Unit 1 CHEM - Exam Prep.

Thermal Energy

Total potential and kinetic energy of a substance

Heat

Transfer of thermal energy from a warm object to a cool object (vice versa).

Temperature

The average kinetic energy of molecules (does NOT reflect the total thermal energy of a system).

What are the differences between thermal energy, temperature and heat?

Temperature is the average kinetic energy, whereas thermal energy is the total kinetic and potential energy of a substance. Heat is the transfer of thermal energy from a warm object to a cool one.

What is a chemical system? How many system types are there?

A composition of studied reactants and products. There are three types.

What is a **closed** system?

A system with energy being exchanged with its surroundings, but NOT matter.

What is an **open** system?

A system with energy AND matter that’s exchanged with its surroundings

What is an **isolated** system?

A system that CANNOT exchange matter or energy with its surroundings.

What is an **endothermic** reaction?

Chemical reaction where thermal energy is **absorbed** from surroundings to **break** reactant bonds.

What is an **exothermic** reaction?

Chemical reaction where thermal energy is **released** into surroundings from the **forming** of product bonds.

How can you tell whether or not a reaction is endo- or exothermic when looking at a skeletal equation?

Endothermic: +energy is on the reactant side

Exothermic: +energy is on the product side

What is **calorimetry**?

Process of measuring energy changes during physical and chemical change.

What is a **calorimeter**?

A device used to measure an energy change.

What is **specific heat capacity**?

Quantity of thermal energy required to raise the temperature of 1g of a substance by 1 degree Celsius . (UNIT: J/g x Degree Celsius)

What has a higher heat capacity: plastic or metal slide?

**Plastic** slide because it is harder to raise its temperature.

What is the **First Law of Thermodynamics**?

Energy is conserved. Energy of universe = constant

What is the **Second Law of Thermodynamics**?

When two objects are in thermal contact, heat is always **transferred from** the object at a **higher** temperature **to** the object at a **lower** temperature **until** the two objects are at the **same** temperature.

What is **enthalpy**?

Total energy (heat content) of the system plus the pressure times the volume.

Is it possible to measure the **total** energy of a system?

**No**, but it is possible to measure the change (delta) in the enthalpy system.

What is the change in pressure times volume assumed to be in a lab?

0 (zero) because there is no change.

What does it mean when the change in enthalpy is **greater than zero**?

The system absorbs thermal energy from its surroundings and the reaction is endothermic.

What does it mean when the change in enthalpy is **less than zero**?

The system releases thermal energy into its surroundings and the reaction is exothermic.

What are three ways to represent changes in enthalpy?

1. Within the skeletal equation (enthalpy is on the product or reactant side depending on type of reaction (endo-/exo-).
2. As a separate expression (displayed next to the skeletal equation as: change in enthalpy = _______).
3. As a potential energy diagram.

What is a **Potential Energy Diagram**?

Diagraming displaying change in enthalpy by having reactants and products at different potential energy levels as a reaction progresses.

What does the difference between the reaction and product of a Potential Energy Diagram represent?

Change in enthalpy.

When the **reactant** is **above** the **product** in a potential energy diagram, what type of reaction is present?

**Exothermic** reaction, as more energy has been released to form the bonds of the products.

When the **product** is **above** the **reactant** in a potential energy diagram, what type of reaction is present?

**Endothermic** reaction, as more energy has been absorbed to break the bonds to build the product.

What is **molar enthalpy**?

Enthalpy change per mole of a substance.

What are **enthalpies of formation/solution**?

The hypothetical change in enthalpy for the formation of one mole of a compound in its standard state (most stable form at SATP) from the elements in their standard states.

What is Hess’s Law?

Change in enthalpy is the same no matter the number of steps needed to find it. Method of obtaining the enthalpy change from reactants to products.

“The enthalpy change for the conversion of reactants to products is the same whether the conversion occurs in one step or several steps.”

Why would you use Hess’s Law rather than a calorimeter?

Some reactions occur too fast or too slow. Some are too dangerous.

What is SATP?

25 Dgr C and 100 kPa

What is **chemical kinetics**?

Study of reaction rate and reaction mechanisms.

What are **reaction rates**?

The **rate** at which a **reactant** is **consumed** or a **product** is **formed** over a **series of intervals**.

Where can we find the **average rate of reaction** on a graph?

Slope of line between two points.

What is an **instantaneous rate**? Where can it be found on a graph?

Rate at a particular time. Slope of tangent.

What is the **time** of the **initial** rate?

Time = 0(zero); it is the instant the reactants are mixed, at the very beginning.

What is **Collision Theory**?

Theory to describe, explain, and predict characteristics of chemical reactions.

What are **three** main points of Collision?

1. A chemical reaction must involve **collisions** of reactant particles.
2. An *effective* collision requires **sufficient energy** (to break bonds/form new bonds).
3. An *effective* collision requires the **correct orientation** (collision geometry) of the colliding entities.

What is **activation energy**?

**Minimum** collision **energy** required for the reaction to take place (Ea).

What are **examples** of activation energy?

**Push** of a rock to roll down a hill or **spark** to start a combustion reaction.

What is an **activated complex**?

**Unstable molecule** containing partially broken and partially formed bonds representing the **maximum potential energy point** in the change.

What is a **catalyst**? Are they used up throughout the reaction? What do they not do?

* A substance that **speeds up** a chemical reaction/ provides an **alternative pathway** for the reaction which has a **lower activation energy**.
* It is **not used up** throughout the reaction, it **remains the same substance** throughout.
* **Does not** increase the **number of collisions** between reactant entities or increase their **kinetic energies**.