Download Module 6 Chemical Reactions

Module 6
Chemical Reactions
6.01 – Chemical Reactions
• A chemical reaction, or chemical change,
occurs when one or more substances react
and change into completely different
substances.
• Signs of a Chemical Reaction
• Formation of a new substance with properties
different from its components.
• a change in color is often an indication that a
chemical reaction has occurred
• the production of a gas from solids or liquids
• the release of energy in the form of heat, light, or
both.
6.01 – Chemical Reactions
• Chemists use statements called equations to
represent chemical reactions
• Parts of a Chemical Reaction
• Reactants – starting substances
• Products – substances formed in the reaction
• Subscripts – small numbers showing how many
atoms of each element
• Coefficients -a whole number placed in front of a
formula in a chemical equation.
6.01 – Chemical Reactions
6.01 – Chemical Reactions
• Recall, that matter cannot be created nor destroyed during a
chemical reaction.
• Chemists use coefficients to satisfy the law of conservation
of mass. They add coefficients to reactants and products in a
chemical equation until there is the same number of each
kind of atom on both sides of the equation.
• This is called balancing chemical equations.
6.01 – Chemical Reactions
• Steps to balancing chemical equations
1.Create a T-chart and List the elements in the chemical
formulas on each side in the SAME order
2.Count the number of atoms of each element on each side
3.See what elements don’t match in your T-chart
4.Add coefficients in front of one substance at a time,
•Start by balancing the most complex compound first,
•general rule of thumb is to save Hydrogen and Oxygen
until the end to balance.
5.recount number of atoms of each element
6.Repeat 3, 4 & 5 until all elements are balanced.
6.01 – Chemical Reactions
•____ N2 + ____ H2  ____ NH3
6.01 – Chemical Reactions
•____ NaF + ____ Br2  ____ NaBr + ____ F2
6.01 – Chemical Reactions
• ____ AgNO3 + ____ Cu  ____ Cu(NO3)2 + ____ Ag
6.01 – Chemical Reactions
•____ CH4 + ____ O2  ____ CO2 + ____ H2O
6.02 – Classifying Chemical Reactions
• Chemical reactions take place everywhere
• chemical reactions according to their similar properties.
• One way to categorize chemical reactions is to examine the characteristics
of the reactants and products.
• six main categories:
•
•
•
•
•
•
combustion reactions
synthesis reactions
decomposition reactions,
single replacement reactions
double replacement reactions
acid–base reactions.
6.02 – Classifying Chemical Reactions
• In a combustion reaction, a substance combines with
oxygen and releases energy in the form of heat and
light. We use combustion reactions every day to
produce energy.
• This type of reaction can produce enough energy to
send a shuttle into space.
• Most combustion reactions occur between oxygen and
an organic compound
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
6.02 – Classifying Chemical Reactions
•In a synthesis reaction, two or more
reactants combine to form one new
product. Chemists often use the following
generic equation to represent a synthesis
reaction:
•A + B → AB
2Cu + O2 → 2CuO
6.02 – Classifying Chemical Reactions
•In a decomposition reaction, one reactant
breaks down to form two or more products.
•AB → A + B
•a decomposition reaction is the reverse of a
synthesis reaction.
2H2O → 2H2 + O2
6.02 – Classifying Chemical Reactions
• In a single–replacement reaction, one element
replaces another element in a compound to
form a new substance.
• Replacement reactions are also called
displacement reactions.
• A + BC → AC + B
Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
6.02 – Classifying Chemical Reactions
• In a double–replacement reaction, the ions of two
different compounds in an aqueous solution exchange
places to form two new compounds.
• The other product usually remains dissolved in the
solution. The general form for a double-replacement
reaction is shown below.
• AC + BD → AD + BC
Pb(NO3)2 (aq) + 2KI(aq) → 2KNO3(aq) + PbI2(s)
6.02 – Classifying Chemical Reactions
• In an acid–base reaction, an acid and a base react to form salt
and water. (also called a neutralization reaction)
HCl + NaOH → NaCl + H2O
acid + base → salt + water
• We can rewrite the equation to show the ions involved in this
reaction.
