Controlling the Rate

Question 1

What is the rate of a reaction proportional to?

Answer 1

Time

Question 2

What is the equation of rate?

Answer 2

Rate is equal to 1 divided by time

Question 3

How can the rate of reaction be increases?

Answer 3

• decreasing particle size
• increasing concentration of a reactant in solution
• increasing the temperature at which the reaction occurs
• use of a catalyst
• increasing pressure

Question 4

What is the collision theory?

Answer 4

For a chemical reaction to occur, reactants must come into contact (collide). Not all collisions are successful for the reaction to occur.

Question 5

What are the two conditions that are required for a successful collision?

Answer 5

• The correct collision geometry must collide with the right orientation.
• Enough energy to break the existing bonds within the reactant molecules.

Question 6

Why does the increase in temperature cause an increase in the rate of reaction?

Answer 6

There are more particles with energy equal or greater than the activation energy.

Question 7

What is activation energy?

Answer 7

Activation energy is the minimum kinetic energy required by Colliding particles before the reaction will occur.

Question 8

What will happen during an unsuccessful Collision?

Answer 8

The reactants will collide, there is not enough energy to break the bonds so the reactants will move apart.

Question 9

What will happen during a successful collision?

Answer 9

The reactants will collide, there is enough energy to break the existing bonds that is greater or equal to the activation energy, the products move apart.

Question 10

What is temperature?

Answer 10

Temperature is a measure of the average kinetic energy of the particles of a substance.

Question 11

What is the effect of temperature on the reaction rate?

Answer 11

There is an increased number of particles with energy greater than the activation energy. With a higher temperature on a energy distribution diagram, the higher temperature will have a smaller peak and more the the right of the lower temperature.

Question 12

What do energy distribution diagrams show?

Answer 12

The effect of changing temperature on the kinetic energy of particles

Question 13

What is an exothermic reaction?

Answer 13

An endothermic reaction causes heat to be released to the surroundings. The products have less potential energy than the reactants. The enthalpy change for exothermic reactions are negative.

Question 14

What is an endothermic reaction?

Answer 14

An endothermic reaction causes absorption of heat from the surroundings. The products contains more potential energy than the reactants. The enthalpy change is positive.

Question 15

What is enthalpy change?

Answer 15

Enthalpy change is the energy difference between the reactants and products.

Question 16

What are units for enthalpy change?

Answer 16

kJmol^-1

Question 17

What is the pathway of a chemical reaction?

Answer 17

In the reactants the bonds break, this forms an activated complex, the new bonds then form forming the product.

Question 18

What is the energy required to break the bonds in the reactants known as?

Answer 18

Activation energy.

Question 19

What is an activated complex?

Answer 19

An activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier during a reaction.

Question 20

When will a reaction be exothermic?

Answer 20

If the total energy change for the bond breaking step is less than for the bond making step.

Question 21

When will a reaction be endothermic?

Answer 21

If the total energy change for the bond breaking step is more than for the bond making step.

Question 22

What does the higher the higher barrier on potential energy diagrams?

Answer 22

The higher the barrier the slower the reaction.

Question 23

What is a catalyst?

Answer 23

A catalyst is a substance which alters the rate if reaction (decreases the activation energy) without being used up in the reaction.

Question 24

What are the two types hat catalysts can be classified as?

Answer 24

• Homogenous

- heterogenous

Question 25

How to heterogeneous catalysts work?

Answer 25

The reactants bind to the active site. The bonds break. The new bonds reform and the product is released.

Question 26

What effect does a catalyst have on activation energy?

Answer 26

It lowers the activation energy.

Question 27

What are bond enthalpies?

Answer 27

The energy required to break a special bond.

Question 28

What is the mean bond enthalpy?

Answer 28

The average energy required to break a bond calculated from a different environment.

Question 29

How to calculate the enthalpy change?

Answer 29

1. Calculate the bond breaking step.
2. Calculate the bond making step.
3. Do enthalpy change = bond making + bond breaking
4. If the bond breaking value is greater than the bond making step then the reaction is endothermic. If the bond breaking value is less than the bond making step then the value is exothermic.
5. Put the appropriate sign
6. Units at kilo Jules per mole

Question 30

What happens when carbon and hydrogen or carbon, hydrogen and oxygen reaction?

Answer 30

They form carbon dioxide and water.

Question 31

What is equilibrium?

Answer 31

In a closed system, reversible reactions attain a state of dynamic equilibrium when the rates of the forward and reverse reactions are equal.

Question 32

What happens when you increase the concentration of the reactants on the LHS of the equation?

Answer 32

This increases the rate of the forward reaction and therefore shifts the equilibrium to the right.

Question 33

What happens when you decrease the concentration of reactants on the LHS of the equation?

Answer 33

This decreases the rate of the forward reaction and the equilibrium shifts to the left.

Question 34

What does Le Chatelier’s principle state?

Answer 34

Le Chatelier’s principle states that if a dynamic equilibrium is disturbed by changing the conditions the position of the equilibrium shifts to counteract the change to re-establish the equilibrium.

Question 35

What happens to the position of the equilibrium when the temperature is increased?

Answer 35

Increasing the temperature will favour the endothermic reaction.

Question 36

What happens to the position of the equilibrium if the temperature decreases?

Answer 36

Decreasing the temperature will favour the exothermic reaction.

Question 37

What happens to the position of the equilibrium if the pressure increases?

Answer 37

Increasing the pressure favours the side with less moles of gas molecules.

Question 38

What happens to the position of the equilibrium when the pressure is decreased?

Answer 38

Decreasing the pressure favours the side with more moles of gas molecules.

Question 39

What happens if there are the same number of moles of gas on each side of the equilibrium?

Answer 39

The change in pressure will have no effect.

Question 40

What happens to the position of the equilibrium if a catalyst is added?

Answer 40

The addition of the catalyst increase the rate of the forward and reverse reaction equally. The catalyst increases the rate at which the equilibrium is achieved but does not have affect on the position of equilibrium.