• Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons;

• This is done by chemical bonding, where old bonds are broken, and new bonds are formed;

• This process involves the transfer of thermal energy into and out of the reaction;

• The terms used to describe this are the system (what hapens in the chemical reaction) and the surroundings (the enviorment around the reaction);

• Chemical bonds are tiny stores of chemical energy.

The 3 most basic types of bongs are:

→ Mettalic bonding

→ Ionic bonding

→ Covalent bonding

Exothermic Reactions

• Exothermic means Exits of energy.

• In exothermic reactions, thermal energy is released (is transferred to the surroundings), so the temperature of the surroundings increases.

• Making new chemical bonds gives out energy to the surroundings. Therefore, making new bonds is an exothermic reaction.

• Combustion, oxidations, and neutralisation reactions are typical exothermic reactions.

Surroundings include:

→ The test tube where the reaction takes place;

→ The air around the test tube;

→ The thermometer in the test tube.

Endothermic Reactions

• Endothermic means Enters of energy

• In endothermic reaction, thermal energy is taken in from the surroundings, so the temperature of the surroundings decrease (energy is absorbed) and old bonds are broken.

• Breaking chemical bonds takes in energy from the surroundings. Therefore, breaking bonds is an endothermic reaction.

• These types of reaction are much less common than the exothermic reactions.

• Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions.

Energy Level Diagrams

Also called reaction pathway diagrams, show the energy levels of the reactants and product in chemical reactions and also if the reaction is Exothermic or Endothermic.

The energy of the reactants and products are displayed on the y-axis, and the reaction progress is shown on the x-axis.

The difference in height between the energy of reactants and products represtents the overall energy change of a reaction.

Arrows also indicate whether the reaction is exothemic (the arro points down, showing that the system has lost energy) or endothermic (the arrow points up, showing that the system hass gained energy).

The reactants are always shown on the left and the products on the right.

Energy Level Diagrams of Exothermic Reactions

The reactants have a larger overall energy thant the products. So, during the reaction the energy is released to the surroundingd as heat. This is shown by the arrow pointing down.

An example of an Exothermic reaction is the reaction between methane (CH4) and Oxygen (O2), that is a combustion.

The reactants have less energy than the product. So, during the reaction, the enrgy is absorbed from the surrounding as heat. This is shown by the arrown pointing up.

Energy Level Diagrams of Endothermic Reactions

An example of an Endothermic reaction is the reaction between Nitrogen (N2) and Oxygen (O2) to produce nitrogen monoxide.

The energy of the reactants is lower than the energy of the products, that is why energy is taken in (absorbed) during the reaction. The arrow is pointing upwards to show that.

Activation Energy

• The initial increase in energy represents the activation energy (Ea), which is the minimum energy that a reaction needs to begin (to happe)

• The greater the initial curve, the more enery is required for the reaction to begin (more heat is needed).

• Even exothermic reactions needs energy at the beginning. Because initially some bonds need to be broken before any new bonds can be formed (new products).

• Stronger bonds require more energy to be broken (Bond Energy).

Activation energy definition - is the minimum energy required to start a chemical reaction. For a reaction to begin, the particles must collide with eachother and for that they need a certain energy (Ea).

Enthalpy and enthalpy change (∆H)

• Enthalpy is the amount of heat (thermal energy) in a system.

• The transfer of thermal energy during a reaction is called the enthalpy change, ∆H.

• ∆H is either a negative or a positive value depending on whether the reaction is exothermic or endothermic.

EXOTHERMIC:

→ Heat is released from the system to the surroundings, so the Enthalpy change is negative (-∆H).

ENDOTHERMIC:

→Heat is taken in from the surroundins to the system so the Enthalpy of the system increases, and the Enthalpy change is positive (∆H).

Enthalpy change (∆H) = Energy taken in - Energy given out

Energy Level Diagrams and Bonds

Exam tip:

• Making bonds between people is very good and positive, and that warms up our hearts, making bonds is Exothermic (releases heat)

• Breaking bonds between people consume a lot of energy from the people involved, so breaking bonds is Endothermic (absorbs energy)

Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed.

• Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds.

• Bond making is always an exothermic process as energy is transfered to the surroundings as the new bond is transformed.

Making bonds = Exothermic Reaction

Breaking bonds = Endothermic Reaction