing bonds
• Rearrangement of particles
• Formation of new bonds

Question 2

What is the heat content (H) or enthalpy of a substance?

Answer 2

It is the chemical energy of a substance.
change in H = H(products) - H(reactants)
change in H = amount of energy released or absorbed in kJ per mol combusted.
Exothermic reactions release heat to the surroundings:
change in H<0, H(products) < H(reactants)
Endothermic reactions absorb heat from the surroundings:
change in H>0, H(products) > H(reactants)

Question 3

What is the rate of reaction? Describe rate graphs. How can concentrations during reactions be determined?

Answer 3

The rate of a reaction is described as the change in concentration of a reactant or a product per unit of time (usual unit is M/s).
The graph can show the volume of a substance on the vertical axis and the time on the horizontal. The gradient of these graphs are used to determined the rate of reaction.
Concentration can be determined by looking at:
- colour changes
- pH
- volume of gases

Question 4

Outline the collision theory.

Answer 4

The collision theory is based on the assumption that for a reaction to occur it is necessary for the reacting species to come together or collide with one another. Thus, according to the collision theory, the reaction rate is equal to the frequency of effective collisions between reacting particles.
Successful collisions occur if particles:
- collide with each other
- have the correct orientation
- have enough kinetic energy to break bonds (otherwise they will bounce off and no reaction occurs)

Question 5

Why is molecular orientation important within reactions?

Answer 5

Successful collisions occur when particles collide at the correct orientation with sufficient energy and thus the reaction can occur. On the other hand, unsuccessful collisions occur if particles collide at the wrong orientation and the reaction cannot occur.

Question 6

What is activation energy and what is its significance in reactions? Describe the process of reaction.

Answer 6

For a reaction to occur, particles must collide with energy equal to or greater than the activation energy (Ea). When the activation energy is absorbed, a new arrangement of atoms occurs. This is known as the transition state (TS) and occurs at the stage of maximum potential energy in the reaction where all bonds of reactants have been broken and they are able to reorient themselves. The atoms then release energy as they form the new bonds of the products.

Question 7

What are the 5 major factors that affect the rate of a reaction?

Answer 7

• The surface area of solids
• The concentration of reactants in solution
• The pressure of gaseous reactants
• The temperature of the reaction
• The presence of catalysts