Energy Changes - 4.5 (Spec)

Exothermic Reactions

A reaction that transfers energy to the surroundings so the temperature of the surroundings increases.

Endothermic Reactions

A reaction that takes in/absorbs energy from the surroundings so the temperature of the surroundings decreases.

Activation Energy

The minimum amount of energy the reactant particles need when they collide with each other in order tor react.

Examples of Exothermic Reactions

• Burning Fuels (Combustion)

• Neutralisation Reactions

• Oxidation

Examples of Endothermic Reactions

• Reactions between citric acid and sodium hydrogencarbonate

• Thermal decomposition reactions

Uses of Exothermic Reactions

• Hand Warmers (Oxidation of iron in air with catalyst)

• Self Heating Cans

Uses of Endothermic Reactions

• Sports Injury Packs (Take in heat and becomes very cold)

Overall energy change of the reaction

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.

Reaction profile

Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

How to draw reaction profile

1. X axis is Progress of reaction

2. Y axis is Energy

3. If exothermic reactants above products

4. If endothermic products above reactants

Bond Energy

Amount of energy required to break one mole of a particular covalent bond.

Breaking Bonds

Endothermic process as energy must be supplied to break existing bonds.

Forming Bonds

Exothermic process as energy must be release to form new bonds.

Overall Energy Change Formula

Energy Change = Energy of bond breaking - Energy of bond making

Steps for Bonds

1. Draw out displayed formula of the molecules so you can clearly see

2. Work out the amount energy used in bond breaking add up

3. Work out amount of energy given out in bond making add up

4. Then use formula

5. If negative result then exothermic. If positive result then endothermic.

Cell

• A system that contains chemicals which react and produce electricity.

• Contains two different electrodes in contact with an electrolyte

Electrolyte

A liquid through which charged ions can flow

Electrodes

• Usually two different metals

• Can conduct electricity

Electrolytes and Voltage

• Voltage of a cell also depends on conc of ions in the electrolyte

• The lower the conc the lower the voltage

• As ions react the conc will slowly decrease (and voltage) eventually to zero and the cell wont work

Battery

Batteries consist of two or more cells connected together in series to provide a greater voltage.

Alkaline/Non Rechargeable Battery

• Alkaline batteries are non-rechargeable.

• In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up.

Rechargeable cells/batteries

Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

Fuel Cells

Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference

Hydrogen Oxygen Fuel Cells

• Electrolyte normally potassium hydroxide

• Electrodes normally porous carbon with a catalyst

• Anode on the left (negative)

• Cathode on the right (positive)

Process of Hydrogen Oxygen Fuel Cells

• Electrolyte usually phospohoric acid and electrodes are often porous carbon with catalyst

• Hydrogen goes into the negative anode and oxygen into the positive cathode (opp to electrolysis)

• Hydrogen loses electrons and form H+ ions (oxidation) and these ions move to the cathode

• Oxygen gain electrons at the cathode and reacts it with H+ ions to make water (reduction)

• Hydrogen + oxygen —> Water

Half equation at the anode

H2 —> 2H+ +2e

Half equation at the cathode

O2 +4H+ +4e- —> 2H20

Overall Redox Reaction

2H2+O2 —>2H20

Advantages of Hydrogen Fuel Cells

• Reaction doesnt produce any pollutants

• Fuel cells last longer than batteries (store more energy)

• Hydrogen and Oxygen are both renewable (rechargable)

Disadvantages of Hydrogen Fuel Cells

• Hydrogen is highly flammable so danger to store explosive with water

• Hydrogen gas requires a large space to store

• It requires energy to produce hydrogen (fossil fuels)

Energy Transfer of Reaction RP4

• Energy transferred is proportional to temperature change of the reaction

How to test for different conc of electrolyte and voltage

• solution as electrolyte and two different metals as eletrodes

• measure voltage

• repeat at different conc by diluting solution with water

• using measured volume of solution and water with a measuring cylinder

• use same two metals

• same volume of electrolyte

Difference in reacitivty with voltage

• Large the difference in reactivity between the two metals the higher the voltage