Thermochemistry Heat and Energy

HEATING The flow of energy from a material with a higher temperature to one with a lower temperature. When energy is transferred (lost or gained), there is a change in the energy within the substance. Most common method of transfer is conduction.

Conduction Conduction involves the transfer of energy by contact. When particles with higher energy (higher T) contact particles with lower energy (lower T), energy is transferred. This continues until all particles have approximately the same energy.

Why do objects feel “hot” or “cold”? A “hot” object is simply one that transfers energy to your finger when you touch it. A “cold” object is one that takes energy from your finger when you touch it. “Hot” and “cold” are relative terms.

System vs. Surroundings The system is what we are talking about: A chemical reaction A piece of metal A beaker of water The surroundings is all the substances around the system.

Direction of Heat Flow Surroundings Sometimes, energy is transferred from the surroundings INTO the system. Sometimes, energy is transferred OUT of the system to the surroundings. System System This process is ENDOTHERMIC This process is EXOTHERMIC Heat (q) – Heat is thermal energy that can be transferred from an object at one temperature to an object at another temperature – Net transfer of thermal energy stops when the two objects reach the same temperature. To study the flow of energy during a chemical reaction, one must distinguish between the system and the surroundings. System — the small, well-defined part of the universe in which we are interested in studying (such as a chemical reaction) Surroundings — the rest of the universe (including the container in which the reaction is carried out) Thermochemical equations are chemical equations in which heat is shown as either a reactant or a product. Exothermic reaction — process in which heat (q) is transferred from the system to the surroundings: q < 0 Endothermic reaction — process in which heat is transferred to the system from the surroundings: q > 0

Enthalpy, H

Enthalpy is a measure of the stored energy in a system. Enthalpy cannot be measured. Instead, we measure the change in enthalpy (ΔH) as a process occurs. We will learn ways to do this later in the unit. For now, you will be given ΔH, as you will see in the example problems.

Amount of energy The total amount of energy that is exchanged through heating is abbreviated with a lower case q. In an exothermic process, q < 0. (As is ΔH) In an endothermic process, q > 0. (As is ΔH) The sign of q represents direction of energy flow. -180 J means 180 J of energy are flowing out of the system (and into the surroundings). +120 J means 120 J of energy are flowing into the system (from the surroundings).

q sounds a lot like ΔH ?! ΔH refers to a specific process. For that process, ΔH doesn’t change. ΔH is a ratio. q is simply a measure of the total heat exchanged. q = nΔH This formula may cause problems; you are better off using unit analysis like you did in Unit 6.

q sounds a lot like ΔH?! As an analogy, ΔH is like molar mass and q is like mass. You can have any mass of water, but the molar mass will always be 18.0 g/mol. You can release any amount of energy while burning methane (and therefore, get any value for q), but the ΔH for this process will always be -882.0 kJ/mol methane.

Look at the following reaction Fe2O3 + 3 CO  2 Fe + 3 CO2 ΔH = -23 kJ/molrxn Is this reaction endothermic or exothermic? How do you know? Is energy absorbed or released by the system?

Fe2O3 + 3 CO  2 Fe + 3 CO2 ΔH = -23 kJ/molrxn For every 1 mole of Fe2O3 that reacts, 23 kJ of energy are released. For every 3 moles of CO that react, 23 kJ of energy are released. For every 2 moles of Fe that form, 23 kJ of energy are released. For every 3 moles of CO2 that form, 23 kJ of energy are released.

Fe2O3 + 3 CO  2 Fe + 3 CO2 ΔH = -23 kJ/molrxn What is the value of q when 3.50 moles of iron (III) oxide react? According to the balanced equation, 23 kJ are released for every 1 mol of Fe2O3. ΔH = -23 kJ/mol Fe2O3

Fe2O3 + 3 CO  2 Fe + 3 CO2 ΔH = -23 kJ/molrxn How much energy is released when 3.50 moles of iron (III) oxide react? How is this question different? Let’s use unit analysis this time.

Fe2O3 + 3 CO  2 Fe + 3 CO2 ΔH = -23 kJ/molrxn 1. How much energy is released when 14.5 grams of carbon dioxide are formed? 2. How many grams of Fe2O3 must react to release 140 kJ of heat?