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Flashcards in Energy changes Deck (22)
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What is an exothermic reaction?

A reaction which transfers energy to the surroundings (usually by heating - shown by a rise in temperature)


Name some examples of exothermic reactions

Combustion (burning fuels)
Neutralisation reactions (acid+alkali)


What is an endothermic reaction?

A reaction which takes in energy from the surroundings
(usually shown by fall in temperature)


Name some examples of endothermic reactions

citric acid and sodium hydrogencarbonate
thermal decomposition


Is bond breaking endothermic or exothermic?

because energy must be supplied in order to break bonds


Is bond forming endothermic or exothermic?

because energy is released when bonds are formed


What do reaction profiles show?

the relative energies of the reactants and products in a reaction and how the energy changes throughout a reaction


What is the activation energy?

the minimum amount of energy the reactants need to collide with each other and react
the initial rise in the line represents the energy needed to break the old bonds and start the reaction


How would you calculate an overall energy change using bond energies?

left - right


What is an electrochemical cell made from?

Two different electrodes in contact with an electrolyte


What are electrodes?

Metals that conduct electricity


What is an electrolyte?

A liquid containing ions that react with the electrodes (in the same cell as it)


How can electricity be produced by a cell?

By connecting the electrodes with a wire so that charge can flow


What does the voltage of a battery depend on?

type of electrodes used (the bigger the difference between their reactivity, the bigger the voltage)
type of electrolyte used (different ions will react differently with different electrodes)
how many cells are connected (he more cells that are connected in series, the bigger the voltage because the voltages of each of the cells is combined)


Why can non-rechargeable batteries not be used multiple times?

the chemical reactions that happen at the electrodes are irreversible
over time, the reactants get used up (as they become products)
once one of the reactants is used up, the reaction can't happen so no more electricity is produced
the products can't be turned back into the reactants so the reaction can't be reversed so the cells can't be recharged


How can a rechargeable battery be recharged?

the reaction can be reversed by connecting the cells to an external electric current


What is a fuel cell?

an electrical cell supplied by a fuel and oxygen (or air) that produces electrical energy efficiently by using energy from the reaction


Why are hydrogen-oxygen fuel cells viewed as better for the environment that other fuels?

water is the only byproduct
it is not polluting so has no bad impact on the environment


What is a redox reaction?

a reaction where electrons are transferred
either an oxidation or reduction reaction (OIL RIG)


What is an oxidation reaction?

reaction where electrons are lost


What is a reduction reaction?

reaction where electrons are gained


How does a hydrogen-oxygen fuel cell work?

hydrogen goes into the anode (negative electrode) and oxygen goes into the cathode (positive electrode)
the hydrogen loses electrons to produce H+ ions (oxidation)
these H+ ions move to the cathode, where the oxygen gains electrons and reacts with the H+ ions to make water (reduction)
the electrons flow through an external circuit from the anode to the cathode, this is the current