T5:Energy Changes

What happens to the energy in a chemical reaction?

Energy is conserved in chemical reactions.

What must be true in a reaction?

If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred

What is an exothermic reaction?

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases

What are three examples of exothermic reactions?

• combustion

• many oxidation reactions

• neutralisation.

What are everyday uses of exothermic reactions?

Self-heating cans and hand warmers.

What is an endothermic reaction?

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

What are three examples of endothermic reactions?

• Thermal decompisition

• Reaction of citric acid and sodium hydrogencarbonate.

What is an everyday uses of an endothermic reactions?

• Sports injury packs.

When is the only time that chemical reactions can occur?

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.

What is minimum amount of energy that particles must have to react called?

The activiation energy.

Draw the reaction profile for an exothermic reaction:

Draw the reaction profile for an endothermic reaction:

Energy must be supplied to….

Break bonds in the reactants

Energy is released when bonds …….

In the products are formed.

What is the overall energy change of a reaction?

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.

What is the relationship between bonds and energy in an exothermic reaction?

In an exothermic reaction, the energy released from forming new

bonds is greater than the energy needed to break existing bonds.

What is the relationship between bonds and energy in an endothermic reaction?

In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds

What si the non-official formula to calculate the overall energy change of a reaction?

Overall energy change=bonds broken-bonds formed

Is the reaction exothermic or endothermic if the total energy change is negative?

Exothermic.

Is the reaction exothermic or endothermic if the total energy change is positive?

Endothermic.

What do cells contain?

Cells contain chemicals which react to produce electricity.

What 2 things is the voltage produced by a cell dependent on?

• The type of electrode

• The electrolyte

How can a simple cell be made?

A simple cell can be made by connecting two different metals in contact with an electrolyte.

What are batteries?

Batteries consist of two or more cells connected together in series to provide a greater voltage.

How do you know what the more reactive element is in an electrochemical cell?

• It is the neagtive element.

What happens in a non-rechargeable battery?

In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up.

What is an example of a non-rechargeable battery?

An alkaline battery.

Why can rechargeable cells an batteries be reacharged?

Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

What are fuel cells supplied by?

Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air.

What happens to the fuel within the fuel cell?

The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

What does the overall reaction in an hydorgen fuel cell involve?

The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.

What can hydrogen fuel cells offer?

Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries

What is the balanced half equation for a hydrogen feul cell at the Cathode?

What is the balanced half equation for a hydrogen feul cell at the Anode?

What is the equation for the overall reaction in a hydrogen fuel cell?

What are some advantages of hydrogen fuel cells?

• Don’t produce as much pollutants as other fuels

• The nly by-products are water and heat

• Feul cells are less expensive than batteries

• Fuel cells store more enrgy than batteries

What are some disadvantages of hydrogen fuel cells?

• Hydorgen is a gas so takes up more space to store

• Hydorgen is hard to store safely as it is explosive when mixed ith air

• The hydorgen fuel cell is made from fossil feuls