H+ + Cl− + Na+ + OH− → Na+ + Cl− + H+ + OH−
• Remember that most acids contain H+ and most bases contain
OH−.
HCl + NaOH → NaCl + H2O
6.03 – Chemical Reactions and Energy
• Energy comes from
chemical reactions.
• Potential energy is
stored in the chemical
bonds in compounds.
• When a chemical
reaction occurs, some
chemical bonds break,
and others form.
6.03 – Chemical Reactions and Energy
• Recall the law of conservation of energy states that energy cannot be
created or destroyed. Energy can only change forms.
• the reactants and products in a chemical reaction have different
amounts of energy.
• If the products have more energy than the reactants. The law of
conservation of energy states that energy cannot be created out of
nothing. So, the extra energy in the products must have come from
somewhere in the environment.
6.03 – Chemical Reactions and Energy
• All chemical reactions either release or absorb
energy. Chemists classify chemical reactions into
two groups based on this observation:
-Exothermic Reactions
-Endothermic Reactions
6.03 – Chemical Reactions and Energy
• Characteristics of an exothermic
reaction
• Chemical reactions that result in a
net release of energy.
• The products are lower in energy
than the reactants.
• The excess energy is released in the
form of heat and/or light as the
reaction proceeds.
• The rusting of metal is an exothermic
reaction. It occurs so slowly that we
don’t notice the heat being released.
6.03 – Chemical Reactions and Energy
• An endothermic reaction absorbs
heat from its environment as a way
to make this happen.
• Endothermic reactions result in
products that have more energy
than the reactants.
• Photosynthesis is an example of an
endothermic reaction.
6.03 – Chemical Reactions and Energy
• Exothermic reactions produce the majority of heat — and light.
• Heat is produced in these reactions and then transferred to the
environment in three basic ways:
• radiation – energy as waves or
particles
• conduction – direct contact of
particles
• convection – movement of
particles
6.03 – Chemical Reactions and Energy
• Recall that the kinetic molecular theory
states that particles of matter are in constant motion.
• For a chemical reaction to occur, the reactant particles have to come in
contact with each other.
• The rate at which a chemical reaction proceeds depends on three main
factors:
• Temperature
• Concentration
• Catalysts and inhibitors
6.03 – Chemical Reactions and Energy
• An increase in temperature will increase the rate of reaction.
A decrease in temperature will decrease the rate of reaction.
6.04 – Reaction Rates and Temperature
• Concentration is a measure of the amount of a substance in a mixture.
• An increase in concentration will increase the rate of reaction.
A decrease in concentration will decrease the rate of reaction.
6.04 – Reaction Rates and Temperature
• a catalyst can be used to increase the rate of a reaction.
• A catalyst is a substance that speeds up a reaction, but does not itself
participate in the reaction.
• Catalysts work by lowering the activation energy - initial energy
needed to begin a chemical reaction.
• Biological catalysts called enzymes control the rates of chemical
reactions in living things.
6.04 – Reaction Rates and Temperature
• An inhibitor is a substance that slows down a chemical reaction, but is
not itself consumed during the reaction.
• Preservatives are inhibitors that are used in the food industry to slow
down reactions that lead to food spoilage.
• Catalysts increase the rates of reactions.
Inhibitors decrease the rates of reactions
6.05 – Radioactivity
• Remember that atoms are made up of
smaller particles: protons, neutrons, and
electrons.
• The number of protons in an atom
nucleus is its atomic number. Every
element has a unique atomic number.
• In addition to protons, an atom's nucleus
also contains neutrons. The number of
neutrons can vary between atoms of a
given element.
• Scientists use a special shorthand
notation to represent different isotopes
of an element.
6.05 – Radioactivity
• Not all Atoms are Created Equal
• The nucleus contains protons and neutrons.
• The number of protons in an atom of an element is the same for each
atom of that element.
• all atoms of the same element do not contain the same number of
neutrons.
6.05 – Radioactivity
• Isotopes are atoms of the same element that have different numbers
of neutrons. The carbon atoms mentioned here are called carbon–11,
carbon–12, carbon–13, and carbon–14. The number tells you the
total number of protons and neutrons in the nucleus.
6.05 – Radioactivity
• Protons and neutrons are packed
tightly in the nucleus and are held
together by strong nuclear force.
• The presence of neutrons in the
nucleus contributes to strong
nuclear force and helps hold the
nucleus together. More protons in
the nucleus, the more neutrons are
needed balance the repulsive forces.
• Unstable nuclei can break down into
more stable nuclei in the process of
radioactive decay
6.05 – Radioactivity
• When an unstable nucleus releases and proton, the atomic number of
the nucleus changes. If the atomic number changes, the atom of one
element becomes an atom of a different element.
• This process is called transmutation.
• An unstable nucleus can emit different kinds of particles to become
stable: alpha particles, beta particles, and gamma rays.
6.05 – Radioactivity
• An alpha particle is a particle emitted from the nucleus during certain
kinds of radioactive decay.
• Alpha particles are positively charged & made up of two protons and
two neutrons; they are also referred to as helium nuclei
• alpha particles can be stopped by a sheet of paper
• As positively charged alpha particles travel through air, they attract
electrons, slow down quickly, and become harmless helium atoms.
• if alpha particles are inhaled, swallowed, or injected into the
bloodstream, they can damage living tissue
6.05 – Radioactivity
• Emission of Alpha
Particle
• Loses 2 protons and 2
neutrons (alpha =
helium nuclei)
• Atomic Mass will
decrease by 4
• Atomic Number will
decrease by 2
Alpha Decay Practice
210
84
222
86
238
90
Po
Rn
Th
6.05 – Radioactivity
• A beta particle is an electron emitted from the nucleus during certain
kinds of radioactive decay.
• An unstable neutron breaks apart to form a proton and an electron.
The electron is released from the nucleus along with a large amount
of energy. The proton remains in the nucleus.
• Beta particles move faster in air than alpha particles and are harder to
stop. Can pass through materials such as paper and clothing,
penetrate skin and pass fairly deeply into the human body. Cannot
penetrate denser materials, such as aluminum.
6.05 – Radioactivity
• Emission of Beta
Particle
• Unstable neutron
breaks apart to form a
proton and an electron
• Atomic Mass will stay
the same
• Atomic Number will
increase by 1
Beta Decay Practice
14
C
6
40
19
K
90
Sr
38
6.05 – Radioactivity
• Some transmutation processes (emitting alpha or beta) leave the nucleus in
an excited state. The excited nucleus emits energy in the form of gamma
rays.
• Gamma rays, like visible light, are a form of electromagnetic radiation. They
have no mass or charge. Gamma rays can be released along with alpha or
beta particles during radioactive decay.
• Gamma rays have no mass. They are made entirely of energy and can
penetrate most materials. Very dense materials, such as lead, are able to
stop gamma rays. Gamma rays are more harmful to humans than alpha or
beta particles.
6.05 – Radioactivity
• Emission of Gamma
• transmutation
processes (emitting
alpha or beta) leave
the nucleus in an
excited state.
• The excited nucleus
emits energy in the
form of gamma rays.
• Atomic Mass will stay
the same
• Atomic Number will
stay the same
6.05 – Radioactivity
• Naturally Occurring Radiation
• low levels of radiation from the
natural environment (background
radiation)
• The Sun emits small amounts of
gamma radiation and other kinds of
harmful radiation.
• Rocks and soil naturally contain
small concentrations of radioactive
elements, such as uranium.
• Some kinds of rock release radon, a
radioactive gas. Radon gas can be
dangerous to people if it becomes
concentrated in a small area.
