Heat in Chemical Reactions

Theory of Heat

The theory of heat is based in how particles move This theory is called Kinetic Molecular Theory

Essentially, the faster a particle is moving, the more heat the particle contains The greater the kinetic energy of a particle, the greater the heat of the

particle

Not all particles in a system are at the same heat so they do not all move at the same speed

Theory of Heat

Temperature is an AVERAGE of every particles kinetic energy (heat) in a system

When you take the temperature of a liquid, you are finding the average speed the particles are moving in the liquid

Theory of Heat

Heat is transmitted to other objects by particles striking each other and transferring kinetic energy

A system wants to reach Thermal EquilibriumAll of the particles in a system will collide and

transfer energy until they all have the same kinetic energy

Theory of Heat

Thermal Equilibrium Example:

Imagine filling a cup with hot coffee. Immediately the cup heats up and the air above the coffee heats up. This is because those are the particles directly touching the hot coffee. The hot particles in the coffee strike the cup and the air above and transfer their kinetic energy. Eventually, the surrounding air continues to strike the cup and energy is transferred throughout the entire surrounding particles. This happens until the coffee, the air, and the cup all reach the same heat levels.

Theory of Heat

Because heat is kinetic energy an is transferred by particles striking and increasing the speed of others, we use an important convention in thermodynamics:

Heat ALWAYS flows from HOT to COLD

You can never let the cold out, you can only let the heat in!

Heat – A TRANSFER of Energy

Heat is the transfer of energy. Note that heat always flows from hot to cold, never from cold to hot. For example, imagine you had an ice cube in your

hand. The heat from your hand gets transferred to the ice cube.

That is, fast moving particles hit the slow moving particles in the ice cube.

This interaction slows the particles down in your hand (making it cold) and speeds up the particles in the ice (making it warm).

Thus, thermal energy is transferred from your hand into the ice cube (coldness does not travel from the ice cube to your hand).

Bonds are Energy

Compounds are held together with bondsThese bonds are potential energy

When a bond is formed, energy is releasedWhen a bond is broken, energy is absorbed

Bonds in Chemical Reactions

During a chemical reaction, bonds are broken, formed, or both

In the reaction in the picture, some bonds are broken and some are formed

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

Energy in Chemical Reactions

In the reaction on the side, the products have less energy than the reactants

An EXOTHERMIC REACTION is one that gives off energy Energy is given off when the products

have less energy than the reactants

Graphing Exothermic Reactions

The energy of reactions is sometimes graphed

In an exothermic graph, the products are ALWAYS lower in energy than reactants

The difference between products and reactants is the Enthalpy

Energy in Chemical Reactions

In the reaction at the right, the products are at a higher energy than the products

An ENDOTHERMIC REACTION is one that takes in (absorbs) energy The products are at a higher energy

than the reactants

2 H2O(l) → 2 H2 (g) + O2 (g)

Graphing Endothermic Reactions

When graphing an endothermic reaction, the products will always be higher in energy than the reactants

Enthalpy

Enthalpy is the energy difference between products and reactants It is ALL of the energy (potential and kinetic)

Enthalpy is a measurement of energy and is measured in Kilojoules (kJ) (sometimes in calories like with your food)

Enthalpy has gets the symbol ∆H

Enthalpy in Equations

Enthalpy can be written in 2 ways1. In a chemical reaction

For exothermic reactions, enthalpy is a PRODUCT

For endothermic reactions, enthalpy is a REACTANT

In both cases, it is ALWAYS written as a positive number

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g) + 2043.9 kJ

2 H2O(l) + 571.6 kJ → 2 H2 (g) + O2 (g)

Enthalpy in Reactions

Enthalpy can also be written another way2.Outside of a chemical reaction

For exothermic reactions, enthalpy is a NEGATIVE

For endothermic reactions, enthalpy is a POSITIVE

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g) ∆H = - 2043.9 kJ

2 H2O(l) → 2 H2 (g) + O2 (g) ∆H = +571.6 kJ

Stoich with Heat

Since we know molar ratios and the energy released/absorbed in a chemical reaction, we can use stoichiometry to calculate various things

Example If 14.7g of methane are burned, how much heat is given off?

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g) + 2043.9 kJ