Define chemical reaction.
A process that leads to the transformation of one set of chemical substances to another.
What do chemical reactions involve?
The breaking and formation of chemical bonds.
What can chemical reactions be represented by?
Examples of chemical reactions.
Burning Magnesium Iron rusting Acid dissolving Copper Oxide
Define chemical equation.
A symbolic representation of a chemical reaction using formulae.
How are chemical equations set out?
Reactants appear on the left side and products on the right with them separated by an arrow.
Examples of chemical equations.
N₂(g) + 3H₂(g) ➝ NH₃(g)
2Mg(s) + O₂(g) ➝ 2MgO(s)
What do balanced equations indicate?
The relative numbers of particles (atoms, molecules or ions) involved in the reaction.
The starting materials for a chemical reaction that appear to the left of the arrow in an equation.
The substances formed as a result of a chemical reaction that appear to the right of the arrow in the equation.
Define phase change.
A physical change where a substance changes between the solid, liquid, gaseous or aqueous state. Also called a change of state.
Examples of phase change.
Labelled chemical equation diagram
Define enthalpy (H).
The stored chemical potential energy in a substance (often called the heat content).
What do reactions often involve?
A change in enthalpy
∆H = Hproducts - Hreactants
Example of enthalpy.
Combustion of Ethanol
∆H = -1370kJ
Law of conservation of energy.
The total amount of energy within a system remains constant (is conserved), although the energy within the sysem can be changed from one form to another.
Example of the law of conservation of energy.
Combustion of Ethane.
Energy is released as heat but only because the products have less chemical energy than the reactants.
Law of conservation of mass.
In a closed system mass will remain constant over time.
Mass will only change if matter is added or removed.
Mass (and energy) can neither be created nor destroyed.
Example of the law of conservation of mass.
H2 + O2 → 2H20
4g 32g 36g
Define exothermic reaction.
A reaction where the system realeases energy to the surroundings (as heat).
The reactants have more energy than the products and the exess is released as heat.
Equation for exothermic reactions.
∆H = -ve
Examples of exothermic reactions.
Adding acid to water
Define endothermic reaction.
A reaction where the system absorbes energy from the surroundings (as heat).
The products have more energy than the reactants.
Equation for endothermic reactions.
∆H = +ve
Examples of endothermic reactions.
Cracking an ice-pack
Dissolving NH4NO3 in water
States of matter.
Change in state during condensation.
Gas to liquid
Change in state during boiling.
Liquid to gas.
Change in state during evaporation.
Liquid to gas (