the changes in energy that occur during chemical reactions can be illustrated using an energy...

The changes in energy that occur during chemical reactions can

be illustrated using an energy diagram:

Many reactions take place in solution:

e.g.

2AgNO3(aq) + Na2CO3(aq) Ag2CO3(s) + 2NaNO3(aq)

The co-efficients in equations allow us to determine the

relative amounts of products and reactants.

# moles of Ag2CO3 = ½(# moles of AgNO3)

# moles of Na2CO3 = ½(# moles of AgNO3)

# moles of NaNO3 = # moles of AgNO3


In the case of this reaction all reagents are in the form of

aqueous solutions.

If we combine two solutions of known volume of the reagents

how could we calculate the amount of product produced?

If we know the molarity of the reagent solutions we can use the

relationship between molarity and volume to determine the

number of moles of each reagent.

M = n/V

n = M x V

We can then determine the yield of product as we would for any

other stoichiometry calculation.

Would anyone like to do an example?

Luckily I thought that might be the case so I prepared a slide!

Consider the following reaction:


If I combined 20 mL of 0.20 molL-1 AgNO3(aq) solution with

10mL of 0.20 molL-1 Na2CO3(aq) what mass of Ag2CO3 would be

produced?

# of moles of AgNO3 = M x V

= 0.2 molL-1 x 20 mL x (1 x10-3 LmL-1)

= 0.004 moles of AgNO3

# moles of Ag2CO3 = ½(# moles of AgNO3)

# moles of Ag2CO3 = ½ x 0.004 moles

If the limiting reagent is AgNO3 0.002 moles of Ag2CO3 will be

produced.

# of moles of Na2CO3 = M x V

= 0.2 molL-1 x 10 mL x (1 x10-3 LmL-1)

= 0.0020 moles of Na2CO3

# moles of Ag2CO3 = # moles of Na2CO3

# moles of Ag2CO3 = 0.0020 moles

If the limiting reagent is Na2CO3 0.0020 moles of Ag2CO3 will be

produced.

Conclude that we produce 0.0020 moles of Ag2CO3 in this

reaction. However question asked for what mass.

Mass = n x MW

Mass of Ag2CO3 = 0.002mol x (2x107.9 + 12.01 + 3x16.00)gmol-1

Mass of Ag2CO3 = 0.55 grams

The reaction 20 mL of 0.20 molL-1 AgNO3(aq) solution with

10mL of 0.20 molL-1 Na2CO3(aq) would produce 0.55 g of

Ag2CO3.

Pure water conducts electricity poorly.

Addition of a solute can affect the ability of water to conduct

electricity.

• solutes that result in aqueous solutions that conduct

electricity well are called strong electrolytes.

• solutes that result in aqueous solutions that conduct

electricity poorly are called weak electrolytes.

• solutes that result in aqueous solutions that do not

conduct electricity are called nonelectrolytes.

The ability of a solute to conduct electricity increases with the

extent to which it dissociates into ions.

Soluble ionic compounds tend to completely dissociate into

ions and are strong electrolytes:

NaCl(aq) Na+(aq) + Cl-(aq)

Polar covalent compounds dissociate depending on how polar

the bonds they contain are:

HCl(aq) H+(aq)+ Cl-(aq)

Non-polar covalent compounds are nonelectrolytes.

Lets do some practice problems now.

Some properties of solutions do not depend on the chemical

nature of the solute but only on the concentration of the solute.

•These kinds of properties are called colligative properties

We will discuss three colligative properties:

• Boiling point elevation

• Freezing point depression

• Osmotic pressure

The vapor pressure above a solution is lower than that above

the pure solvent.

This has some interesting effects:

• The boiling point of a solution is higher than the pure solvent

(boiling point elevation).

• The freezing point of a solution is lower than the pure solvent

(freezing point depression).

The change in boiling point and freezing point can be

calculated using the following similar equations:

Δ tf =nKfM

Δtb = nKbM

Where:

Δtb = change in boiling poing

Δtf = change in freezing point

n = number of moles of solute particles put in to solution

when 1 mole of solute is dissolved.

