All metals show certain physical & chemical properties like malleability,
acids. But the reactivity of metals towards various reactants is not the same.
Some metals like alkali & alkaline earth metals (group-1 & 2) are very reactive &
react vigorously with a reactant. But some metals like gold & platinum are least
reactive and passive for almost all reactants. Some metals like copper release
hydrogen gas with dilute acid. Hence, there must be some criteria for understanding
the reactivity of different metals and predicting the products of different reactions.
Reactivity Series of Metals
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Elements are mainly classified under metals & non-metals. There are some
elements, which have intermediate features. They are known as metalloids.
Malleable & ductile in nature
Good conductor of heat & electricity Insulator in nature
Form ionic compounds
Form Covalent compounds
Have lustre surface
Have high melting point
Usually solid at room temperature
They are good reducing agent
Form basic oxides
Have low electronegativity
10 Have a tendency to lose electrons
Have a tendency to gain electrons
Almost all metals are reactive and react vigorously with various compounds. In
The reactivity series or activity series is an empirical arrangement of metals, in
order of "reactivity" from highest to lowest. In other words, the most reactive metal is
presented at the top and the least reactive metal at the bottom.
Hydrogen gas H
Hence potassium is the most reactive metal and platinum the least reactive one. In
Carbon helps in predicting the products formed during the extraction of iron in blast
furnace and hydrogen is included because non-metals below it will not react with
In the reactivity series, as we move from bottom to top, the reactivity of metals
form positive ions and corrode or tarnish more readily. They require more energy to
be separated from their ores, and become stronger reducing agents, while metals
present at the bottom of the series are good oxidizing agent.
By using the reactivity series, one can predict the products of displacement
any of the elements above it. For example, magnesium metal can displace zinc ions in
The interval between metals in the reactivity series represents the reactivity of those
If the interval between elements is larger, they will react more vigorously. The
topmost five elements, form lithium to sodium are known as very active metals;
hence they react with cold water to produce the hydroxide and hydrogen gas. For
example, sodium forms sodium hydroxide and hydrogen gas with cold water.
they will react with very hot water or steam and form the oxide and hydrogen gas.
For example, aluminum reacts with steam to form aluminum oxide and hydrogen gas.
Hydrochloric acid, dilute sulfuric and nitric acids. Oxides of these metals undergo
reduction when heated with hydrogen gas, carbon, or carbon monoxide. Till copper,
metals can combine directly with oxygen and form metal oxide. Elements present at
the bottom from mercury to gold are often found in the native form in nature and
their oxides show thermal decomposition under mild conditions.
Reactivity Series Chart
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We can summarize the reactivity of different metals in a reactivity series chart.
gas from water, steam and
gas from steam and acids
Smelting with coke
gas from acids only and
included for comparison
combines with O
to form oxides and
All metals have a tendency to lose electrons and form metal ions. In other
The reactivity series of elements can be shown in another way, which includes
oxidation reaction of each metal to the respective metal ion. It gives information
regarding the reducing power of the metal atom and the oxidation number of the
Exothermic and endothermic reactions
All processes can be classified into one of two categories: exothermic and
endothermic. In an exothermic process, energy is released, while in an endothermic
process, energy is stored. This section will specifically cover exothermic and
endothermic chemical reactions, but almost any process can be described as releasing
or storing energy.
The concept of giving off or storing energy can sometimes be a bit confusing,
so let's go over some of the basic types of energy that you'll encounter in your
chemistry class, and what it means to give off and store each type of energy.
Heat: Heat energy is the energy that accompanies temperature changes. If heat
energy is being released then the reaction from which it is released will become
hotter. If heat energy is being stored, then reaction will become colder.
Light: If light energy is being given off, then the reaction will glow. If it's
being stored, then the reaction will seemingly proceed on its own without any catalyst
present without any heat being evolved or absorbed.
Mechanical energy: If mechanical energy is being stored, then the volume
and/or pressure of the reaction will get smaller. If mechanical energy is being given
off, then the opposite will be true.
The most common change in energy that you'll witness in your chemistry class
will be changes in heat energy. It can be measured with a bomb calorimeter. Energy
released or stored in a reaction will often be expressed written as
ΔH, or a change in
enthalpy. A positive
ΔH means that energy is stored and the reaction is endothermic.
