Rates of reactions

Define rate of reaction

Change in concentration per unit time of any one reactant or product

How are rates of reaction measured?

a) Formation of a product per unit time
b) Loss of a reactant per unit time

Average rate of reaction formula

Amount of substance formed/lost / Time taken

How to find Instantaneous rate of reaction

1) Draw a tangent to curve at time asked

2) Choose two good points on tangent

3) Find slope of tangent

Instantanous rate= slope of tangent

How to find initial rate of reaction

1) Draw tangent to curve from (0,0)

2) Choose two good points on tangent
3) Find slope of tangent

Initial rate= slope of tangent

Outline basic assumptions of collision theory

• For a reaction, reactant particles collide with each other

• Reactants have energy

• If reactants collide with correct orientation and meet required energy it will be an effective collision

• Effective collision causes chemical bonds to be broken and new bonds to form resulting in formation of products.

• Rate of a reaction depends on number of effective collision per second

What is activation energy

Minimum combined energy of colliding particles necessary for a reaction.

What is meant by an effective collision

Results in the formation off products

Factors necessary for an effective collision

Reactants have correct orientation

Reactants must collide with energy that equals or exceeds activation energy.

Why does large activation energy=slow rate of reaction

Low number of effective collision per second

Why does small activation energy= fast rate

High number of effective collision per second

What is an exothermic reaction

Produces heat and gives it out

In exothermic reaction is change in energy positive or negative

negative

What is an endothermic reaction?

Takes heat in

Is change in energy in endothermic reaction positive or negative

positive

Factors affecting rate of a reaction

Nature of reactants (ionic or covalent)

Particle size/ surface area

Concentration

Temperature

Pressure

Catalyst

Why are reactions including ionic compounds in solution faster

In solution, ionic compounds are already dissociated as free ions
No bond breaking is required, activation energy is lower and reactions are fast

Why are reaction involving covalent compounds slower

Covalent compounds do not dissolve
Bonds need to be broken, activation energy is higher and reactions are slow.

How does particle size affect rate of reaction and why

Smaller particle size increases rate

larger particle size decreases rate

Because smaller particles have greater surface area which means more collisions meaning more effective collisions per unit time.

Dust explosions present a risk in industry. Give five conditions necessary for a dust explosion to occur

1) Dust must be combustible
2) Dust must be dry

3) Source of ignition
4) Oxygen must be present

5) Must be in an enclosed space

How does concentration of reactants affect rate of reaction

Increasing concentration increases rate

Decreasing concentration decreases rate

Relationship between rate of reaction and concentration

Rate is directly proportional to concentration

Why does increased concetrationn increase rate

More concetrated solution has greater number of particles

Means more collision meaning more effective collision occur per unit time

Explain why the rate of a chemical reaction is always fastest at the beginning and then slows down as the reaction proceeds?

At the beginning of a reaction, concentration of reactants are highest.
As reaction proceeds, reactants are used up and concentration decreases- rate of reaction decreases

In what situations does concentration NOT have an effect on the rate of a reaction?

1) If reactants in very high concentration and one reactant is saturated, maximum number of collisions are already occuring

2) If chemical reaction produces an immediate precipitate

How does tempertature affect rate of reaction

Increasing temperature increases rate

Decreasing temperature decreases rate

Relationship between temperature and rate of reaction

As temperature increases, rate increase exponentially

Why does increases temperature increase rate of reaction

Increasing temperature increases energy of particles so more effective collisions per unit time.

Increasing temperature increases velocity meaning more collisions meaning more effective collisions per unit time

: Which of these two reasons is more significant in temperature increasing the rate of reaction? Explain.

• Increasing energy of particles

• For same temperature rise, increase due to higher velocity is small compared to increase due to particles acquiring more energy

What type of reaction does pressure effect

Only effects gaseous reactions

How does pressure affect rate of a gaseous reaction?

Increasing pressure increases rate

Decreasing pressure decreases rate

Why does increased pressure effect rate of gaseuous reaction

Increased pressure, gas particles compressed into smaller volume. This means more collisions meaning more effective collisions per unit time

What is a catalyst

Substance that alters the rate of a reaction without being used up.

Explain why the presence of a catalyst can speed up the rate of a chemical reaction

Lowers activation energy providing an alternative pathway for a reaction
More effective collisions per unit time

Properties of catalysts

1) Chemically unchanged at end of reaction

2) Specific to reaction
3) Can be inactivated by catalytic poisins

4) Brings reversible reaction to equilibrium more quickly but doesn’t affect state of equilibrium

5) Only required in small amounts

What is an enzyme

Biological catalyst

Examples of enzymes

Catalase and amylase

How could you reduce the rate of a chemical reaction that takes place in solution

1) Lowering temperature- cooling
2) Lowering concentration- diluting

3) Adding inhibitor

Experiment to monitor rate of production of oxygen from hydrogen peroxide using mangasnes dioxide as a catalyst PROCEDURE

1) Add hydrogen peroxide to conical flask, clamp horizontally and add manganese dioxide to neck of flask

2) Stopper the flask and connect to inverted graduated cylinder
3) Turn flask vertical, start stopwatch
4) Measure volume of oxygen gas at regular intervals
5) Graph volume of oxygen vs time

What is appearance of manganese dioxide catalyst

Black powder

How is it ensured the stopwatch is started at exactly the same time as the reaction starts

• Add hydrogen peroxide to conical flask, clamp horizontally and add manganese dioxide to neck of flask.
• Turn flask vertical and start stopwatch

What does this mean

Oxidiser- Keep away from other substances

What does this mean

Irritant- wear gloves and use spatula

What effect would increasing the mass of the catalyst have on the rate of reaction? Explain

No effect

Catalysts required in very small amounts