6. The rate and extent of chemical change (the rate of reaction, reversible reactions and dynamic equilibrium)

Question 1

How is rate of reaction calculated?

Answer 1

rate of reaction = amount of reactant or product used/time
rate of reaction = mols of reactant used or product formed/time

Question 2

What are the various units for rate of reaction?

Answer 2

g/s, cm3/s or mol/s

Question 3

Name three common ways of measuring the rate of reaction

Answer 3

• loss in mass of reactants
• volume of gas produced
• time for a solution to become opaque

Question 4

Describe measuring the rate by monitoring mass loss

Answer 4

• Place a reaction flask on a balance. In these reactions, a gas is given off, so record the decrease in mass in time intervals
• Plot a graph of mass vs time

Question 5

Describe measuring the rate by monitoring the volume of a gas

Answer 5

• connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals
• plot a graph of volume vs time

Question 6

Describe measuring the rate by monitoring the disappearance of a cross?

Answer 6

Take a piece of paper and mark a cross on it. Put a reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross

Question 7

Explain the graphical method for determining the rate of reaction.

Answer 7

• draw tangents to curves and use the
  slope of the tangent
• calculate the gradient of the tangent on the curve on these graphs

Question 8

List the five factors that can influence the rate of a chemical reaction

Answer 8

• concentration
• surface area
• pressure
• temperature
• catalysts

Question 9

What is collision theory?

Answer 9

chemical reactions can occur only when reacting particles collide with each other and with sufficient energy

Question 10

What is activation energy?

Answer 10

the minimum amount of energy that particles must have to react

Question 11

Describe and explain the effect of increasing temperature on the rate of reaction?

Answer 11

• If temperature increases, there is a faster rate of reaction
• because as the temperature increases, kinetic energy of the particles increases so there are more energetic collisions
• and the particles move faster, so they collide more frequently

Question 12

Describe and explain the effect of increasing concentration on the rate of reaction

Answer 12

• when concentration increases, the rate of reaction also increases.
• because more reactants means more frequent collisions

Question 13

Describe and explain the effect of increasing the pressure of a gas on the rate of reaction?

Answer 13

• Increasing the pressure of reacting gases, increases the number of gas molecules in the same volume.
• so it increases the frequency of collisions and therefore the rate of reaction

Question 14

What are volume and pressure to one another?

Answer 14

inversely proportional

Question 15

Describe and explain the effect of increasing surface area

Answer 15

Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction

Question 16

Do solid reactants in solid pieces have a greater surface area?

Answer 16

yes

Question 17

What are catalysts?

Answer 17

substances that speed up chemical reactions without being changed or used up during the reaction

Question 18

How does a catalyst increase the rate of reaction?

Answer 18

by providing a different pathway for the reaction that has a lower activation energy

Question 19

What is an enzyme?

Answer 19

a molecule that acts as a catalyst in a biological system

Question 20

How does a catalysts affect the reaction profile?

Answer 20

The reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy)

Question 21

What is a reversible reaction?

Answer 21

A reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants

Question 22

When is dynamic equilibrium reached?

Answer 22

In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant

Question 23

Describe Le Chatelier’s principle

Answer 23

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change and restore the equilibrium

Question 24

Describe the effect of changing the concentration of reactants and products on the position of the equilibrium?

Answer 24

• If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium, and the concentrations of all the substances will change until equilibrium is restored
• if the concentration of a reactant is increased, more products will be formed until equilibrium is reached again. If the concentration of a product is decreased, more reactants will react until equilibrium is reached again

Question 25

Describe the effect of changing the temperature on the position of the equilibrium?

Answer 25

If the temperature of a system at equilibrium is increased
- the relative amount of products at equilibrium increases for an endothermic reaction
- the relative amount of products at equilibrium decreases for an exothermic reaction

Question 26

Describe the effect of changing pressure on the position of the equilibrium?

Answer 26

• When pressure increases, the equilibrium shifts towards the side with fewer molecules, as indicated by the symbol equation.
• When pressure decreases, the equilibrium to shift towards the side with more molecules, as shown by the symbol equation for the reaction.

Question 27

Describe the effect of a catalyst on the position of the equilibrium?

Answer 27

• No effect
• it just speeds up both forward and backward reactions equally