Chemical Reactions

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Chemical ReactionsSarah Jones

Chemical Reaction Indicators• Colour change• Evolution of heat, light or sound• Gas produced• Formation of a precipitate

A yellow precipitate of solid lead(II) iodide forms immediately when solutions of lead(II) nitrate and potassium iodide are mixed.

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Law of Conservation of Mass

Antoine Lavoisier discovered the Law of Conservation of Mass.

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http://en.wikipedia.org/wiki/Law_of_conservation_of_mass

The total mass of the reactants or starting materials must be equal to the mass of the products.

Chemical Equations

Meaning of Chemical Formula

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Showing States

(s) Solid (l) Liquid (g) Gas (aq) Aqueous

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http://en.wikipedia.org/wiki/Chemical_equation

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Visualising a Chemical Reaction

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Classifying Chemical Reactions

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Decomposition Reactions• When a single reactant breaks apart to form

several products.

XY X + Y • Soft drinks contain carbonic acid. This

decomposes to form water and carbon dioxide gas.

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Thermal Decomposition• When a substance will only decompose if

exposed to high temperatures.• Thermal decomposition happen to sodium

azide when inflating vehicle airbags. When heated it decomposes to form sodium metal and nitrogen gas.

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Combination Reactions• When two reactants combine to form a single

product.

X + Y XY

• These reactions are important in industry – hydrogen gas + chlorine gas react to form hydrogen chloride gas. This gas is then bubbled through de-ionised water to produce hydrochloric acid.

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Precipitation ReactionsWhen two clear solutions are mixed together and they form a solid e.g. scale (calcium carbonate the builds up in a jug).

A yellow precipitate of solid lead(II) iodide forms immediately when solutions of lead(II) nitrate and potassium iodide are mixed.

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Oxidation ReactionsWhen a substance combines with oxygen during a chemical reaction – combustion or corrosion.

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During combustion reactions substance burn rapidly in oxygen and produce large amounts of heat and light.

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Reduction Reactions• When a compound loses oxygen atoms.

2CuO + C 2Cu + CO2

• The copper oxide is reduced and the carbon is the reducing agent. Also, the carbon is oxidised and the copper (II) oxide is the oxidising agent.

• Oxidation and reduction occurs in pairs – Redox Reactions.

Copper (II) Oxide Carbon Copper Carbon Dioxide

• Redox reactions don’t always involve oxygen.• Redox reactions include any reaction where

electrons are transferred from one substance to another.

Oxidation is gain of oxygen or loss of electrons.Reduction is loss of oxygen or gain of electrons.

Metal displacement reactions are redox reactions.

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Activity SeriesShows the tendency for metals to react.

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When do reactions occur?Chemical reactions occur only under certain conditions e.g. iron will not rust without air and water. This is why rusting does not occur on the Moon.

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• You often need heat to cause a chemical reaction e.g. to fry an egg, to get dough to rise, a sparkler to burn etc.

• Sometimes you need electricity e.g. to charge a car battery. The electricity produces changes in the chemicals inside the battery.

Reaction Rate• Some reactions are slow, like rusting, while

others occur quickly.• Explosions are chemical reactions that occur

very quickly and produce large amounts of heat, light and sound.

• The speed of a reaction is called its reaction rate.

Controlling Reaction Rate

• Most reactions can be increased or decreased.• Factors that affect reaction rate– Temperature– Concentration– Surface Area– Agitation– Catalysts

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TemperatureIncreasing temperature normally increases reactions rate.– increase in particle speed - increased collisions.

The particles have more energy so when they collide the chemical bonds are more likely to break and easily form products.

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Lowering temperature can decrease reaction rate. – Refrigeration

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Concentration• Concentration refers to the amount of

reactants present.• Higher concentration = faster reaction

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AgitationStirring reactants can increase reaction rate.

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Surface AreaIncreasing surface area means more particles are reacting at the same time.

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CatalystsChemicals that speed up reactions but are not consumed during the reactions.

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A solution of hydrogen peroxide (H2O2) decomposes in water so slowly that the change is not noticeable (left). Iodide ion acts as a catalyst for the decomposition of H2O2, producing oxygen gas. The solution turns brown because of the reaction of H2O2 with I−, which generates small amounts of I3

− (center). The enzyme catalase is about 3 billion times more effective than iodide as a catalyst. Even in the presence of very small amounts of enzyme, the decomposition is vigorous (right).

Enzymes• Enzymes are biological catalysts.• The enzyme amylase is found in your saliva.• Similar enzymes are secreted by you

pancreases and small intestine.

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