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Chemistry IGCSE Chapter 6 – Chemical Changes

Exothermic Reactions

·         These are reactions that give out energy (mostly in the form of heat)

·         E.g. Respiration, Combustion, Neutralisation, Hydration

Endothermic Reactions

·         Reactions that take in energy from the environment/energy supplied.

·         E.g. Photosynthesis, Thermal Decomposition

Chemical Reactions/Bonds

·         In chemical reactions, atoms are rearranged (not created/lost).

·         To break the bonds between these atoms, energy must be supplied

·         When bonds are created, energy is given out

·         In a chemical reaction, both these processes occur

·         Because energy is given out when new bonds are made, bond creation is exothermic

·         Likewise, energy is taken in to break old bonds, bond breaking is endothermic

·         Collectively, if in a reaction:

o        More overall energy is used in breaking bonds than in creating, the reaction is endothermic (energy taken in is greater than energy given out)

o        More overall energy is used in creating bonds than in breaking, the reaction is exothermic (energy given out is greater than energy taken in)

Exothermic Reactions: Burning Fuels for Heat

·         Heat energy is produced in the COMBUSTION of the following fuels:

o        Coal (carbon + hydrocarbons)

o        Natural Gas (largely methane, CH₄)

o        Petrol (a mixture of hydrocarbons e.g. octane)

·         An oxidation reaction in which heat is given out is combustion

·         Combustion accompanied by a flame is burning

·         A substance which is oxidized with the release of energy is a fuel

Energy/Profile Diagram

·         Diagram which shows the energy content of the reactants and the products

·         Shows that at the end of the reaction was less than energy at beginning. Also, more energy was given out during the reaction. (EXOTHERMIC)

·         Energy at the end of the reaction was more than energy at beginning. Less energy was given out, more absorbed. (ENDOTHERMIC)

·         The enthalphy change or the energy change of the reaction is given by:

∆H = Energy of Products – Energy of Reactants

·         For endothermic reactions this is positive

·         For exothermic reactions this is negative

Activation Energy

·         This is the energy which the reactants must gain to overcome the energy barrier

·         Minimum energy required for an effective reaction

·         This energy comes from chemical bonds

·         Bond energy is the energy absorbed when one mole of covalent bonds breaks to form free gaseous atoms

Using Bond Energy Values

·         A standard table has been drawn up which consists of the values of enthalpy change for various types of bonds

Production of Electricity from Simple Cells

·         When a metal is dipped into an electrolyte or water, some ions of the metal (e.g. zinc) pass into the solution, leaving their electrons behind on the metal.

·         Strip of metal becomes negatively charged. This charge builds up such that no more cations can leave the solution.

·         In the reactivity series of metals, Zinc is high up, suggesting it ionizes more readily. However, Copper is one of the less reactive metals which means it ionizes less.

·         Therefore a strip of zinc and copper in solution, connected to each other would cause electrons to flow in the external circuit from zinc (negatively charged) to copper (less charged).

·         A chemical cell is a device or a system for converting the energy of a chemical reaction into electricity or electrical energy. E.g. car battery

·         Direction of flow is from metal higher in series to metal lows in series.

                                                                                                         Zn(s) à Zn²⁺ (aq) + 2e^-

                                                                                                         Cu²⁺ (aq) + 2e^- à Cu (s)

                                                                                                                   Zn(s) + Cu²⁺ (aq) à Zn²⁺ (aq) + Cu(s)

                                                                                                                        Reduction

                                                                                                                         Oxidation

Zinc = negative electrode

Copper = positive electrode

Note: paired with a more reactive metal e.g. zinc, copper is the positive electrode. Paired with a less reactive metal e.g. silver, copper is the negative electrode

• Where there is reduction of copper ions, that is the cathode
• Where there is oxidation of zinc ions, that is the anode

However, IN AN ELECTROCHEMICAL CELL, UNLIKE IN ELECTROLYSIS,

The anode is –vely charged and the cathode is +vely charged

• The difference in electric potential between the two electrodes is called the electromotive force of a cell (e.m.f)
• The further apart the metals on the reactivity series, the larger the e.m.f

Factors that affect the magnitude of ∆H

·         Depends on the conditions under which the measurement was made

1.      Physical state

2.      temperature

3.      pressure

4.      amount (moles)

·         PHYSICAL STATE:

o        Water in the gaseous state has more energy that water in the liquid state

H₂ (g) + ½O₂ (g) à H₂O (l); ∆H = -286 KJ/mol

H₂ (g) + ½O₂ (g) à H₂O (g); ∆H = -242 KJ/mol

If H₂ was in liquid state, then the reaction would start lower in the first place. Therefore there will be less magnitude/change.

Heat Energy/Types

o        Can be evolved from various types of reactions:

HEAT OF COMBUSTION:

o        The heat change which takes place when 1 mole of the substance is completely burned in oxygen. (Heat of combustion is always –ve, ie. exothermic)

E.g. CH₄ (g) + 2O₂ (g) à CO₂ (g)+ 2H₂O (g) ; ∆H = -1560 KJ/mol

HEAT OF NEUTRALIZATION (of acid or base)

o        The heat evolved when an amount of acid or base needed to form 1 mole of water is neutralized

HEAT OF SOLUTION

o        The heat of solution of a solute is the heat change when 1 mole of the solute dissolved in a large volume of water.

Measuring Heat Output

o        Heat output cannot be measured directly

o        Standard method: Analyse effect that energy has on the temperature of a known volume of water.

Weigh burner with fuel

                                                                                     Change in temp. of water is measured.

                                                                         Re-weigh burner          

                            Mass ethanol used: 1.5 g

                            Change in Temp. of water = 33°C

                            S.H.C. of water = 420J/°C

                        Energy = 33°C x 420J/°C

                            = 13,860J

                Energy of Combustion = joules that 1 mol of substance uses

                Moles of ethanol used = 0.033mol

                                                    0.033 mol à 13,860J, THEN 1 mol à -420,000J

                              -420KJ/Mol