C7 Energy Changes
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What is an exothermic reaction
A reaction in which energy is transferred from the reacting substances to the surroundings
What is an endothermic reaction
A reaction in which energy is transferred to the reacting substances from the surroundings
What is an example of an exothermic reaction
Hand warmers and self heating cans
What is an example on an endothermic reaction
Instant cold packs for sports injuries
What type of process is making bonds and why
It is an exothermic process because energy is transferred to the surroundings when they are formed
What type of process is breaking bonds
Endothermic reaction, energy has to be supplied from the surroundings to break the bonds
What does a reaction profile for an endothermic reaction look like
The energy level for the reactants is lower than the energy level for the products
What does the reaction profile for an exothermic reaction look like
The energy level for the reactants in higher than the energy level for the products
How do you determine whether a reaction is endo or exo thermic when given a symbol equation
Add up the energy needed to break each bond of the reactants and calculate the energy for each bond in the products, ( energy for each bond will be given to you in the exam ) subtract the products energy from the reactants energy ( E (reactant) - E (product) ) and if it is negative, it is an exothermic reaction, if it is positive it is an endothermic reaction
What is an example and advantages and disadvantages of non rechargeable cells
E.g double A battery +: cheap -: can pollute if not disposed of the right way Finite life
What is an example and advantages and disadvantages of rechargeable cells
E.g phone battery \+: used many times No emissions -: expensive Takes time to recharge
What is an example and advantages and disadvantages of fuel cells
E.g hydrogen fuel cells \+: only waste product is water Continuous supply of electricity -: expensive Uses hydrogen which is flammable
How does a rechargeable battery work
Chemicals inside produce electricity. Chemicals get used up but can be reproduced when recharged. The chemical reaction can be reversed when connected to an external electric current
How does a fuel cell work
Needs continuous supply of hydrogen and oxygen into the cell. Inside the chemical processes convert hydrogen and oxygen into water. These have a contiuous fuel supply so do not go flat or need recharging
How do non rechargeable batteries work
Flow of electrons in the electrolyte ( metal - non metal ) creates an electrical current. The greater the difference in reactivity, the higher the voltage produced. The reaction stops because one of the reactants has been used up, voltage drops
How do chemical cells work
Anode: less reactive metal Cathode: more reactive metal More reactive metal wants to lose electrons and the electrons have gone round to the anode which wants to gain electrons. This creates an electric current
How do hydrogen fuel cells work
Hydrogen gas is supplied as fuel to negative electrode. It diffuses through graphite electrode and reacts with hydroxide ions to form water. It provide a source of electrons to an external cicuit. Oxygen gas supplied to positive electrode. It diffuses through graphite electrode and reacts to form hydroxide ions, accepting electrons from the external circuit
What is the half equation for the negative electrode ( anode ) in a hydrogen fuel cell
H2 = 2H+ + 2e-
What is the half equation for the positive electrode ( cathode ) in a hydrogen fuel cel
O2 + 4H+ + 4e- = 2H2O
What happens to heat energy in an exothermic and endothermic reaction
Exo= given out Endo= taken in
What happens to the temperature of the reaction mixture during an exothermic and endothermic reaction
Exo: increases Endo: decreases