Topic 6 The Rate and Extent of Chemical Change

Question 1

How is the rate of reaction calculated?

Answer 1

g/cm^2 - Amount of product or reactants / time
mols/sec - Moles of reactants or products / time

Question 2

What are the various units for rates of reaction?

Answer 2

mass/time
volume/time
moles/time

Question 3

Name three common ways of measuring rates of reaction

Answer 3

• Loss of mass in reactants
• Volume of gas produced
• Time for a solution to become opaque

Question 4

Describe how to measure the rate of reaction by monitoring mass loss

Answer 4

Place the reaction flask on a balance. In these reactions (metal carbonate + acid) a gas is given off so record the decrease in mass in time intervals. Plot a graph of mass Vs time.

Question 5

Describe how to measure the rate of reaction by monitoring the volume of a gas produced

Answer 5

Connect a gas syringe to a reaction flask and measure the volume of gas formed at time intervals. Plot a graph of volume Vs time.

Question 6

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

Answer 6

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

Question 7

How do you find the rate of reaction at some point on a graph of the amount of reactant Vs time?

Answer 7

• Pick the point corresponding to the time and find the tangent to the curve.
• The tangent is the gradient of the graph (tells you how fast the reaction proceeds at this point).
• The steeper the gradient the faster the rate of reaction.

Question 8

State five factors affecting the rate of a chemical reaction

Answer 8

• Concentration
• Pressure
• Surface Area
• Temperature
• Presence of a catalyst

Question 9

What is the Collision Theory?

Answer 9

Chemical reactions only occur when reacting particles collide with sufficient energy (more than or equal to the activation energy).

Question 10

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

Answer 10

• As the temperature increases the rate also increases
• When the temperature increases the particles have more kinetic energy and will collide faster and more often
• There is no direct correlation between temperature and rate (they are not directly proportional)

Question 11

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

Answer 11

Concentration increases = More particles
More particles to react = Faster reaction

Question 12

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

Answer 12

• Same effect as increasing concentration
• Increasing the number of molecules in a set volume means that there will be more collisions and therefore a higher rate of reaction
• Volume and pressure are indirectly proportional to one another (increasing the volume will slow the reaction)

Question 13

Describe and explain the effect of increasing the surface area

Answer 13

Increased surface area = increased collisions and therefore increases the rate of reaction

Question 14

What is a catalyst and how does it work?

Answer 14

A catalyst increases the rate of reaction by providing an alternate reaction pathway that requires less activation energy. It does not get used up in the reaction.

Question 15

How does a catalyst affect the reaction profile?

Answer 15

It lowers the maximum of the curve

Question 16

What is an enzyme?

Answer 16

A protein molecule that acts as a catalyst in biological reactions

Question 17

What is a reversible reaction?

Answer 17

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

Question 18

When is dynamic equilibrium reached?

Answer 18

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

Question 19

Describe Le Chatelier’s Principal

Answer 19

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 equilibrium.

Question 20

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

Answer 20

• If the concentration is changed the system will no longer be at equilibrium
• If the concentration of reactants is increased more products will form and vice versa until equilibrium is reached again

Question 21

Describe the effect of changing temperature on the position of equilibrium

Answer 21

• If the temperature is increased: the endothermic reaction increases to cool the temperature
• if the temperature decreases: the exothermic reaction will be favoured to raise the temperature

Question 22

Describe the effect of changing pressure on the position of equilibrium

Answer 22

• If the pressure increases: the reaction with fewer products will be favoured to decrease the pressure
• If the pressure decreases: the reaction with more products will be favoured to increase the pressure again

Question 23

Describe the effect of a catalyst on the position of equilibrium

Answer 23

No effect. It just speeds up the rate of both the forward and reverse reaction equally (equilibrium is achieved faster)