The rate and extent of chemical change

Question 1

How is rate of reaction calculated?

Answer 1

Rate of reaction = amount (such as grams, cm³) of reactant used or product formed / time

Rate of reaction (mol/s) = moles of reactant used or product formed / time

Question 2

What are the various units for rate for reaction?

Answer 2

g/s, cm³/s or mol/s

This is because mass/time, volume/time, moles/time

Question 3

Name 3 common ways of measuring rate of reaction

Answer 3

• Change in mass.
• Volume of gas produced.
• Time it takes for cross to disappear.

Question 4

How to find a rate of reaction at some time, t, from a graph of amount of reactant vs time?

Answer 4

• Pick a point corresponding to the time, t, and find a tanged to the curve at this point.
• The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the gradient, the faster the rate. Gradient of tangent can be expressed in change in y value / change in x value.

Question 5

State 5 factors affecting the rate of a chemical reaction

Answer 5

• Concentration of reactants.
• Pressure of gases (volume).
• Surface area.
• Temperature.
• Catalysts.

Question 6

What is the collision theory?

Answer 6

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

Question 7

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

Answer 7

Temperature increase = faster reaction.
As temperature increase, kinetic energy of particles increases (more energetic collisions). Also, they move faster, so they collide more frequently. It also provides enough activation energy.

However, there is no straight line relationship between rate and temperature, they are no proportional to one another.

Question 8

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

Answer 8

Concentration increase = faster reaction

More reactant particles means there are more frequent collisions, increasing rate of reaction.

Question 9

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

Answer 9

Increasing the pressure of reacting gases, is the same as increasing concentration. It increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction.

Question 10

Describe and explain the effect of increasing surface area

Answer 10

If solid reactant are in smaller pieces, they have a greater surface rea. Increasing the surface area of solid reactant increases the frequency of collisions and so increases the rate of reaction. For example, a block of magnesium reacts slower than magnesium powder.

Question 11

What is a catalyst and how does it work? How does it affect the reaction profile?

Answer 11

A catalyst speeds up the rate of reaction without being used up. It increases rate by providing a different pathway for the reaction to have lower activation energy.

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

Question 12

What is an enzyme?

Answer 12

An enzyme is a molecule that acts as a catalyst in a biological system.

Question 13

What is a reversible reaction?

Answer 13

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

Question 14

When is dynamic equilibrium reached?

Answer 14

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 15

Describe Le Chatelier’s principle

Answer 15

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

Question 16

Describe the effect of changing the concentration of reactant and product on the position of equilibrium

Answer 16

If the concentration of one of reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again. 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 17

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

Answer 17

If the temperature of a system at equilibrium is increased:

• The relative amount of product at equilibrium increases for an endothermic reaction.
• The relative amount of products at equilibrium decreases from an exothermic reaction.

Question 18

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

Answer 18

This applies to reactions that involve gases.
An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction. A decrease in pressure causes the the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction. Pressure has no effect on the reaction where the number of gas molecules are equal on both sides of the equation.

Question 19

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

Answer 19

No effect.

It just speeds up both the forward and backward reactions equally which allow equilibrium to be reached faster.`