Energy Changes

What is an exothermic reaction?

A reaction that transfers energy to the surroundings - temperature of surroundings increases.

What is an endothermic reaction?

A reaction that takes in energy from the surroundings - temperature of surroundings decreases.

What happens to the surroundings during an exothermic reaction?

They get warmer.

What happens to the surroundings during an endothermic reaction?

They get colder.

Give examples of exothermic reactions.

Combustion, many oxidation reactions + neutralisation.

Give examples of endothermic reactions.

Thermal decomposition + reaction of citric acid with sodium hydrogencarbonate.

What are everyday uses of exothermic reactions?

Self-heating cans + hand warmers.

What are everyday uses of endothermic reactions?

Instant cold packs.

What is a reaction profile?

Diagram showing energy changes during a chemical reaction.

What is shown on the y-axis of a reaction profile?

Energy.

What is shown on the x-axis of a reaction profile?

Progress of reaction.

What is activation energy?

Minimum energy particles must have for a reaction to occur.

Where is activation energy shown on a reaction profile?

Energy difference between reactants + top of the energy barrier.

What does an exothermic reaction profile show?

Products at a lower energy than reactants.

What does an endothermic reaction profile show?

Products at a higher energy than reactants.

What is the overall energy change in an exothermic reaction?

Energy is transferred to the surroundings.

What is the overall energy change in an endothermic reaction?

Energy is taken in from the surroundings.

What happens to bonds during a chemical reaction?

Bonds in reactants are broken + new bonds form to make products. (HT)

Does breaking bonds require or release energy?

Requires energy - endothermic. (HT)

Does forming bonds require or release energy?

Releases energy - exothermic. (HT)

Why is a reaction exothermic in terms of bond energies?

More energy is released forming new bonds than is required to break existing bonds. (HT)

Why is a reaction endothermic in terms of bond energies?

More energy is required to break existing bonds than is released forming new bonds. (HT)

What is the equation for overall energy change using bond energies?

Energy change = energy needed to break bonds - energy released when bonds form. (HT)

How do you calculate energy needed to break bonds?

Add the bond energies of all bonds broken in the reactants. (HT)

How do you calculate energy released by bond formation?

Add the bond energies of all bonds formed in the products. (HT)

What does a negative calculated energy change mean?

Exothermic reaction - more energy is released than taken in. (HT)

What does a positive calculated energy change mean?

Endothermic reaction - more energy is taken in than released. (HT)

What units are bond energies usually given in?

kJ/mol.

What is the method for a bond-energy calculation?

Count bonds broken - calculate energy needed - count bonds formed - calculate energy released - broken minus formed. (HT)

RP4 - What does the temperature-change practical investigate?

Temperature changes when substances react in solution.

RP4 - What equipment can reduce heat transfer to the surroundings?

Polystyrene cup with a lid.

RP4 - Why is a polystyrene cup used?

It is a thermal insulator - reduces unwanted energy transfer.

RP4 - Why is a lid used?

Reduces energy transfer to the surroundings + reduces evaporation.

RP4 - What should be measured before mixing the reactants?

Initial temperature.

RP4 - What should be measured after mixing the reactants?

Highest or lowest temperature reached.

RP4 - How is temperature change calculated?

Temperature change = final temperature - initial temperature.

RP4 - How can the effect of concentration on temperature change be investigated?

Change concentration of one reactant - keep volumes/other variables constant - measure temperature change.

RP4 - What should be controlled when investigating concentration?

Volumes, concentration of other solution, starting temperature + apparatus.

RP4 - Why should the mixture be stirred?

Ensures even temperature throughout the solution.

RP4 - Why should the experiment be repeated?

Identify anomalies + calculate a mean for more reliable results.

RP4 - What is a major source of error in this experiment?

Energy transfer between the reaction mixture + surroundings.

RP4 - How can heat loss be reduced?

Use insulated cup + lid and take temperature readings promptly.

RP4 - What graph can be plotted when concentration is changed?

Temperature change against concentration.

What is an electrochemical cell?

Two electrodes connected by an electrolyte that produce a potential difference. (Chemistry only)

What can simple cells be made from?

Two different metals connected by an electrolyte. (Chemistry only)

Why does a simple cell produce a potential difference?

Different metals have different tendencies to lose electrons. (Chemistry only)

What affects the voltage produced by a simple cell?

Electrode materials + electrolyte. (Chemistry only)

How does the difference in reactivity of the two metals affect voltage?

Greater difference in reactivity generally produces a larger voltage. (Chemistry only)

What happens if the same metal is used for both electrodes?

No potential difference is produced. (Chemistry only)

What is a battery?

Two or more cells connected together in series to provide a greater voltage. (Chemistry only)

What happens in a non-rechargeable cell when its reactants are used up?

The reaction stops and the cell no longer produces a voltage. (Chemistry only)

Why can rechargeable cells be recharged?

An external electrical current reverses the chemical reactions. (Chemistry only)

What happens when a rechargeable cell supplies electricity?

Chemical reactions occur that produce a potential difference. (Chemistry only)

What is a fuel cell?

Cell supplied continuously with fuel + oxygen to produce electrical energy. (Chemistry only)

What fuel is commonly used in a hydrogen fuel cell?

Hydrogen.

What is the overall reaction in a hydrogen fuel cell?

Hydrogen + oxygen - water.

What is the balanced equation for a hydrogen fuel cell?

2H2 + O2 - 2H2O.

What are the products of a hydrogen fuel cell?

Water + electrical energy.

What are advantages of hydrogen fuel cells?

Produce water at point of use - can operate continuously while fuel is supplied - no recharging needed.

What are disadvantages of hydrogen fuel cells?

Hydrogen must be produced + stored - storage is difficult - hydrogen production may use fossil fuels.

Why is hydrogen difficult to store?

It is a gas with low density - requires high-pressure tanks or other specialised storage.

Why are hydrogen fuel cells potentially cleaner than petrol/diesel at point of use?

Their product is water rather than carbon dioxide and air pollutants.

Why are hydrogen fuel cells not necessarily completely environmentally friendly?

Environmental impact depends on how hydrogen is produced + energy source used.

How do rechargeable cells compare with fuel cells?

Rechargeable cells store reactants internally and need recharging - fuel cells are supplied continuously with fuel.