T5 energy changes

Question 1

what is the conservation of energy principle

Answer 1

energy is conserved in chemical reactions

amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes places

Question 2

what is an exothermic reaction

give examples

Answer 2

reaction where energy is transferred to the surroundings so that the surrounding temperature increases

combustion, oxidation reactions, neutralisation

negative sign of energy change

Question 3

what is an endothermic reaction

give examples

Answer 3

reaction where energy is taken in from the surroundings so that the surrounding temperature decreases

thermal decomposition, reaction of citric acid and sodium hydrogencarbonate

Question 4

what is activation energy

Answer 4

minimum amount of energy that particles need to react

Question 5

what is a reaction profile

Answer 5

graph which shows the relative energies of reactants and product, as well as activation energy of the reaction

Question 6

what occurs in a chemical reaction in terms of bond energies

describe exothermic and endothermic reactions in terms of bond breaking/forming

Answer 6

energy is supplied to break bonds and energy is released when bonds are made

exo: energy released from forming bonds is greater than that needed to break the bonds
endo: energy needed to break bonds is greater than energy released making them

Question 7

equation to find enthalpy change in terms of bond energies

Answer 7

energy of reaction = sum of bonds broken - sum of bonds made

Question 8

what is a cell

Answer 8

composed of 2 electrons dipped in an electrolyte solution

produces electricity from a chemical reaction

Question 9

what is a battery

Answer 9

2 or more cells connected in series

Question 10

what determines the voltage obtained from a cell

Answer 10

identities of metals used as electrodes and the identity and concentration of an electrolyte

Question 11

advantages and disadvantages of using cells and batteries

Answer 11

quite cheap, some are rechargeable, a convenient source of electrical energy

harmful chemicals

Question 12

describe rechargeable and non-rechargeable cells

Answer 12

r - chemical reactions are reversed when an external current is supplied

n - reactants are used up, can’t be recharged

Question 13

what is a fuel cell

Answer 13

supplied by fuel and oxygen to oxidise the fuel to generate electricity

Question 14

what is the overall reaction in a hydrogen fuel cell

what are the half equations

Answer 14

cathode: 2H2 > 4H+ + 4e-
anode: O2 + 4H+ + 4e- > 2H2O
overall: > 2H2 + O2 > 2H2O

Question 15

advantages and disadvantages of hydrogen fuel cells

Answer 15

a: no pollutants, no recharging
d: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis