C7-Chemical Changes

The minimum amount of energy for particles to collide with in order for a successful reaction to occur.

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

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

Cells contain chemicals which react to produce electricity

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.

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

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

Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place

combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative sign of energy change.

thermal decomposition, reaction of citric acid and sodium hydrogencarbonate. Positive sign of energy change.

Energy of reaction=sum of bonds supplied-sum of bonds released

A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.

Identities of metals used as electrodes and the identity and concentration of an electrolyte.

(+) more or less cheap, some are rechargeable, a convenient source of electrical energy (-) harmful chemicals

The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water. Cathode: H2 → 2 H+ + 2 e- Anode: O2 + 4 H+ + 4 e-→ 2 H2O Overall: →2 H2 +O2→2H2O

Advantages: no pollutants, no recharging,hydrogen is readily available,only water is made,renewable,very efficient Disadvantages: flammable, H, difficult to store, fossil fuel production, toxic chemicals, expensive production of H, by electrolysis,explosive

Everyday uses of exothermic reactions include self-heating cans and hand warmers.

Some sports injury packs are based on endothermic reactions.

To monitor the temperature change of a chemical reaction to classify it as exothermic or endothermic.

• eye protection • two 50 cm3 measuring cylinders • polystyrene cup • 250 cm3 beaker • weighing boat • balance • spatula • 0–110 °C thermometer • stirrer • stopwatch • 1.00 mol/dm3 hydrochloric acid • 1.00 mol/dm3 nitric acid • 1.00 mol/dm3 sodium hydroxide • iron filings • 1.00 mol/dm3 potassium hydroxide • 1.00 mol/dm3 copper(II) sulfate solution

Wear chemical splash-proof eye protection and wash hands after the practical.

1 Use the first measuring cylinder to measure 25 cm3 of the sodium hydroxide solution and pour this into the polystyrene cup. 2 Stand the polystyrene cup in the 250 cm3 beaker. 3 In the second measuring cylinder, measure 25 cm3 of hydrochloric acid. 4 Using the thermometer, measure the temperature of the sodium hydroxide every 30 seconds whilst gently stirring. 5 After exactly 2 minutes add the hydrochloric acid and continue to stir and to record the temperature of the solution every 30 seconds for 10 minutes. 6 Repeat this experiment twice: * with 25 cm3 of copper(II) sulfate and iron filings • with 25 cm3 potassium hydroxide and nitric acid.

The energy required to break bonds in the reactants is greater than the energy released when new bonds are formed. The surroundings lose energy, so they yui cooler.