C9, C10, C11, C12

Acids, bases and salts, rates of reaction, energetics and electrolysis

C1 - ACIDS BASES AND SALTS

Acid

• A substance that forms an aqueous solution with a pH of less than 7

• Contains an excess of H+ ions and is an H+ donor

• One mole of some acidic substances can donate more than one mole of H+ ions

  • Mono, di, tri and polyprotic

Base

• A substance with a pH of more than 7

• Contains an excess of OH- ions

• OH- acceptors

• Alkali are type of base

  • Soluble in water

pH scale

• More hydrogen, lower pH

• Measure of H+ ion concentration

Neutralisation reactions

• Positive H+ ions react with negative OH- ions to form water- often exothermic

• Acid + base --> salt + water

  • Acid + metal oxide --> salt + water

  • Acid + metal hydroxide --> salt + water

  • Ionic

    • H+ + OH- --> H2O

• Acid + metal carbonate --> salt + water + CO2

• Acid + metal --> salt + hydrogen

Oxide reactions

• Some oxides of non-metals can react with water to form acidic solutions

• Some oxides and hydroxides of metals react with water to form alkaline solutions

Strong acid/base

• Ionise COMPLETELY

• All acid particles will dissociate

• HCl, HNO3

Weak acid/base

• Don’t ionise completely

• Not all acid particles dissociate

• REVERSIBLE

• Ethanoic acid, citric acid

Concentration of acid/base

• How much acid there is for a certain volume

• Strong acid --> lower pH

  • More H+ ions, which determines pH

C10 - RATES OF REACTION

Factors: concentration

• More reactant particles

• Are closer together

• Frequency of successful collisions increases

• Rate increases

Pressure

• Reactant particles are closer together

• More frequent collisions

• Rate of reaction increases

• Graph

  • Steeper line = greater pressure

SA:V

• Lumps vs powders

  • Large lumps have small surface area to volume ratio

  • If grounded:

    • Volume stays the same

    • Area of surface increases

    • Ratio increases

• Large SA:V

  • More reactant particles are exposed at surface

  • Frequency of collisions increases

  • Rate increases

Temperature

• Reactant particles move more quickly

• Energy of particles increases

• Frequency of collisions increases

• Rate increases

Catalyst

• Substance that

  • Increases rate of reaction

  • Does not change chemically or in mass after reaction

  • Does not alter products

    • Just produces them at quicker rate

  • Do not affect the position of the equilibrium

• How

  • Provide an alternate reaction pathway

    • Lower activation energy

  • Does not change frequency of collisions

    • Changes frequency of successful collisions

    • More will exceed lower activation energy

• Enzyme:

  • Biological catalyst

  • Important in cell reactions

  • Used fermentation of sugars

Measuring rate of reaction

• Can be done in two main ways

  • Measuring loss of a reactant or gain of a product

  • Measurement of a physical property

• When to use

  • Measuring gain of product:

    • Gas like CO2

  • Physical property

    •  Loss/formation of precipitate

    • X disappears

Collision theory

• Conditions of a reaction

  • Particles must collide

    • Successful collision (enough energy to react)

  • Enough energy

    • Activation energy (min energy for reaction to occur)

Measuring rate again

• Depends on reactants and products involved

• How long reaction takes

• Measuring mass

  • Change in mass of reactants

  • Good for CO2 because of high Mr

  • g/s

• Measuring volume

  • Change in volume of reactant

  • Gas syringe

  • cm³/s

Reaction profiles

• Shows how energy of reactants and products changes

• Includes activation energy

  • The hump which stats at the energy of reactants

C11 - ENERGETICS

Exothermic graph

Endothermic graph

Catalyst graph

Enthalpy change

• When chemical bonds break or form, energy is often released or absorbed as heat

• At constant pressure the energy change is enthalpy change

Exothermic

• Release heat energy

• Have a negative enthalpy change

• Temperature of surroundings increase

Endothermic

• Absorb heat energy from surroundings

• Have a positive ΔH

• Temperature of surroundings decreases

Reversible reactions

If a reversible reaction is exothermic in one direction, it is endothermic in the other direction

Calculating enthalpy change

• Q = mcΔT

• ΔH = q/n

Breaking and forming bonds

• BENDO - MEXO

• Overall enthalpy change of a reaction depends on the difference between energy needed to break and form bonds

  • If more energy is needed to break bonds rather than forming new ones, ΔH is positive

  • Vice verse means exothermic as ΔH is negative

C12 - ELECTROLYSIS

Molten electrolysis

Done when ion is insoluble

Electrolyte

• Ionic compounds molten or dissolved in water

• Molten when ionic compound is insoluble

• Ions are free to move

Electrolysis

• Electrical energy from DC decomposes the electrolyte

• Ions are attracted to oppositely charged electrode

Movement of ions and production in molten electrolysis

• Movement of ions

  • Cations migrate to the cathode (negatively charged electrode)

  • Anions migrate to the anode (positively charged electrode)

• Production at the electrodes

  • Ions are discharged (lose their charge) and become atoms or molecules of elements

  • Gain electrons from the inert electrodes so as not to react in the reaction

Lead bromide

• Lead will form at the cathode because it has positively charged ions

• Bromine gas will form at anode because of the anions that are attracted to it and will be discharged

Reduction and oxidation

• OILRIG

• Oxidation occurs at anode

• Half equation for oxidation

Electrolysis in aqueous solutions

• Dissolved in water

• Means in the solution there are the ionic compound's ions and H+ and OH-

Aqueous electrolysis with cathode

• Attracts positive ions

• Metal ions + H+ ion

• If metal is more reactive than H+ ion, hydrogen will form

• If not, metal is formed

• Least reactive ion forms

Aqueous anode

• Negative ions

• Non-metal ion + OH- ion

• If halide ion, it will be discharged and be the product

  • Reason for change in mass

AC vs DC

• Ions oscillate back and forth in AC which disrupts the electrochemical reaction needed

• DC has a consistent polarity on the electrodes

Electroplating

• Involves using electrolysis to put a thin layer of metal on object

  • Cathode

    • Iron / steel object

  • Anode

    • Plating metal

  • Electrolyte

    • Ions of plating metal

• Improves corrosion resistance and appearance