Unit 5 Chemical Change

Disciplinary Core Ideas

Pieces of the DCI taken from the FRAMEWORK. The entire DCI is not unpacked, just those pieces related to this unit.

PS1.A: Structure and Properties of Matter

“The periodic table places elements with similar chemical properties in columns.”

• Elements with similar properties have same number of outer electrons
• The number of valence electrons for an element is based on their position in the periodic table
• The number of valence electrons largely determines the chemical properties of an element

“The repeating patterns of the periodic table reflect patterns of outer electron states.”

• Chemical reactions involve a change in the position of outer electrons in the forming and breaking of chemical bonds between atoms
• Atoms will transfer electrons to/from another atom or share electrons with another atom
• Often atoms react with other atoms to achieve an octet (8 electrons in outermost shell)
• It is the number of electrons in the outermost shell (valence electrons) that is most important in determining how an atom will react with another atom

   

PS1.B: Chemical Reactions

“The fact that atoms are conserved.”

• The total mass of the reactants was the same as the total mass of the products.
• Chemical equations must be balanced to obey the law of conservation of mass
• A balanced equation has the same number of atoms of each element on the reactants side as the products side.
• Coefficients are small whole numbers placed in front of the formulas in a skeleton equation in order to balance it

   

“Chemical properties of the elements involved, can be used to describe and predict chemical reactions.”

• Two general types of bonds form during chemical reactions: ionic and covalent
• Ionic bonding is the complete transfer of valence electron(s) between atoms. It is a type of chemical bond that generates two oppositely charged ions.
• In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons to become a negatively charged anion.
• Covalent bonding is the sharing of electrons between atoms. This bonding occurs primarily between nonmetals

“Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules.”

• The energy transfer between system and surroundings occur by molecular collisions
• Thermal transfer of energy occurs through random collisions of neighboring molecules
• Reaction rate is expressed as the change in the amount of reactant or product per unit time
• Collision theory is used to explain how reaction rates can differ.
• Molecules that collide can break bonds and form new bonds, producing new molecules
• The minimum energy that colliding particles must have in order to react is called activation energy
• In general, as kinetic energy of colliding particles increases and the number of collisions increases, the reaction rate increases
• There are 4 major factors that influence reaction rate. They are temperature, concentration, particle size, and catalysts.
• An increase in temperature causes particles to move faster, increasing the frequency of collisions and the number of particles that reach the activation energy, thus forming products faster.
• A higher concentration will allow for a greater frequency of collisions and also increase reaction rate
• The smaller the particle size, the greater the surface area and thus an increase in frequency of collisions and reaction rate.
• A catalyst can increase reaction rate by permitting reactions to proceed along a lower energy path

“…and the rearrangements of atoms into new molecules.”

• chemical changes is any change that results in the formation of new substances
• The atoms of the reactants are only rearranged to form new products during a chemical reaction
• There are 5 fundamental types of reactions (synthesis, decomposition, single replacement, double replacement, and combustion)
• Synthesis – 2 or more substances react to form a single new substance
• Decomposition – a single compound breaks down into two or more simpler products (most require energy)
• Single replacement – a chemical change in which one element replaces a second element in a compound
• Double replacement – a chemical change involving an exchange of positive ions between two compounds. Generally take place in aqueous solution and often produce a precipitate, a gas, or water
• Combustion – a chemical change in which an element or a compound reacts with oxygen often producing energy in the form of heat and light. The reactants are usually hydrocarbons with oxygen

“The sum of all bond energies in the set of molecules that are matched by changes in kinetic energy.”

“A stable molecule has less energy than the same set of atoms separated; one must provide at least this energy in order to take the molecule apart.”

• There is energy stored in chemical bonds. It is called chemical potential energy.
• The kinds of atoms and the arrangement of atoms determine the amount of chemical potential energy stored in a substance
• During a chemical reaction, atoms are rearranged into new groupings that have different relative potential energies
• The change in potential energy is either a result of absorption of energy from the surroundings (endothermic) or the release of energy to the surroundings (exothermic)
• Bond breaking is endothermic and bond making is exothermic
• The total energy change of the chemical reaction system is matched by an equal but opposite change of energy in the surroundings
• A reaction is overall exothermic if more energy is released when new bonds form in the products than is used when bonds in the reactants are broken
• If it takes more energy to break bonds in the reactants than is released when new bonds form in the products then the reaction will be overall endothermic

“A dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present.”

• Some reactions are reversible This can be indicated with a double arrow in a chemical equation
• A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur at the same time
• Chemical equilibrium is established when the rates of the forward and reverse reactions are equal (dynamic equilibrium)
• A shift in the equilibrium position can occur when stresses are added to the system
• LeChatelier’s Principle states when a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress
• The stresses that upset the equilibrium of a chemical system include changes in concentration of reactants or products, changes in temperature, and changes in pressure
• Changing the concentration of a chemical will shift the equilibrium to the side that would reduce that change in concentration
• Increasing the temperature causes the equilibrium of a reaction to shift in the direction that absorbs heat