chem kinetics

Chemical change

Involves a chemical reaction to produce a new product (def)

Chemical bonds are broken and reformed to make new molecules

Eg. Baking a cake, cooking an egg, burning a candle

Some are reversible

Physical change

A change in matter that alters its form but not its chemical identity , size and shape can change but no chemical reaction occurs , mostly reversible

Eg. Breaking a water , boiling water , melting an ice cube

Signs of chemical reaction

1. Temperature change

2. Light

3. Colour change

4. Bubbling (effervescence)

5. Odour

6. Formation of precipitate

To distinguish chemical and physical change

1. Whether there is a new substance that wasn’t there before

2. Chemical reaction = chemical change

Kinetics studies of chemical reactions provides information on

1. How fast a Chemical reaction occurs (rate of reaction)

2. How economically a product can be made

Collision theory

The rate of a chemical reaction is directly proportional to the frequency of defective collision

Conditions for collision theory

1. Reacting particles must collide with each other

2. Must collide with sufficient energy to overcome the activation energy of the reaction

3. Correct orientation

   all of these = effective collision

To find rate of a reaction

1. Measuring the amount of products formed per unit time = amount of products / time

2. Measuring the amount of reactants used up per unit time = amount of reactants used up / time taken

Steepest gradient means

Fastest rate of reaction (gentlest gradient =slowest rate of reaction)

Gradient is always steepest at the start

Over time, why does the rate of reaction decrease

The concentration of reactants decrease over time. When the rate of reaction is 0, all the limiting regent has been used up

If the graph does not end at 0

There is still some excess reactant left

If a reaction gives off gas , how do you find the rate of reaction

Use a gas syringe to measure the volume of gas given off at regular time intervals

What does it mean when the graph is horizontal?

No more gas is produced , reaction has stopped

To increase rate of reaction involving acid

Use an acid that produces more h+ ions , eg sulfuric acid and not hydrochloric acid

(If the reactant stays the same and only the acid is changed to something similar , the similarity between both experiments is that the same vol of gas will be produced)

Factors affecting rate of reaction

1. Concentration

2. Pressure (gaseous reactants only)

3. Particle size (solid reactants only)

4. Temperature of the reacting mixture

5. Catalyst

Concentration of reactants

Increasing the conc = increasing the number of reactants per unit volume

• when the concentration of the reactants increases , the number of reacting particles per unit volume increases , so they collide more often. As a result, the frequency of effective collision increases, hence an increase in rare of reaction

Pressure of gaseous reactants

As pressure of gas increases, gas particles are forced closer so they are able to collide more . This increases the number of gas particles per unit volume, increasing the frequency of effective collision between the gas particles, increasing the overall rate of reaction.

• pressure increase = rate of reaction increase

Particle size of solid reactant

Smaller the particle size = larger the surface area= faster rate of reaction

• a large substance has a smaller surface area as the collisions are only limited to the outer surface

• When the particle size decreases, the total surface area for reaction to take place increases, as a result, the frequency of effective collision increases which leads to an increase in the rate of reaction

Temperature of the reacting mixture

• an increase in temperature means that the kinetic energy of the reacting particles increases, they move faster and more vigorously so they collide more often. More particles have energy that is more than or equal to the activation energy . As a result, the frequency of effective collision increases, increasing the rate of reaction

Catalyst

• In a catalysed reaction, more particles will have energy that is more than or equal to the activation energy. Hence, increasing the frequency of effective collision and rate of reaction

• Only change rate not the yield of the experiment

Catalyst def

A substance that increases the rate of a chemical reaction . It remains chemically unchanged at the end of the reaction and provides alternative reaction pathway of lower activation energy

Which factors affect the frequency of collisions

Everything but catalyst ( temp, conc , pressure , particle size )

What affects energy of collsion

Only temp and catalyst