ΔH means that energy is released and the reaction is exothermic. It is
usually expressed in kilojoules (kJ) or joules (J).
Why Exothermic Or Endothermic?
If you understand the above section, then you can now identify whether a
reaction is exothermic or endothermic. If it gives off one of the above three types of
energy then it's exothermic, if it absorbs it, then it's endothermic. The question that
still hasn't been answered, though is why? Why are some reactions exothermic while
others are endothermic, and why does energy have to be absorbed or released at all?
The answer lies in chemical bonds. Chemical bonds have bond energies
associated with them. This bond energy is the amount energy that it takes to break the
bonds, and also the amount of the energy that is released when the bonds are formed.
Consequently, if the bonds in your reactants have a higher total bond energy than
your products, the reaction will be endothermic. If they have a lower total bond
energy, it will be exothermic.
The reason for this is the law of conservation of energy, which states that
energy cannot be created or destroyed; it can only change forms. In this case, it would
mean that whatever energy was used to break the bond will be released if the bond is
reformed. For example:
Suppose we have a C-H bond somewhere, and we wanted to break that bond
apart into a C and an H. We'd have to put in some amount of energy. Let's call this
amount 'x'. Once we put in x energy, by say, adding heat, the C-H bond will break
apart. What happened to 'x' though? The conservation of energy law says that 'x'
didn't just disappear; it just took on another form, in this case exciting the electrons in
C and H. Some of the energy went to the C atom and some went to the H atom. If the
C-H bond reformed, then 'x' would be released again. If the C went off and
recombined with a different molecule (let's say a Cl), and so did the H (with an F, for
instance). Then the energy released from the new pairings would be 'x' plus whatever
energy the Cl and F had stored.
Many chemical reactions release energy in the form of heat, light, or sound.
These are exothermic reactions. Exothermic reactions may occur spontaneously and
result in higher randomness or entropy (
ΔS > 0) of the system. They are denoted by a
negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (
0). In the lab, exothermic reactions produce heat or may even be explosive.
These are endothermic reactions. Endothermic reactions cannot occur spontaneously.
Work must be done in order to get these reactions to occur. When endothermic
reactions absorb energy, a temperature drop is measured during the reaction.
Endothermic reactions are characterized by positive heat flow (into the reaction) and
an increase in enthalpy (+
An exothermic reaction is one in which heat is produced as one of the end
the burning of a candle, wood, and neutralization reactions. In an endothermic
reaction, the opposite happens. In this reaction, heat is absorbed. Or more exactly,
heat is required to complete the reaction. Photosynthesis in plants is a chemical
endothermic reaction. In this process, the chloroplasts in the leaves absorb the
sunlight. Without sunlight or some other similar source of energy, this reaction
cannot be completed.
In exothermic reactions the enthalpy change is always negative while in
endothermic reactions the enthalpy change is always positive. This is due to the
releasing and absorption of heat energy in the reactions, respectively. The end
products are stable in exothermic reactions. The end products of endothermic
reactions are less stable. This is due to the weak bonds formed.
‘Endo’ means to absorb and so in endothermic reactions, the energy is
absorbed from the external surrounding environment. So the surroundings lose
energy and as a result
the end product has higher energy level than the reactants. Due to this higher
not spontaneous. ‘Exo’ means to give off and so energy is liberated in exothermic
reactions. As a result, the surroundings get heated up. And most exothermic reactions
When we light a matchstick, it is an exothermic reaction. In this reaction, when
we strike the stick, stored energy is released as heat spontaneously. And the flame
will have lower energy than the heat produced. The energy being released is
previously stored in the matchstick and thus do not require any external energy for
the reaction to occur.
When ice melts, it will be due to the heat around. The surrounding environment
will have a higher temperature than the ice and this heat energy is absorbed by the
ice. The stability of the bonds is reduced and as a result and the ice melts into liquid.
Some exothermic reactions in our lives are the digestion of food in our body,
combustion reactions, water condensations, bomb explosions, and adding an alkali
metal to water.
