C2-C6 IGCSE Combined Science

Electrolysis

A process by which an electric current breaks chemical bonds in ionic compounds in molten state or aqueous solutions.

Electrolyte

A substance that dissolves in water to give a solution that conducts electric current

Non-electrolyte

A substance that dissolves in water to give a solution that does not conduct an electric current

Electrical cell

Produces an electric current using the chemical or physical properties of different materials.

Decomposition

A chemical reaction that breaks down compounds into simpler products.

Anode

Positive electrode

Anions

Negatively charged ions

Cathode

Negative electrode

Cations

Positively charged ions

Inert electrode

An electrode that does not react with the other parts of the cell. Example carbon or graphite electrodes

Electrolysis of molten lead bromide

PbBr₂ (l) ⎯→ Pb(s) + Br₂ (l)

Brown fumes of bromine gas is formed at the anode. Silvery lead is deposited at the cathode.

Electrolysis of dilute sulfuric acid

Hydrogen gas is produced at the cathode and oxygen gas is produced at the anode.

Electrolysis of sodium chloride solution

Hydrogen is produced ta the cathode and chlorine gas is produced at the anode.

Physical change

A change that does not produce a new substance and is usually easy to reverse

Chemical change

A change that produces a new substance with different properties and is usually difficult to reverse

Signs of chemical change

Indicators such as colour change, temperature change, or effervescence (fizzing)

Rate of reaction

The speed at which reactants are converted into products

Effect of concentration on rate

Increasing concentration increases rate because more particles are present leading to more collisions

Effect of pressure on rate

Increasing pressure increases rate because particles are closer together leading to more collisions

Effect of surface area on rate

Increasing surface area increases rate because more particles are exposed for collisions

Effect of temperature on rate

Increasing temperature increases rate because particles have more kinetic energy and collide more frequently with greater energy

Catalyst

A substance that increases the rate of reaction and is unchanged at the end of the reaction

Effect of catalyst on activation energy

A catalyst lowers the activation energy of a reaction

Collision theory

A theory stating that reactions occur when particles collide with sufficient energy and correct orientation

Successful collision

A collision between particles with energy greater than the activation energy resulting in a reaction

Activation energy

The minimum energy that particles must have to react

Redox reaction

A reaction in which oxidation and reduction occur simultaneously

Oxidation (electron definition)

The loss of electrons or an increase in oxidation number

Reduction (electron definition)

The gain of electrons or a decrease in oxidation number

Roman numerals in ion names

Indicate the oxidation number of a transition metal ion such as iron(II) or iron(III).

What is an exothermic energy transfer?

Energy transfer to the surroundings

What happens to the temperature in an exothermic reaction?

It increases

What is an endothermic energy transfer?

Energy transfer from the surroundings

What happens to the temperature in an endothermic reaction?

It decreases

how do you calculate enthalpy change

energy of products-energy of reactants

Give five examples of exothermic chemical reactions

Neutralization, respiration Self-heating cans, hand warmers & combustion

Give four examples of endothermic chemical reactions

Thermal decomposition, sports injury pack, electrolysis & photosynthesis

What is a reaction pathway?

Diagram showing how the energy changes in a reaction

what direction do exothermic reaction go in reaction pathways

downwards

what direction do endothermic reaction go in reaction pathways

upwards

What is activation energy?

The minimum energy with which particles must collide in order to react

What is bond energy?

The energy required to break a bond or the energy released when a bond is formed

What are the units for bond energy?

kJ/mol

In terms of energy transfer, bond breaking is

Endothermic

In terms of energy transfer, bond making is

Exothermic

Draw and label a reaction pathways for an exothermic reaction

heat given out to surroundings = enthalpy change

Draw and label a reaction pathways for an endothermic reaction

heat absorbed = enthalpy change

chemical symbol

a letter or group of letters representing an element in a chemical formula

compound ion

an ion made up of several different atoms covalently bonded together and with an overall charge

giant metallic lattice

a three-dimensional structure of positive ions and delocalised electrons, bonded together by strong metallic bonds; elements that are metals will be structured this way

giant covalent structure

a large lattice of covalently bonded atoms, can be found in covalent compounds and some non-metallic elements

simple molecules (elements)

elements found in nature covalently bonded to at least one other atom of the same type

side of the periodic table that has metals

left

noble gases

found in nature as single atoms and in group 8 of the periodic table

carbonate

CO3 2-

nitrate

NO3-

sulfate

SO4 2-

ammonium

NH4+

stoichiometry

the relationship between the relative quantities of substances taking part in a reaction or forming a compound

valency

the number of bonds an atom can form

how to determine valency of elements in groups 1-4

it equals the group number

how to detemine the valency of elements in groups 5-7

it is equal to 8-the group number

valency of group 8 elements (the noble gases)

0; they don't form bonds

cross over method for working out chemical formulae for simple covalent compounds with a central atom

write down the chemical symbols, write the valencies of each element, "cross over", simplify if needed

molecular formula

a chemical formula that shows the actual number of atoms of each element present in a molecule of the compound

cross over method for working out chemical formulae for ionic compounds

write down the chemical symbols, write the charges of each ion, "cross over" charges to get the number of opposite atoms present

charge of metal ions

positive

charge of hydrogen ion

positive

charge of non-metal ions

negative

how to determine charge of ions from elements in groups 1-4

charge equal to group number

how to determine charge of ions from elements in groups 5-7

8-group number for charge

charge of transition metals

given in brackets with a name, such as copper (I) or Cu (II) so that the charge is equal to the number in the bracket

word equation

an equation in which the reactants and products in a chemical reaction are represented by words

reactants

the substances that take part in and undergo change during a chemical reaction

products

the substance(s) produced by a chemical reaction

balanced chemical equation

statement of a chemical reaction with the number of each type of atom equalized for both the products and reactants

state symbols

solid (s), liquid (l), gas (g), and aqueous (aq)

aqueous

dissolved in water

ionic equation

the simplified equation for a reaction involving ionic substances: only those ions which actually take part in the reaction are shown

Element

a substance that cannot be separated or broken down into simpler substances by chemical means

Compound

A substance made up of atoms of two or more different elements joined by chemical bonds

mixture

material composed of two or more elements or compounds that are physically mixed together but not chemically combined

Electron

A subatomic particle that has a negative charge

Proton

A subatomic particle that has a positive charge and that is found in the nucleus of an atom

Nucleus

Center of an atom

Neutron

A subatomic particle that has no charge and that is found in the nucleus of an atom

atomic number

the number of protons in the nucleus of an atom

Atomic Mass

Number of protons and neutrons

electron shells

An energy level representing the distance of an electron from the nucleus of an atom.

Electron Configuration

the arrangement of electrons in the orbitals of an atom

group number

the number of the vertical column that an element is in on the Periodic Table

period number

A horizontal row of elements in the periodic table

Noble Gases

the elements in Group 8 of the periodic table, very unreactive

ionic bonding

Chemical bonding that results from the electrical attraction between cations and anions

ions

charged particles

Diatomic Molecules

molecules made up of two atoms of the same element

Covalent Bonding

results from the sharing of electron pairs between two atoms

Dot and cross diagram

A diagram to show the electron sharing in a molecule

Max Number of electrons first valance shell

2

Max Number of electrons valance shell 2 and higher

8