physical & chemical change 2026

Physical change

Change in shape, state, colour, or electrical conductivity, and no new substances are formed

Exothermic reaction

Reactions which transform chemical potential energy into thermal energy

Endothermic

Reactions which transform thermal energy into chemical potential energy

Heat of reaction (🔼H)

Net change of the chemical potential energy of the system

Activation energy

The minimum energy required to start a chemical reaction

Activated complex

High energy, unstable, transition state between the reactants and the products

Characteristics of physical change (5)

1. No new substances formed

2. No change in chemical properties of the substance

3. Physical dimensions might change

4. Electrical conductivity may change

5. Involves small energy changes, energy is either absorbed or released

Catalyst

A substance that increases the rate of the reaction but remains unchanged at the end of the reaction

Decomposition reaction

Compound breaks into simpler substances

Synthesis reaction

Two or more substances combined chemically to form more complex compounds

Displacement reaction

Cations swap places to bond with a different anion

Single displacement / redox reaction

Atom swap places with a cation to bond with an anion

Redox

Transfer of electrons

Law of conservation of energy

The total energy in a system cannot be created nor destroyed, only transformed from one form to another

Negative 🔼H

If heat flows from the system to its surroundings the chemical potential energy of the system decreases

Positive 🔼H

If heat flows from surroundings to the system, the chemical energy increases

Bond breaking (3)

1. Input energy

2. Endothermic

3. Absorb energy

Bond forming (2)

1. Release energy

2. Exothermic

   
   

Spontaneous reactions

1. Reactants have enough energy for a reaction to start by itself

2. Activation energy supplied by reactants themselves

Non-spontaneous

1. reactants do not have enough energy to start a reaction

2. Additional energy needed in the form of light or heat energy

Heterogeneous catalyst

Not the sane state as the reactants

The law of constant composition

A specific chemical compound always consists of the same elements in exactly the same ration

The law of conservation of mass

Matter cannot be created nor destroyed in chemical reactions

Label parts A to F

A. Energy of reactants

B. Activation energy/ energy absorbed for forward reactions

C. Activated complex, for reaction without catalyst

D. Activation energy for catalysed forward reaction

E. Energy of products

F. Activated complex for reaction with catalyst