Energy changes Flashcards
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?
endothermic
because energy must be supplied in order to break bonds
Is bond forming endothermic or exothermic?
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