Reaction rate, the speed at which a chemical reaction proceeds. It is often
that is formed in a unit of time or the concentration of a reactant that is consumed in a
unit of time. Alternatively, it may be defined in terms of the amounts of the reactants
consumed or products formed in a unit of time. For example, suppose that the
balancedchemical equation for a reaction is of the formA + 3B
−da/dt, −db/dtwhere t is the time, [A], [B], and [Z] are the
concentrations of the substances, and a, b, and z are their amounts. Note that these six
expressions are all different from one another but are simply related. Chemical
reactions proceed at vastly different speeds depending on the nature of the reacting
substances, the type of chemical transformation, the temperature, and other factors. In
general, reactions in which atoms or ions (electrically charged particles) combine
occur very rapidly, while those in which covalent bonds(bonds in which atoms share
electrons) are broken are much slower. For a given reaction, the speed of the reaction
will vary with the temperature, thepressure, and the amounts of reactants present.
Reactions usually slow down as time goes on because of the depletion of the
reactants. In some cases the addition of a substance that is not itself a reactant, called
a catalyst, accelerates a reaction. The rate constant, or the specific rate constant, is the
proportionality constant in the equation that expresses the relationship between the
rate of a chemical reaction and the concentrations of the reacting substances. The
measurement and interpretation of reactions constitute the branch of chemistry
known as chemical kinetics.
The reaction rate (rate of reaction) or speed of reaction for a reactant or product
in a particular reaction is intuitively defined as how fast or slow a reaction takes
place. For example, the oxidative rusting of iron under Earth's atmosphere is a slow
reaction that can take many years, but the combustion of cellulose in a fire is a
reaction that takes place in fractions of a second.
Chemical kinetics is the part of physical chemistry that studies reaction rates.
The concepts of chemical kinetics are applied in many disciplines, such as chemical
engineering, enzymology and environmental engineering.
Formal definition of reaction rate
Consider a typical chemical reaction:
→ p P + q Q
The lowercase letters (a, b, p, and q) represent stoichiometric coefficients,
while the capital letters represent the reactants (A and B) and the products (P and Q).
According to IUPAC's Gold Book definition
chemical reaction occurring in a closed system under isochoric conditions, without a
build-up of reaction intermediates, is defined as:
where [X] denotes the concentration of the substance X. (Note: The rate of a
reaction is always positive. A negative sign is present to indicate the reactant
concentration is decreasing.) The IUPAC
recommends that the unit of time should
increase of concentration of a product P by a constant factor (the reciprocal of its
stoichiometric number) and for a reactant A by minus the reciprocal of the
stoichiometric number. Reaction rate usually has the units of mol L
. It is
important to bear in mind that the previous definition is only valid for a single
reaction, in a closed system of constant volume. This usually implicit assumption
must be stated explicitly, otherwise the definition is incorrect: If water is added to a
pot containing salty water, the concentration of salt decreases, although there is no
For any open system, the full mass balance must be taken into account: in
− consumption = accumulation
is the inflow rate of A in molecules per second, F
the outflow, and
in a given differential volume, integrated over the entire system volume V at a given
moment. When applied to the closed system at constant volume considered
previously, this equation reduces to:
where the concentration [A] is related to the number of molecules N
by [A] =
is the Avogadro constant.
derivative of the extent of reaction with respect to time.
is the stoichiometric coefficient for substance i, equal to a, b, p, and q
in the typical reaction above. Also V is the volume of reaction and C
concentration of substance i.
When side products or reaction intermediates are formed, the IUPAC
recommends the use of the terms rate of appearance and rate of disappearance for
Reaction rates may also be defined on a basis that is not the volume of the
reactor. When a catalyst is used the reaction rate may be stated on a catalyst weight
) basis. If the basis is a specific catalyst site
that may be rigorously counted by a specified method, the rate is given in units of s
Factors influencing rate of reaction
- The nature of the reaction: Some reactions are naturally faster than others.
The number of reacting species, their physical state (the particles that form solids
move much more slowly than those of gases or those in solution), the complexity of
the reaction and other factors can greatly influence the rate of a reaction.
- Concentration: Reaction rate increases with concentration, as described by
the rate law and explained by collision theory. As reactant concentration increases,
the frequencyof collision increases.
- Pressure: The rate of gaseous reactions increases with pressure, which is, in
fact, equivalent to an increase in concentration of the gas.The reaction rate increases
in the direction where there are fewer moles of gas and decreases in the reverse
direction. For condensed-phase reactions, the pressure dependence is weak.
