4.5 Energy Changes

What is an exothermic reaction?

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

What does the overall reaction in a hydrogen fuel cell involve?

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

∆H

bonds broken - bonds formed

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 examples of endothermic reactions?

Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate.

How are fuel cells supplied?

Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

What offers a potential alternative to rechargeable cells and batteries?

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

What must have happened if a reaction transfers energy to the surroundings?

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

What is the activation energy?

The minimum amount of energy that particles must have to react is called the activation energy.

What are everyday uses of endothermic reactions?

Some sports injury packs are based on endothermic reactions.

What is the voltage produced by a cell dependent on?

The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.

What are batteries?

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

What do cells contain?

Cells contain chemicals which react to produce electricity

What has to happen for chemical reactions to occur?

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

What happens during a chemical reaction?

During a chemical reaction: • energy must be supplied to break bonds in the reactants • energy is released when bonds in the products are formed.

What happens to energy in a chemical reaction?

Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.

In endothermic reactions is the energy released from forming new bonds greater or less than the energy needed to break existing bonds?

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

In exothermic reactions is the energy released from forming new bonds greater or less than the energy needed to break existing bonds?

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

How do you calculate the energy change for a reaction?

1. Draw the molecules 2. Calculate the energy needed to break bonds 3. Calculate the energy released in forming new bonds 4. Calculate the energy change (∆H)

What is the method for RP4?

1. Measure 25cm³ of copper sulfate solution into the coffee cup 2. Measure the initial temp. of the solution 3. Add 1g of magnesium powder to the cup and quickly put on the lid and stir 4. Take the temperature every 20 seconds for three minutes

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 everyday uses of exothermic reactions?

Everyday uses of exothermic reactions include self-heating cans and hand warmers.

What are examples of exothermic reactions?

Exothermic reactions include combustion, many oxidation reactions and neutralisation.

What type of batteries are non-rechargeable?

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

What does the energy profile of an exothermic reaction look like?

Products are lower than reactants because energy has been given out

What can reaction profiles be used to do?

Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

What type of batteries are rechargeable?

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

What is the overall energy change in 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 is the overall energy change of the reaction.

What can be calculated using bond energies?

The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.