Chemical Changes
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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
? 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.
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) + ?O2 (g) ? H2O (l); ∆H = -286 KJ/mol
H2 (g) + ?O2 (g) ? 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) ? 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 ? 13,860J, THEN 1 mol ? -420,000J
-420KJ/Mol
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by: Admin
Total views: 3452
Word Count: 5141
Date: Sun, 21 May 2006 Time: 12:00 AM
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