- Order: The order of the reaction controls how the reactant concentration (or
pressure) affects reaction rate.
- Temperature: Usually conducting a reaction at a higher temperature delivers
more energy into the system and increases the reaction rate by causing more
collisions between particles, as explained by collision theory. However, the main
reason that temperature increases the rate of reaction is that more of the colliding
particles will have the necessary activation energy resulting in more successful
collisions (when bonds are formed between reactants). The influence of temperature
is described by the Arrhenius equation.
For example, coal burns in a fireplace in the presence of oxygen, but it does not
when it is stored at room temperature. The reaction is spontaneous at low and high
temperatures but at room temperature its rate is so slow that it is negligible. The
increase in temperature, as created by a match, allows the reaction to start and then it
heats itself, because it is exothermic. That is valid for many other fuels, such as
methane, butane, and hydrogen.
Reaction rates can be independent of temperature (non-Arrhenius) or decrease
with increasing temperature (anti-Arrhenius). Reactions without an activation barrier
(e.g., someradical reactions), tend to have anti Arrhenius temperature dependence:
the rate constant decreases with increasing temperature.
- Solvent: Many reactions take place in solution and the properties of the
solvent affect the reaction rate. The ionic strength also has an effect on reaction rate.
- Electromagnetic radiation and intensity of light: Electromagnetic radiation is
a form of energy. As such, it may speed up the rate or even make a reaction
spontaneous as it provides the particles of the reactants with more energy. This
energy is in one way or another stored in the reacting particles (it may break bonds,
promote molecules to electronically or vibrationally excited states...) creating
intermediate species that react easily. As the intensity of light increases, the particles
absorb more energy and hence the rate of reaction increases.
For example, when methane reacts with chlorine in the dark, the reaction rate is
very slow. It can be sped up when the mixture is put under diffused light. In bright
sunlight, the reaction is explosive.
- A catalyst: The presence of a catalyst increases the reaction rate (in both the
forward and reverse reactions) by providing an alternative pathway with a lower
For example, platinum catalyzes the combustion of hydrogen with oxygen at
- Isotopes: The kinetic isotope effect consists in a different reaction rate for the
same molecule if it has different isotopes, usually hydrogen isotopes, because of the
relative mass difference between hydrogen and deuterium.
- Surface Area: In reactions on surfaces, which take place for example during
heterogeneous catalysis, the rate of reaction increases as the surface area does. That is
because more particles of the solid are exposed and can be hit by reactant molecules.
- Stirring: Stirring can have a strong effect on the rate of reaction for
- Diffusion limit: Some reactions are limited by diffusion.
All the factors that affect a reaction rate, except for concentration and reaction
order, are taken into account in the reaction rate coefficient (the coefficient in the rate
equation of the reaction).
For a chemical reaction a A + b B
→ p P + q Q, the rate equation or rate law is
a mathematical expression used in chemical kinetics to link the rate of a reaction to
theconcentration of each reactant. It is of the kind:
For gas phase reaction the rate is often alternatively expressed by partial
In these equations k(T) is the reaction rate coefficient or rate constant,
although it is not really a constant, because it includes all the parameters that affect
reaction rate, except for concentration, which is explicitly taken into account. Of all
the parameters influencing reaction rates, temperature is normally the most important
one and is accounted for by the Arrhenius equation.
The exponents n and m are called reaction orders and depend on the reaction
mechanism. For elementary (single-step) reactions the order with respect to each
reactant is equal to its stoichiometric coefficient. For complex (multistep) reactions,
however, this is often not true and the rate equation is determined by the detailed
mechanism, as illustrated below for the reaction of H
coefficient are both equal to the molecularity or number of molecules participating.
For a unimolecular reaction or step the rate is proportional to the concentration of
molecules of reactant, so that the rate law is first order. For a bimolecular reaction or
step, the number of collisionsis proportional to the product of the two reactant
concentrations, or second order. A termolecular step is predicted to be third order, but
also very slow as simultaneous collisions of three molecules are rare.
By using the mass balance for the system in which the reaction occurs, an
expression for the rate of change in concentration can be derived. For a closed system
with constant volume, such an expression can look like