Chapter 5: Energy Changes Flashcards Preview

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Flashcards in Chapter 5: Energy Changes Deck (34):

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