Chemistry 11 - Chapter 4 (Classifying Chemical Reactions)

toth time once again

What is chemistry?

The study of matter and its changes.

What are chemical changes?

Any change in which experimental evidence indicates that original substances (reactants) have been consumed and new substances (products) have been produced. A chemical change involves a reorganization of atoms, ions, or molecules by the breaking and reforming of chemical bonds.

What are the five different kinds of experimental evidence?

1. Noticeable energy change
2. Change in color
3. Formation of a gas in the absence of boiling
4. Formation of a precipitate from two solutions
5. Relatively difficult to reverse

Why do we balance chemical equations?

Because of Lavoisier’s Law of Conservation of Mass

Why is it helpful to classify reactions?

Classifying reactions helps us predict the products of chemical changes and predict whether or not a reaction is likely to occur.

What are the six major categories of chemical reactions?

1. Synthesis
2. Decomposition
3. Single replacement
4. Double replacement
5. Neutralization
6. Combustion

What is a synthesis reaction?

Two or more simple substances (elements or compounds) combine to form one more complex substance.

Are synthesis reactions endothermic or exothermic?

Exothermic, often accompanied by a significant release of heat and light.

What is the energy required to initiate a chemical reaction called?

Activation energy

When a metal oxide combines with water, what forms?

A base

When a non-metal oxide combines with water, what forms?

An acid

What is a decomposition reaction?

When a complex compound is broken down or decomposed into two or more simpler substances (element or compound)

Are decomposition reactions endothermic or exothermic?

Endothermic (reverse of synthesis reactions)

What do decomposition reactions generally involve?

They generally involve one of the following:

* An acid decomposing to form water and non-metal oxides
* A base decomposing to form metal oxides and water
* Ternary salts decomposing to form oxides of metals and non-metals

What is a combustion reaction?

The reaction of a hydrocarbon, carbohydrate, or alcohol reacting with oxygen gas to produce carbon dioxide gas and water.

Are combustion reactions endothermic or exothermic?

Exothermic, releasing a significant amount of energy in the form of heat, light, and/or sound

What is a single replacement reaction?

A reaction between a compound and an element so that the element replaces an element of the same type in the compound, resulting in a new compound and a new element.

What loses and what gains electrons in a single replacement reaction?

The thing doing the replacing loses electrons and the thing getting replaced gains electrons.

How do we determine if a single replacement reaction will proceed?

We must compare the chemical reactivity of the element to that of the element it will replace using the activity series table.

What is chemical reactivity?

The tendency of a substance to undergo a chemical change.

What is a double replacement reaction?

A chemical reaction between two compounds that trade cations or anions with one another.

What are the three categories of double replacement reactions?

1. Precipitation reaction
2. Neutralization reaction
3. Gas formation reaction

What is a precipitation double replacement reaction?

When two freely soluble salts trade partners, two new compounds are formed, one of which is a precipitate.

What is immediate evidence that a precipitation reaction has occurred?

The reaction ends with a solid suspended in solution.

How do we know which product forms a precipitate in a precipitation reaction?

Verify using the solubility table

What is a neutralization reaction?

When an acid and a base react to form a salt and water.

How do we test that a neutralization reaction occurred?

With a pH test or litmus paper to see that the solution in neutral (no more acid or base in the solution)

How do we know that the acid is gone (completely neutralized)?

We see bubbles when pouring the base over if the acid is still there. When we stop seeing bubbles, the acid is gone.

Is a neutralization reaction generally endothermic or exothermic?

Exothermic, as detectable heat is usually released.

What is a gas formation reaction?

A double replacement reaction where one of the products is not very stable and spontaneously decomposes to form water and a gas.

What are the two most common gas formation reactions?

* Carbonic acid, decomposing to form carbon dioxide gas and water
* Ammonium hydroxide, decomposing to form ammonia gas and water

What is enthalpy?

Potential energy that may be gained or stored, usually lost as heat.

