An atom has a tiny, dense nucleus (protons + neutrons) surrounded by electrons in shells.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons, so they have different mass numbers. Isotopes have the same chemical properties (same electron arrangement) but slightly different physical properties like mass.
Electrons fill shells from the nucleus outwards, lowest energy level first.
Elements are arranged in order of increasing atomic number, in rows called periods and columns called groups. Elements in the same group have the same number of outer electrons, so react in similar ways.
There are three main types of strong chemical bonding: ionic, covalent and metallic. All involve electrons being transferred or shared to give atoms full outer shells (a stable electron structure like a noble gas).
Ionic compounds form giant ionic lattices. They have high melting and boiling points because there are very many strong electrostatic forces between ions that need lots of energy to break. They conduct electricity only when molten or dissolved in water, because then the ions are free to move and carry charge. Solid ionic compounds do NOT conduct, as the ions are fixed in place.
Covalent substances come in two forms.
Common mistake: students say 'weak covalent bonds' cause low boiling points in simple molecules, but it's actually the weak forces BETWEEN molecules that break, the covalent bonds stay intact.
Metals conduct electricity and heat because the delocalised electrons carry charge/energy through the structure. Metals are malleable because the layers of ions can slide over each other, with the sea of electrons holding them together. Alloys are mixtures of metals (or metal plus another element) that are harder than pure metals, because the different-sized atoms distort the layers and stop them sliding easily.
The three states are solid, liquid and gas; changes of state are physical changes (no new substance made). Particle theory limitations: it assumes particles are solid spheres with no forces between them, which is a simplification. Nanoparticles are 1-100 nanometres in size, with a very high surface area to volume ratio, giving them different properties to bulk materials (e.g. in sun creams and catalysts).
In a closed system, no atoms are lost or made, so the total mass of reactants equals the total mass of products.
Add up the relative atomic masses (Ar) of all atoms in the formula.
A mole is a fixed amount of substance: 6.02 x 10^23 particles (Avogadro's constant).
Concentration is measured in g/dm3 or mol/dm3.
Mole ratios from a balanced equation tell you how much of each substance reacts or forms.
The reactant that gets used up first is the limiting reactant - it controls how much product can form.
percentage yield = (actual yield / theoretical yield) x 100
percentage atom economy = (Mr of desired product / sum of Mr of all products) x 100
Used to find unknown concentrations by neutralising a measured volume of acid or alkali.
An exothermic reaction transfers energy to the surroundings, so the temperature goes up. Examples: combustion, neutralisation, most oxidation reactions, and hand warmers.
An endothermic reaction takes in energy from the surroundings, so the temperature goes down. Examples: thermal decomposition, and the reaction between citric acid and sodium hydrogencarbonate used in some sports cold packs.
A reaction profile is a graph of energy against progress of reaction. For exothermic reactions the products have less energy than the reactants, and energy is released, shown as a downward step on the diagram. For endothermic reactions the products have more energy than the reactants.
Activation energy is the minimum energy that colliding particles need for a reaction to happen. It is shown as the initial energy rise (the 'hump') from reactants to the peak, even in exothermic reactions. A common mistake is forgetting that exothermic reactions still need an initial activation energy input.
Breaking bonds requires energy (endothermic), while forming bonds releases energy (exothermic). To calculate overall energy change: energy change = energy to break bonds in reactants minus energy released forming bonds in products.
If breaking bonds needs more energy than forming bonds releases, the reaction is endothermic overall (positive value). If forming bonds releases more energy than breaking bonds needs, the reaction is exothermic overall (negative value). Always show your working line by line and keep units in kJ/mol.
A simple cell is made from two different metals in an electrolyte, connected by a wire, producing a voltage. The bigger the difference in reactivity between the two metals, the bigger the voltage produced.
A battery is two or more cells connected together in series to increase the total voltage supplied.
Non-rechargeable cells stop producing a voltage once one of the reactants has been fully used up, because the chemical reaction cannot be reversed.
Rechargeable cells can be recharged because passing a current through them in reverse reverses the chemical reactions.