Kb = a constant characteristic of the solvent

Kf = a constant characteristic of the solvent

M = molarity

What is the boiling point of a 0.1 molL-1 solution of MgCl2?

Δtb = nKbM

Δtb = ?

n = 3 (2 x Cl- and 1 x Mg2+)

Kb = 0.52 oC/M (from Table 7.6)

M = 0.1 molL-1

Δtb = 3 x 0.52 x 0.1 = 0.156 oC

Boiling point = 100oC + 0.156oC = 100.2oC

When solutions having different concentrations of solute are

separated by a semipermeable membrane, solvent flows

through the membrane from the less concentrated solution into

the more concentrated solution this is called osmosis.

This will ultimately result in unequal amounts of liquid on each

side of the membrane.

A pressure will be exerted hindering the movement of solvent

across the membrane. Eventually the pressure will prevent the

movement of solvent across the membrane. This is called the

osmotic pressure and is given by equation:

π= nMRT

Where:

π = osmotic pressure

n = number of moles of solute particles put in to solution

when 1 mole of solute is dissolved.

M = molarity (molL-1)

R = universal gas constant

T = temperature (Kelvin)

A process very similar to osmosis is dialysis. In dialysis a

membrane is used that allows the smaller molecules to cross

but do not the larger ones.

Dialysis is used extensively for purifying solutions of large

biomolecules.

e.g. proteins

A commonly known use of dialysis is in purifying the blood of

people whose kidneys do not function correctly.

If we now replace the outer solvent we can repeat the

process until we have the desired purity in the dialysis bag.

We have all observed how some processes tend to

spontaneously occur while others don’t.

e.g. Objects roll downhill but not up.

Our rooms effortlessly become disorganized but don’t

spontaneous tidy themselves up.

We age rather than grow younger.

It would be very useful to predict what processes will occur

and what ones won’t?

We have already classified chemical processes as either being

endothermic (heat in) or exothermic (heat out) processes.

HINT: if you are having trouble remembering what these things

mean think about the words entrance and exit.

Heat is only one form of energy, others include light, sound,

kinetic energy etc.

In a more general sense, taking all forms of energy into

account, we can classify reactions as being exergonic (energy

out) or endogonic (energy in).

When a chemical reaction occurs there will be a change in the

order of the system as well as energy.

e.g. The system could become more organized, such as liquid

water freezing to become ice.

H2O(l) H2O(s)

The system could become less organized such as a reaction

where from one mole of reactants forms several moles of

products are formed.

2HgO(s) 2Hg(l) + O2(g)

The degree of order or randomness in a system is called the

entropy.

Spontaneous reactions tend to release energy and have an

increase in entropy.

If these conditions don’t exist then there must be a larger change

in the other parameter to off set the effect of the other in order for

the process to be spontaneous.

When looking at a reaction there are only several possible ways

in which the energy and entropy can change that allow us to

predict the spontaneity of the process.

1. If the reaction has no change in energy or gains energy from

the surroundings. It will only be spontaneous if there is an


2. A process will always be spontaneous if the reaction

releases energy to the surroundings and is accompanied by

an increase in entropy.

3. A process in which the entropy decreases will only be

spontaneous if accompanied by a release of energy.



1. If there is no change in energy or a gain of energy from the

surroundings. Process will only be spontaneous if there is an


Can anyone think of a spontaneous process that falls into

this case?

H2O(s) + energy H2O(l)



2. A process will always be spontaneous if the reaction

releases energy to the surroundings and is accompanied by

an increase in entropy.


this case?

NaOH(s) Na+(aq) + OH-

(aq) + heat



3. A process in which the entropy decreases will only be

spontaneous if accompanied by a release of energy.


this case?

H2O(g) H2O(l) + heat

A reaction mechanism describes the pathway or process by

which a reaction occurs.

This is different from a reaction rate we discussed previously

which just tells you how quickly the reaction occurs.

e.g. Comparing the difference between reaction rate and

mechanism is like comparing how long it takes to complete a

journey and the directions to a location.

e.g. “ it takes 4.5 hours to fly to Chicago and Chicago lies to the

east of Seattle.”