What is ΔH?

A change in energy

What is bond energy?

A measure of the bond strength of a chemical bond, and is the amount of energy needed to break the atoms involved in a molecular bond into free atoms. This is determined by the position of the negative electrons in an atom relative to the nucleus and the other nearby nuclei in a molecule.

What is bond energy a result of?

The chemical potential energy of intramolecular bonds.

What are intramolecular bonds?

The bonds between atoms in a molecule.

What are intermolecular bonds?

The bonds that exist between molecules.

What is the difference between the potential bond energy of reactants before and after a chemical or physical change called?

The enthalpy change, or ΔH value.

How does the energy change when breaking bonds differ from the energy change when forming bonds?

Bond breaking requires energy input while bond forming results in energy release.

What is the definition of an endothermic reaction?

If the energy necessary to initiate a chemical change (that is break the bonds in the reactants) is greater than the energy released when the bonds in the products form, the reaction is endothermic and requires a net input of energy (you spend more than you get).

What does an endothermic reaction entail in terms of energy?

* Net absorption of energy
* Gain in enthalpy
* Positive ΔH value

What is the definition of an exothermic reaction?

If the energy necessary to initiate a chemical change (that is break the bonds in the reactants) is less than the energy released when the bonds in the products form, the reaction is exothermic and releases a net gain or energy (you get more than you spend).

What does an exothermic reaction entail in terms of energy?

* Net release of energy
* Decrease in enthalpy
* Negative ΔH value

Is this reaction endothermic or exothermic?

Endothermic

Is this reaction endothermic or exothermic?

Exothermic

How do we find the ΔH value?

P.E. of products - P.E. of reactants (P.E. can also be called enthalpy here)

What is the activated complex?

The state where the reactants are no longer reactants but not yet a product.

How do we find the activation energy?

P.E. of activated complex - P.E. of reactants?

How does energy in a compound correspond to attractive forces?

The less energy stored in a compound, the stronger the attractive forces. The more energy stored in a compound, the weaker the attractive forces. (Whenever attractive forces increase, energy is released)

Thermochemical equations vs. ΔH notation

Thermochemical equations:

* energy change is present in the equation itself as if it were a reactant itself

ΔH notation:

* specifies energy change, but written separate from the equation
* important to know whether it is a positive or negative value

Are energy (enthalpy) changes intensive or extensive properties?

Extensive properties.

What is important to remember when multiplying a reaction with energy notation involved?

When multiplying a chemical reaction to have whole number coefficients, whatever we multiply the equation by, we must also multiply the energy by.

What is stoichiometry used for?

It is used to study the relationships between the various elements and compounds involved in chemical reactions.

What does a balanced equation indicate?

It indicates the relative amounts of the chemicals involved in the reaction.

What do molar ratios tell us about the volume of gases?

Volumes of gases react in the same proportion as moles of gases. (mole ratios = volume ratios)

Why is mixing two solutions to react them so efficient?

The solvent pulls the ions apart. Mobile ions react faster with each other. Mixing two solutions is one of the fastest reactions we know of.

What is a limiting reactant or reagent?

The reactant that is totally consumed after a reaction. It limits the amount of product that can be formed.

What is the excess reactant or reagent?

The reactant that remains once the limiting reactant is completely consumed.

What are AMR and RMR?

AMR is the Actual Molar Ratio (what we currently have). RMR is the Required Molar Ratio (as per the balanced chemical equation).

What is the percentage purity of a chemical?

(mass of pure chemical/mass of impure sample) x 100%

Why is percentage purity important?

The only thing that goes in and out of a stoichiometric reaction is pure, so we need to know how much of a chemical we really have.

What yield does stoichiometry give us and why is it inaccurate?

Stoichiometry gives us the theoretical yield. It is inaccurate as most reactions complete themselves only partially, resulting instead in a percentage yield.

What is the percentage yield?

(Actual Yield/Theoretical Yield) x 100%