Chemical Fundamentals: Bonding, Reaction Rates, and Energy Transfer

Fundamental Chemical Concepts

Diffusion is the movement of particles through a liquid or gas.

• Elements consist of only one type of atom.
• Compounds are substances that are chemically bonded.
• Mixtures are easily separated, unlike compounds.

Chemical Bonding and Atomic Structure

Covalent Bonding and Structures

A covalent bond is a shared pair of electrons.

Types of Covalent Structures

• Simple Molecular Substances: The atoms within a molecule are held together by very strong covalent bonds.
• Giant Covalent Structures: Similar to ionic structures, except that there are no charged ions. All the atoms are bonded to each other by strong covalent bonds.

Isotopes and Relative Atomic Mass

Isotopes are different atomic forms of the same element. They have the same number of protons but different numbers of neutrons.

Relative Atomic Mass takes all stable isotopes into account.

Reaction Kinetics: Speed and Measurement

Reactions can proceed at various rates.

Factors Affecting Reaction Rate

The rate of a reaction depends on four key factors:

1. Temperature
2. Concentration (or pressure of gases)
3. Catalyst
4. Size of Particles (or surface area)

Methods to Measure Reaction Speed

Three common ways to measure the speed of a reaction:

• Precipitation: This occurs when the product of the reaction is a precipitate which clouds the solution.
• Change in Mass: Usually measured when a gas is given off. Measuring the speed of a reaction that produces a gas can be carried out on a mass balance.
• Volume of Gas Given Off: This involves the use of a gas syringe to measure the volume of gas produced.

Example Rate Experiments

• Reaction of hydrochloric acid and marble chips.
• The decomposition of hydrogen peroxide: $2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$

Collision Theory and Catalysis

More collisions increase the rate of reaction:

• Higher temperature
• Higher concentration
• Larger surface area

Catalysts: A solid catalyst works by giving the reacting particles a surface to stick to. They increase the number of successful collisions by lowering the activation energy. Faster collisions also increase the rate of reaction.

Energy Transfer in Chemical Reactions

Energy must be supplied to break bonds.

Exothermic Reactions

In an exothermic reaction, energy is given out to the surroundings, usually in the form of heat, and is typically shown by a rise in temperature.

Endothermic Reactions

In an endothermic reaction, energy is taken in from the surroundings, usually in the form of heat, and is typically shown by a fall in temperature.

Enthalpy Change

The overall change in energy in a reaction is called the enthalpy change.