Chapter 5: Energy Changes Flashcards
What is an exothermic reaction?
A reaction in which thermal energy is transferred from the chemicals to the surroundings, increasing their temperature
What are some uses of exothermic reactions?
Hand warmers, self-heating cans
What are some examples of exothermic reactions?
Oxidation reactions, neutralisation reactions, combustion reactions
What is an endothermic reaction?
A reaction in which thermal energy is transferred from the surroundings to the chemicals, increasing their temperature
What are some uses of endothermic reactions?
Cold packs
What are some examples of endothermic reactions?
Decomposition, reaction of acids with hydrogencarbonates
What is activation energy?
The minimum energy particles must have to react with each other
What is a reaction profile diagram?
A graph of energy and progress of reaction
What are bond energies?
The amount of energy it takes to make/break a bond
How is energy used/released?
Breaking bonds uses up energy, making bonds releases energy (BENDO MEXO)
How do you calculate the total energy change?
(Energy needed breaking bonds in reactants) - (Energy released making bonds in products)
What is an electrolyte?
A liquid that conducts electricity. Ions carry the electric charge through the liquid
What is a cell?
2 electrodes and an electrolyte, used to generate electricity
What is a battery?
2 or more chemical cells connected together
How do chemical cells work?
Electrons flow from the more reactive metal to the less reactive metal because there is a potential difference (voltage) between them, The ions in the electrolyte carry the charge
What is the voltage produced by many chemical cells?
1.5 volts, so multiple cells may need to be connected together in a series to provide a higher voltage
What does it mean when a chemical cell is flat?
The reactants in the cell have been used up and the battery no longer works
How do rechargeable cells work?
Passing an electric current through the cell reverses the reaction
What are the advantages of using chemical cells?
Cheap, convenient, portable source of electricity, some are rechargeable
What are the disadvantages of chemical cells?
Many cells contain environmentally-harmful chemicals, some go flat and must be recharged
How are fuel cells different from chemical cells?
There is a constant supply of chemicals (usually oxygen) into the cell, as well as a fuel (usually hydrogen)
What happens inside a fuel cell?
Hydrogen gas enters the cell at the anode, where a chemical reaction strips it of its electrons and turns it into H+ ions. The electrons pass through a circuit which things can be connected to. The hydrogen ions pass through a proton exchange electrolyte and combine with the returning electrons and oxygen at the cathode
What are the advantages of fuel cells?
Only waste product is water, produce constant supply of electricity
What are the disadvantages of fuel cells?
Expensive, contain toxic chemicals in electrodes and electrolytes, use flammable hydrogen gas, hydrogen is sometimes made in a process that uses fossil fuels
What is a decomposition reaction?
When substances break down into simpler substances (e.g. * carbonate => * oxide + carbon dioxide). These are always endothermic
What is an redox reaction?
When a substance reacts with oxygen and loses electrons, the substance is oxidised and the oxygen is reduced. These are exothermic
What is a combustion reaction?
An exothermic reaction where a substance reacts with oxygen and catches fire
What is a neutralisation reaction?
Exothermic reactions when acids react with bases to form water and a salt
What happens when acids react with metal hydrogencarbonates?
The temperature drops
What is the potential difference of a chemical cell?
How easy it is for charge to flow between the two electrodes
How do you calculate the potential difference?
The difference in how reactive the metals are
What is the overall reaction in a hydrogen cell?
2H2 + O2 -> 2H2O
What is the half equation at the anode?
2H2 -> 4H+ + 4e-
What is the half equation at the cathode?
O2 + 4H+ 4e- -> 2H2O