Some aspects of reaction mechanisms are common to most

reactions:

1. The reactant particles must collide (come into

contact)

Can anyone think of an exception to this rule ?

Decomposition reactions have only one reagent:

A B + C

The second aspect of reaction mechanisms that is common to

most reactions is:

2. The reactant particles must collide with a certain

minimum amount of energy

When a reaction occurs old bonds are broken and often new

ones form. This typically requires an initial input of energy called

the activation energy.

Once the activation energy is overcome the process proceeds

spontaneously.The energy of the reactants is made up of two

components:

1. The kinetic energy of the molecules.

2.The internal energy of the reactant molecules.

The kinetic energy of the reactants describes how fast the

reactant molecules are moving. This will be greater for gases

and liquids than solids and increases with temperature.

The internal energy describes how the atoms are moving within

a molecule. The internal energy of a molecule increases with

temperature.

For many reactions there

is a minimum temperature

below which the reaction

does not occur.

The third aspect of reaction mechanisms that is common to

most reactions is:

3. The reactant particles must collide with a certain

orientation

For polyatomic species the orientation of the reactants may be

significant.

e.g.

AB + CD → AC + BD

In the above A must be near C and B must be near D.

Reactant molecules that are

gases and liquids have a

greater ability to orient

themselves favourably than

reactants that are solids and

have molecules held in fixed

positions.

Good luck in the final !!

Keep watching the website for updates

The energy of products from an exothermic process are lower

in energy than the reactants.

The energy of products from an endothermic process are

higher in energy than the reactants.

The reaction rate of all reactions are affected by:

1. The nature of the reactants

2. The concentration of the reactants

3. The temperature of the reactants

4. The presence of catalysts

Important chemical properties of the reagents that can affect

the reaction rate include:

The type of bonds the reactants contain

The charge of the reactants

The state of the reactants

Reactions that involve the breaking and making of covalent

bonds tend to proceed slowly than those involving ionic species.

Other properties of the type of bonds present in the reactants

that are can affect the reaction rate include:

Shape (VSEPR)

How many bonds

Bond polarity (strength of bond)

Bond order (number of electrons in bond)

Reactions where the reactants are oppositely charged ions will

have a larger number of collisions between reagents that lead

to products than reactants involving reagents with no charge

or the same charge.

If everything other than the charge is equal which of the

following would you expect to have the greatest rate?

A+ + B- AB

A+ + A+ A2 2+

A + B AB

By increasing the temperature of the reagents we increase the

proportion of reagents that have sufficient energy to overcome

the activation energy.

A good rule of

thumb is that for

every 100C rise in

temperature the

reaction rate

doubles.

As we have mentioned before for a reaction to occur the reacting

particles must collide.

As we increase the concentration of the reagents the likelihood

of an effective collision increases and therefore the reaction

rate.

For solids reactions typically take place at the surface of the

reactant. By finely dividing solids we can increase the surface

area and the rate at which they react.

Catalysts are substances that change the rate of a reaction

without being consumed.

Catalysts that slow the rate of a reaction are known as inhibitors.

Catalysts that are in the same state as the reagents and

distributed as individual molecules or ions are known as

homogeneous catalysts.

Many catalysts are solids (often with large surface areas) while

the reactants are gases or liquids, these are known as

heterogeneous catalysts. E.g. catalytic convertors in cars.

Catalysts work by providing an alternative, faster, pathway (or

mechanism) from the reactants to the products.

This alternative mechanism may have a lower activation

energy (EA).

More reactants are

likely to have sufficient

energy to overcome EA

and so the reaction

proceeds quicker.

Solid catalysts can accelerate reactions by binding one of the

reactants in a desirable orientation. Furthermore, molecules

that are not moving are easier targets for collisions.

In this diagram the

catalyst holds the

reactant in an

orientation such that

the yellow atoms are

in a favourable

position in order for

them to react.

the changes in energy that occur during chemical reactions can be illustrated using an energy...

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