Energy changes

Question 1

What is an exothermic reaction?

Answer 1

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

Question 2

Name some examples of exothermic reactions

Answer 2

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

Question 3

What is an endothermic reaction?

Answer 3

A reaction which takes in energy from the surroundings

usually shown by fall in temperature

Question 4

Name some examples of endothermic reactions

Answer 4

citric acid and sodium hydrogencarbonate

thermal decomposition

Question 5

Is bond breaking endothermic or exothermic?

Answer 5

endothermic

because energy must be supplied in order to break bonds

Question 6

Is bond forming endothermic or exothermic?

Answer 6

exothermic

because energy is released when bonds are formed

Question 7

What do reaction profiles show?

Answer 7

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

Question 8

What is the activation energy?

Answer 8

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

Question 9

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

Answer 9

left - right

Question 10

What is an electrochemical cell made from?

Answer 10

Two different electrodes in contact with an electrolyte

Question 11

What are electrodes?

Answer 11

Metals that conduct electricity

Question 12

What is an electrolyte?

Answer 12

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

Question 13

How can electricity be produced by a cell?

Answer 13

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

Question 14

What does the voltage of a battery depend on?

Answer 14

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)

Question 15

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

Answer 15

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

Question 16

How can a rechargeable battery be recharged?

Answer 16

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

Question 17

What is a fuel cell?

Answer 17

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

Question 18

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

Answer 18

water is the only byproduct

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

Question 19

What is a redox reaction?

Answer 19

a reaction where electrons are transferred

either an oxidation or reduction reaction (OIL RIG)

Question 20

What is an oxidation reaction?

Answer 20

reaction where electrons are lost

Question 21

What is a reduction reaction?

Answer 21

reaction where electrons are gained

Question 22

How does a hydrogen-oxygen fuel cell work?

Answer 22

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