6.05 – Radioactivity
• Artificial Sources of Radiation
• X–rays used to diagnose tooth decay and
other diseases.
• Radiation therapy is another source of
radiation.
• Some radioactive materials are used as
tracers to diagnose cancer and other
diseases.
• Radioactive tracers are also used in
agriculture to measure the amount of
fertilizer used by the plant. From these
measurements, they can tell farmers just
how much fertilizer to use on their
crops.
6.06 – Nuclear Reactions
• In the late 1930s, scientist discovered that during a
nuclear reaction - A reaction that changes the number
of protons or neutrons in the nucleus of an atom
• Today, nuclear energy is used as an alternative energy
source and in the future may be used to replace some
common resources such as natural gas, oil, and coal.
• There are two main kinds of nuclear reactions:
• nuclear fission
• nuclear fusion.
6.06 – Nuclear Reactions
• Nuclear fission, or radioactive decay,
• when a heavy atomic nucleus splits into smaller nuclei.
• For example, atoms of uranium–235 can break down into smaller particles. Some of
those particles are atoms of lighter elements. The rest are neutrons.
• During a nuclear reaction, matter changes into energy.
6.06 – Nuclear
Reactions
2
• Einstein's famous equation, E = mc , shows
how matter and energy are related.
• In this equation, E stands for energy, m stands
for mass, and c is the speed of light.
(Remember that the speed of light is about 3 ×
108 m ⁄ s.)
• a small amount of mass is equivalent to an
enormous amount of energy.
• For example, if you converted just 1 g of
matter completely into energy, you would get
9 × 1013 joules of energy. That is an amount of
energy equivalent to about 22 billion
kilocalories.
6.06 – Nuclear Reactions
• If there are enough nuclei packed close together,
the neutrons released by each fission can strike
other nuclei; this is called a chain reaction.
• As in a nuclear reactor or a nuclear bomb, there
are many more millions of uranium nuclei, so
more reactions that can be triggered.
• In a nuclear reactor, the chain reaction is
controlled. Devices called control rods are
inserted into the reactor to absorb some of the
neutrons. This reduces the number of reactions
that occur and controls the amount of energy
the reactor produces. In a nuclear bomb, the
chain reaction is uncontrolled. All of the energy
is released at once.
6.06 – Nuclear Reactions
• Nuclear power plants can generate
large amounts of electricity without
producing any air pollution or
greenhouse gases.
• The waste products of nuclear
reactions remain toxic for thousands
of years, and they must be stored in
special containers to prevent them
from leaking into water and soil.
• The disposal of nuclear waste is one of
the main challenges to using nuclear
fission to generate electricity.
6.06 – Nuclear Reactions
• In April 1986, the nuclear power plant in
Chernobyl, Ukraine, exploded. Almost 5
million people were forced to evacuate and
56 people died as a direct result of this
accident. Several more thousand cancer
deaths may also be attributable to
Chernobyl, although it is difficult to assign
these directly.
• There is a big debate over the use of
nuclear power. Some cite accidents such as
Chernobyl as evidence that nuclear energy
is too risky. Still others insist that accidents
can be averted using strict safety measures,
making nuclear energy one of the cleanest,
safest energy options we have.
6.06 – Nuclear Reactions
• Nuclear fusion occurs when two small nuclei
combine to form one larger nucleus.
• The Sun is an example of a site where nuclear
fusion occurs naturally. Extreme temperatures
are needed to get the nuclei moving fast
enough to fuse.
• Fusion reactions produce mainly helium as a
by–product. Helium is completely non–toxic.
For this reason, nuclear fusion is appealing as
an energy source.
• The temperatures and pressures required to
contain the reaction and harness the energy
produced have been impossible to create in the
laboratory.
6.06 – Nuclear Reactions
• It is important to understand the
differences between chemical
reactions and nuclear reactions..
•part of the atom involved in
reactions.
•amount of energy involved in
each reaction.
•the masses of the reactants
and products.