Types of Types of reactionsreactions

What is a Chemical Reaction?

• A chemical reaction is the process by which one or more substances change into one or more new substances.

• Reactants are the original substances in a chemical reaction.

• Products are the substances that are created in a chemical reaction.

Sample Chemical Reaction

Video 1 Click for sample reaction

• Video 2

Formula for Reaction

• 2Al + 3Br2 2AlBr3

ReactantsProduct

Symbols in A Chemical Reaction

Evidence of a Chemical Reaction

Chemical Reaction Versus Physical Change

• chemical change – new substance forms with properties that differ from original substance– density– boiling point– melting point

• physical change - changes of state – evaporation– condensation– melting– freezing

Types of Reactions• A combustion reaction is a

reaction of a carbon-based compound with oxygen.

• Combustion of propane:

• C3H8 + 5O2 3CO2 + 4H2O

• Combustion of ethanol:

• CH3CH2OH + 3O2 2CO2 + 3H2O

Sample Combustion

Click for combustion reaction

Synthesis Reaction

• In a synthesis reaction a single compound forms from two or more reactants.

• Two elements form a binary compound

• C + O2 CO2

• 2C + O2 2CO

• Two compounds form a ternary compound• CaO(s) + H2O(l) Ca(OH)2(s)

• CO2(g) + H2O(l) H2CO3(aq)

Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.)

reactant + reactant 1 product A + B → AB

Example: 2H2 + O2 2H2O

Example: C + O2 CO2

Sample Synthesis

Click to see the synthesis of hydrogen and oxygen

Decomposition Reactions

• In a decomposition reaction a single compound breaks down, often with the input of energy, into two or more elements or simpler compounds.

• Decomposition of waterelectricity

2H2O(l) O2(g) + 2H2(g)

Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds

1 Reactant Product + Product

In general: AB A + B

Example: 2 H2O 2H2 + O2

Example: 2 HgO 2Hg + O2

Sample Decomposition

Click to see the decomposition of ammonium dichromate

Click to see the continuation of the decomposition

Single Displacement

• In a displacement reaction a single element reacts with a compound and displaces another element from the compound.

•• 2Al(s) + 3CuCl2(aq) 2AlCl3(aq) +

3Cu(s)

• Aluminum displaces copper.

Single Replacement Reactions occur when one element replaces another in a compound.A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-).element + compound product + product A + BC AC + B (if A is a metal) Zn(s) + HCl(aq) ZnCl2 + H2(g)

Sample Single Displacement

Click to see the alkali series with water

Double Displacement

• In a double-displacement reaction two compounds in aqueous solution appear to exchange ions and form two new compounds.

• One of the products must be a solid precipitate, a gas, or a molecular compound, such as water.

• HCl(aq) + NaOH(aq) HOH(l) + NaCl(aq)

Sample Double Displacement

Click to see double displacement

Law of Conservation of Mass• Atoms cannot be created or destroyed, in a chemical

reaction, they are rearranged.• The atoms that make up the reactants are still there

in the products – they are just rearranged.• The total mass of the reactants must equal the total

mass of the products.• When balancing equations, reactant atoms equal the

atoms on the product side.• Cannot change subscripts when balancing

equations. Coefficients are used. (Number added to the front of a compound.

Rates of reaction

Objectives

• To understand that a chemical reaction involves collisions between particles

• To be able to describe the four factors which will affect the rate of a chemical reaction.

Rates of reactionWhy are some reactions faster than others?

What does rate of reaction mean?The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.

What is the rate of these reactions?

The speed of a reaction is called the rate of the reaction.

rusting baking explosion

slow fast very fast

Rates of Reaction

• A chemical reaction involves a collision between particles.

• The particles collide and make new substances

• The particles which react are called the reactants

• The substances which are made are called the products

Reactions, particles and collisions

Reactions take place when particles collide with a certain amount of energy.

The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction.

The rate of a reaction depends on two things:

the frequency of collisions between particles

the energy with which particles collide.

If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

Changing the rate of reactions

increased temperature

increased concentration of dissolved reactants, and increased pressure of gaseous reactants

increased surface area of solid reactants

use of a catalyst.

Anything that increases the number of successful collisions between reactant particles will speed up a reaction.

What factors affect the rate of reactions?

Slower and slower!Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop.

As the reaction progresses, the concentration of reactants decreases.

This reduces the frequency of collisions between particles and so the reaction slows down.

percentage completion of reaction

100%0% 25% 50% 75%

reactants

product

Temperature and collisionsHow does temperature affect the rate of particle collision?

Temperature

• When we increase the temperature we give the particles energy

• This makes them move faster

• This means they collide with other particles more often

• So the reaction goes faster.

Temperature and particle collisions

Surface area

• If we make the pieces of the reactants smaller we increase the number of particles on the surface which can react.

• This makes the reaction faster.

The particles on the surface can react

When cut into smaller pieces the particles on the inside can react

Effect of surface area on rate of reaction

Any reaction involving a solid can only take place at the surface of the solid.

If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction?

The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

This means that there is an increased area for the reactant particles to collide with.

low surface area high surface area

Surface area and particle collisions

Concentration• If we make one reactant

more concentrated (like making a drink of orange squash more concentrated)

• There are more particles in the same volume to react

• So the reaction goes faster.

There are less red particles in the same volume so there is less chance of a collision

There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Bell Work 11/28

1. What does a reaction rate tell you?

2. What 5 factors affect reaction rates?

3. How does a catalyst make a reaction go faster?

Bell Work Review

1. How fast a chemical reaction is going.

2. Temperature, concentration, surface area, stirring and catalysts (pressure can also be listed).

3. By lowering the activation energy.

Effect of concentration on rate of reaction

The higher the concentration of a dissolved reactant, the faster the rate of a reaction.

Why does increased concentration increase the rate of reaction?

At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react.

higher concentrationlower concentration

Concentration and particle collisions

Effect of pressure on rate of reaction

The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.

Why does increasing the pressure of gaseous reactants increase the rate of reaction?

As the pressure increases, the space in which the gas particles are moving becomes smaller.

lower pressure higher pressure

Using a catalyst

• A catalyst is a chemical which is added to a reaction.

• It makes the reaction go faster.

• The catalyst does not get used up in the reaction.

• It gives the reaction the energy to get started

reaction (time)

ener

gy

(kJ)

What are catalysts?Catalysts are substances that change the rate of a reaction without being used up in the reaction.

Catalysts never produce more product – they just produce the same amount more quickly.

Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea).

Ea withcatalyst

Ea withoutcatalyst

Everyday catalysts

Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils).

Many catalysts are transition metals or their compounds. For example:

Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.

Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).

Catalysts in industry

Catalysts are also essential for living cells. Biological catalysts are special types of protein called enzymes.

Why are catalysts so important for industry?

Products can be made more quickly, saving time and money.

Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.