A hydrogen fuel cell uses hydrogen and oxygen to produce a voltage, with water as the only product. This makes fuel cells attractive as they produce no harmful emissions like carbon dioxide.
Advantages of fuel cells over rechargeable cells include no need for recharging and consistent power as long as fuel supply continues. Disadvantages include the need to store and transport hydrogen safely, and hydrogen is often produced from fossil fuels, which reduces the environmental benefit.
Rate of reaction can be measured by how fast a product forms or a reactant is used up, e.g. mass lost (gas escaping) or volume of gas collected over time. Steeper graph gradient = faster rate.
Four factors speed up reactions: higher temperature, higher concentration (or pressure for gases), smaller particle size (larger surface area), and adding a catalyst. Collision theory explains this: particles must collide with enough energy (at least the activation energy) and the correct orientation to react. More collisions, or more successful collisions, means a faster rate.
Raising temperature gives particles more kinetic energy, so they move faster, collide more often, AND more collisions have energy above the activation energy - this is why temperature has such a big effect, roughly doubling rate for every 10C rise as a rule of thumb.
Catalysts speed up reactions by providing an alternative reaction pathway with a lower activation energy. They are not used up and do not appear in the balanced equation. Enzymes are biological catalysts.
A reversible reaction can go forwards and backwards, shown by the symbol with two half-arrows (⇌). At equilibrium, in a closed system, the forward and backward reactions happen at the same rate, so the concentrations of reactants and products stay constant (but are not necessarily equal).
Le Chatelier's Principle: if you change the conditions of a system at equilibrium, the position of equilibrium shifts to counteract the change.
A catalyst speeds up both forward and backward reactions equally, so it does NOT shift the position of equilibrium - it just gets there faster.
Crude oil is a mixture of hydrocarbons, mostly alkanes, separated by fractional distillation using differences in boiling point. Shorter chains have lower boiling points and rise higher up the column.
Alkanes are saturated hydrocarbons with the general formula CnH2n+2 (single bonds only). Alkenes are unsaturated, general formula CnH2n, containing a C=C double bond, which can be detected using bromine water: alkenes decolourise it (orange to colourless), alkanes do not.
Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes, using either high temperature with a catalyst, or high pressure with steam - this meets demand for smaller molecules like petrol.
Common mistakes: forgetting a catalyst doesn't change the equilibrium position (only rate); mixing up 'rate increases' with 'yield increases'; writing CnH2n for alkanes instead of alkenes; and forgetting that bromine water tests for double bonds, not general reactivity.
A common mistake is mixing up hydrogen and oxygen tests. Remember: hydrogen POPS, oxygen RELIGHTS.
Flame tests identify metal ions by colour: lithium gives crimson, sodium gives yellow/orange, potassium gives lilac, calcium gives orange-red, copper(II) gives blue-green. Dip a clean nichrome wire in dilute hydrochloric acid, then into the sample, then into a blue Bunsen flame.
Flame emission spectroscopy identifies metal ions in solution using a machine rather than a flame test by eye. It is faster, more accurate, more sensitive, and can detect mixtures of ions from tiny samples, unlike the older manual tests.
Today's atmosphere is roughly 80% nitrogen and 20% oxygen, with small amounts of carbon dioxide, water vapour and noble gases. Over 4.6 billion years it evolved: early volcanic activity released mostly carbon dioxide and water vapour with almost no oxygen; oceans formed as water vapour condensed; algae and plants evolved and photosynthesis gradually removed carbon dioxide and released oxygen; carbon became locked in fossil fuels and sedimentary rocks (like limestone).
Carbon dioxide, methane and water vapour trap outgoing infrared radiation, keeping Earth warm (the greenhouse effect). Human activity, burning fossil fuels and deforestation, increases carbon dioxide and methane levels, which is linked to global warming. Common mistake: the greenhouse effect itself is natural and essential for life; it is the enhanced effect from human emissions that causes climate change.
Burning fuels releases carbon dioxide, water vapour, carbon monoxide (from incomplete combustion, toxic, colourless, odourless), sulfur dioxide and oxides of nitrogen (cause acid rain and respiratory problems), and soot/particulates (cause global dimming and health issues).