Unit 8: Chemical Reactions

Question 1

Combustion

Answer 1

reaction of a substance with oxygen to form an oxide with heat and light (fire); also burning

Question 2

Reactant

Answer 2

Reactants are substances initially present in a chemical reaction that are consumed during the reaction to make products.

Question 3

Limiting Reactant

Answer 3

The limiting reactant is a reactant that is completely consumed, thus limiting further production.

For example, if your cake requires 2 eggs, 2 cups of flour and 1 cup of milk, how many cakes can you make if you have 2 eggs, 40 cups of flour, and 1 gal of milk? You can make only one cake. In this scenario, eggs are the limiting reactant.

Question 4

Chemical Reaction (Chemical Change)

Answer 4

a process that involves rearrangement of the molecular or ionic structure of a substance, as opposed to a change in physical form or a nuclear reaction.

There are two driving forces for all chemical reactions. The first is enthalpy, and the second is entropy.

Question 5

Spontaneous Reaction (Immediate Reaction)

Answer 5

Spontaneous reactions occur without outside intervention. They may occur quickly, like the combustion of hydrogen, or slowly, like when graphite turns to diamond.

Question 6

Entropy

Answer 6

Entropy (DS) is a measure of the disorder of a system, and systems tend to favor a more disordered system: nature tends toward chaos.

Question 7

Enthalpy

Answer 7

Enthalpy is a thermodynamic property of a system. It is the sum of the internal energy added to the product of the pressure and volume of the system. It reflects the capacity to do non-mechanical work and the capacity to release heat. Enthalpy is denoted as H; specific enthalpy denoted as h. Common units used to express enthalpy are the joule, calorie, or BTU (British Thermal Unit). Enthalpy in a throttling process is constant.

Enthalpy Formula: H = E + PV

where H is enthalpy, E is internal energy of the system, P is pressure, and V is volume

Question 8

Exothermic Reactions

Answer 8

A chemical reaction that releases energy by heat or light (has a negative ΔH).

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 (ΔH < 0).

Question 9

Endothermic Reactions

Answer 9

The term “endothermic” describes a process which absorbs thermal (heat) energy.

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 (+ΔH).

Remember: ‘Endo’ = Greek prefix meaning ‘inside’

Question 10

What does a chemical reactions always result in?

Answer 10

A new substance.

Question 11

Which type of reaction occurs without being started by an outside force?

Answer 11

Spontaneous Reaction.

Question 12

Activation Energy

Answer 12

The minimum energy which must be available to a chemical system with potential reactants to result in a chemical reaction.

Question 13

Chemical Equation

Answer 13

The written expression of what takes place in a chemical reaction; reactants and products are identified by their chemical formulas in chemical equations.

Question 14

Chemical Equation

Answer 14

A chemical equation is a written representation of the process that occurs in a chemical reaction. A chemical equation is written with the reactants on the left side of an arrow and the products of the chemical reaction on the right side of the equation. The head of the arrow typically points toward the right or toward the product side of the equation, although reactions may indicate equilibrium with the reaction proceeding in both directions simultaneously. To be correct, the atoms on the left side must be balanced with the atoms on the right side of the equation.

The elements in an equation are denoted using their symbols. Coefficients next to the symbols indicate the stoichiometric numbers. Subscripts are used to indicate the number of atoms of an element present in a chemical species.

An example of a chemical equation may be seen in the combustion of methane:
CH4 + 2 O2 → CO2 + 2 H2O

Question 15

The __________ on the left side of an equation must be balanced with the atoms on the right side of the equation.

Answer 15

Total number of atoms.

Question 16

Which choice shows the correct way to balance the following equation?

H + O → H2O

Answer 16

2H2 + O2→ 2H2O

Question 17

Stoichiometry

Answer 17

Stoichiometry is the calculation of reactants and products in chemical reactions.

Question 18

Stoichiometric Number (Stoichiometric Coefficient)

Answer 18

Simply put, the stoichiometric coefficient (or stoichiometric number) of any given component is the number of molecules that participate in the reaction as written. If there is no coefficient in front of the molecule, assume the number is 1.

For example, in the reaction
CH4 + 2 O2 → CO2 + 2 H2O,
the stoichiometric coefficient of CH4 is −1,
the stoichiometric coefficient of O2 is −2,
for CO2 it would be +1 and for H2O it is +2.

Question 19

What is the universal solvent?

Answer 19

Water, due to its hydrogen bonds.

Question 20

Solvent

Answer 20

A solvent is a liquid or gas that dissolves a solid, liquid, or gaseous solute. Particle size influences the rate of dissolving.

Question 21

Solute

Answer 21

A solute is a substance that is dissolved into another substance. Temperature, pressure, and what the solute and solvent are composed of all influence solubility.

Question 22

Dissociation

Answer 22

when a chemical combination breaks up into simpler constituent parts; compounds dissociate into ions and dissociation is normally temporary and reversible in that when atoms or ions of the dissociated substance return to their original conditions, they recombine.

Question 23

Electrolytes

Answer 23

A liquid or gel that contains ions and can be decomposed by electrolysis. Electrolytes conduct electric current because electrons can move through them.

Question 24

Electrodes

Answer 24

A conductor through which electricity enters or leaves an object, substance, or region.

Question 25

Cathode

Answer 25

A positively charged electrode.

Question 26

Anode

Answer 26

A negatively charged electrode.

Question 27

Cation

Answer 27

A positively charged ion, i.e., one that would be attracted to the cathode in electrolysis.

Metals form cations in solutions. Cations in metal solutions combine with anions.

Question 28

Anion

Answer 28

A negatively charged ion, i.e., one that would be attracted to the anode in electrolysis.

Question 29

Galvanic Cell (Voltaic Cell)

Answer 29

A galvanic cell, or voltaic cell is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell.

Question 30

Product

Answer 30

Products are substances that form from the chemical reaction.

Question 31

What is the difference between products and reactants?

Answer 31

Reactants are substances that undergo change. Products are substances that form from the chemical reaction.

Question 32

What is striking a match an example of?

Answer 32

It is an example of providing activation energy to a chemical reaction.