C8. Rates and equilibrium

Question 1

How is the rate of reaction calculated?

Answer 1

Rate of reaction = amount 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 of reaction?

Answer 2

Generally, mass/time, volume/time, moles/time

Question 3

Name three common ways of measuring 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 the reaction flask on a balance. In these reactions (e.g. Metal carbonate + acid) a gas given off, so record the decrease in mass in time intervals (note hydrogen is too light). 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 (X) on it. Put the reaction flask on the cross. Mix the reagents and measure how long it takes for a cloudy mixture to conceal a cross

Question 7

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

Answer 7

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 8

State five factors affecting the rate of a chemical reaction.

Answer 8

Concentration of reactants
Pressure of gases
Surface area
Temperature
Catalysts

Question 9

What is the collision theory?

Answer 9

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

Question 10

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

Answer 10

T increases = faster reaction

As T increases, the kinetic energy of particle increases so there is more energetic collisions. The particles move faster so they collide more frequently. However there is no straight line between the rate an temperature, i.e. They are not directly proportional to each other

Question 11

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

Answer 11

Conc. increases = faster reaction,
More reactants = more frequent collisions

Question 12

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

Answer 12

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

Question 13

Describe and explain the effect of increasing surface area?

Answer 13

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 14

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

Answer 14

A catalyst changes the rate of reaction but it is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a maximum of the curve (lower activation energy)

Question 15

What is an enzyme?

Answer 15

A molecule that acts as a catalyst in a biological system

Question 16

What is a reversible reaction?

Answer 16

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

Question 17

When is dynamic equilibrium reached?

Answer 17

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 18

Describe Le Chatelier’s Principle

Answer 18

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 the equilibrium.

Question 19

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

Answer 19

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 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 20

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

Answer 20

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 21

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

Answer 21

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 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 reactions where the numbers of gas molecules are equal on both sides of the equation

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