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The changes in energy that occur during chemical reactions can be illustrated using an energy diagram:. Many reactions take place in solution : e.g. 2AgNO 3(aq) + Na 2 CO 3(aq)  Ag 2 CO 3(s) + 2NaNO 3(aq)

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The changes in energy that occur during chemical reactions can

The changes in energy that occur during chemical reactions can

be illustrated using an energy diagram:


The changes in energy that occur during chemical reactions can

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 relativeamounts of products and reactants.

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

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

# moles of NaNO3 = # moles of AgNO3


The changes in energy that occur during chemical reactions can

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

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?


The changes in energy that occur during chemical reactions can

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?


The changes in energy that occur during chemical reactions can

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

Consider the following reaction:

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

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 beproduced?


The changes in energy that occur during chemical reactions can

# 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.


The changes in energy that occur during chemical reactions can

# 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.


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

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 changes in energy that occur during chemical reactions can

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)


The changes in energy that occur during chemical reactions can

Non-polar covalent compounds are nonelectrolytes.

Lets do some practice problems now.


The changes in energy that occur during chemical reactions can

  • 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 colligativeproperties

  • We will discuss three colligative properties:

    • Boiling point elevation

    • Freezing point depression

    • Osmotic pressure


The changes in energy that occur during chemical reactions can

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 puresolvent (boiling point elevation).

• The freezing point of a solution is lower than the puresolvent (freezing point depression).


The changes in energy that occur during chemical reactions can

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


The changes in energy that occur during chemical reactions can

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


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

This will ultimately result in unequal amounts of liquid on each

side of the membrane.


The changes in energy that occur during chemical reactions can

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)


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

If we now replace theouter solvent we canrepeat the processuntil we have thedesired purity in thedialysis bag.


The changes in energy that occur during chemical reactions can

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?


The changes in energy that occur during chemical reactions can

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).


The changes in energy that occur during chemical reactions can

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 changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

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

increase in entropy.

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.


The changes in energy that occur during chemical reactions can

Spontaneous reactions tend to release energy and have an

increase in entropy.

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

increase in entropy.

Can anyone think of a spontaneous process that falls into

this case?

H2O(s) + energy H2O(l)


The changes in energy that occur during chemical reactions can

Spontaneous reactions tend to release energy and have an

increase in entropy.

2. A process will always be spontaneous if the reaction

releases energy to the surroundings and is accompanied by

an increase in entropy.

Can anyone think of a spontaneous process that falls into

this case?

NaOH(s)Na+(aq) + OH-(aq) + heat


The changes in energy that occur during chemical reactions can

Spontaneous reactions tend to release energy and have an

increase in entropy.

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

spontaneous if accompanied by a release of energy.

Can anyone think of a spontaneous process that falls into

this case?

H2O(g)H2O(l) + heat


The changes in energy that occur during chemical reactions can

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.”


The changes in energy that occur during chemical reactions can

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 changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

Once the activation energy is overcomethe process proceeds spontaneously.The energy of the reactants is made upof two components:

1. The kinetic energy of the molecules.

2.The internal energy of the reactantmolecules.

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 changes in energy that occur during chemical reactions can

The internal energy describes how the atoms are moving within

a molecule.The internal energy of amolecule increases withtemperature.

For many reactions there is aminimum temperature belowwhich the reaction does notoccur.


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

Reactant molecules that are gasesand liquids have a greater ability toorientthemselves favourably thanreactants that are solids andhavemolecules held in fixed positions.


The changes in energy that occur during chemical reactions can

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The changes in energy that occur during chemical reactions can

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 changes in energy that occur during chemical reactions can

  • 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


The changes in energy that occur during chemical reactions can

  • 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)


The changes in energy that occur during chemical reactions can

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


The changes in energy that occur during chemical reactions can

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 isthat for every 100C risein temperature thereaction rate doubles.


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

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.


The changes in energy that occur during chemical reactions can

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 holdsthe reactant in an orientationsuch that the yellow atoms are ina favourable position in order forthem to react.


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