C6: The rate and extent of chemical change

Question 1

How is rate of reaction calculated?

Answer 1

rate of reaction = amount ( eg grams ) of reactant used or product formed / time
rate of reaction ( mol/s) = moles of reactant used or product formed / time

Question 2

give the three various units for rate of reaction?

Answer 2

can include:
- g/s
- cm³/s or moles/time

Question 3

Name three common ways of measuring ate of reaction

Answer 3

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

Question 3

Describe measuring the rate by monitoring mass loss

Answer 3

• Place the 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 3

Describe measuring the rate of reaction by monitoring the volume of a gas

Answer 3

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

Question 4

Describe measuring the rate by monitoring the disappearance of cross

Answer 4

• take a piece of paper and mark a cross (X) on it
• Put the reaction flask on this cross
• Mix the reagents and measure how long it takes for a cloudy mixture to conceal a cross

Question 5

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

Answer 5

• pick a point corresponding to the time t, and find the tangent 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 tangent line , the faster the rate
• gradient of tangent can be expressed in change in y values over change in x values

Question 6

State five factors affecting the rate of a chemical reactions

Answer 6

• concentration of reactants
• pressure of gases ( volumes)
• surface area
• temperature
• catalysts

Question 7

What is the collision theory?

Answer 7

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

Question 8

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

Answer 8

• As temp increases, kinetic energy of particles increases i.e. more energetic collisions.
• also , they move faster, so they collide more frequently

Question 9

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

Answer 9

• More concentrated means more particles in solution,
• therefore more frequent collisions between reactants

Question 10

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

Answer 10

• it increases the number of gas molecules in the same volume
• so increases the frequency of collisions
• therefore increases the rate of reaction

Question 11

Describe and explain the effect of increasing surface area.

Answer 11

• if solid reactants are in smaller pieces, they have a greater surface area
• increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction

Question 12

what is a catalyst and how does it work?

Answer 12

• A catalyst changes the rate of reaction but it is not used up
• They provide another route for the reaction to take place which has a lower activation energy.

Question 13

How does a catalyst affect the reaction profile?

Answer 13

• the reaction profile for a catalysed reaction will have a lower maximum of the curve
• lower activation energy

Question 14

What is an enzyme ?

Answer 14

A molecule that acts as a catalyst in a biological system

Question 15

What is a reversible reaction?

Answer 15

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

Question 16

When is dynamic equilibrium reached?

Answer 16

• 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 17

Describe Le Chatelier’s principle

Answer 17

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

Question 18

describe the effect of changing the concentration of reactant and product on the position of the equilibrium (3)

Answer 18

• The system is no longer at equilibrium
• if the concentration of the reactant is increased , more products will be formed until equilibrium is reached again
• if product is decreased , more reactants will react until equilibrium is reached again

Question 19

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

Answer 19

increasing temp:
- equilibrium moves to the direction of the endothermic reaction
decreasing temp:
- equilibrium moves to the direction of the exothermic reaction
products inc
products dec

Question 20

Describe the effect of an increase in pressure on the position of the equilibrium

Answer 20

this applies to equilibria that involve gases

• an increase in pressure causes the equilibrium position to shift towards the side with the smaller number molecules as shown by the symbol equation for that reaction

Question 21

Describe the effect of a decrease in pressure on the position of the equilibrium

Answer 21

• a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for the reaction

Question 22

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

Answer 22

• no effect
• It just speeds up both forward and backward reactions equally
• i.e. equilibrium is achieved faster

Question 23

A reaction is exothermic in the
forward direction. What will occur if
the temperature is increased?

Answer 23

The backward reaction will increase as it is
endothermic and will reduce the temperature