4.5.1 Exothermic and endothermic reactions

Is energy lost or gained in a chemical reaction? Explain

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.

What is the surroundings in a reaction?

Everything other than the reactants and products (e.g., the test tube, the air, the solvent, the thermometer)

What is the system in a reaction?

The reactants and products.

What is an exothermic reaction?

An exothermic reaction is one where energy is transferred from the system to the surroundings. (so the temp of the surroundings increases)`

What are some examples of exothermic reactions?

Exothermic reactions include
a) combustion
b) many oxidation reactions
c) neutralisation reactions

What are some everday uses of exothermic reactions?

Everyday uses of exothermic reactions include

• self-heating cans

• hand warmers.

What is an endothermic reaction?

An exothermic reaction is one where energy is transferred from the surroundings to the system. (so the temp of the surroundings decreases as energy is taken in from it)

What are some examples of endothermic reactions?

a) thermal decomposition reactions
b) the reaction of citric acid and sodium hydrogencarbonate

What are some everyday uses of endothermic reactions?

Some sports injury packs are based on endothermic reactions.

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

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

When can chemical reactions occur?

• When reacting particles collide with each other

• When reacting particles have sufficient energy to react

Define activation energy

Activation energy is the minimum amount of energy that particles must have to react

What is a Reaction Profile Diagram? And what is it used to show?

A reaction prophile is graphical representation of the energy changes that occur during a chemical reaction.

It shows

• the relative energies of the reactants and products

• the activation energy

• the overall energy change
  of a reaction

What must be labelled on the axes of a Reaction Profile?

Y axis: Energy
X axis: Progress of reaction

How is activation energy displayed on a reaction profile?

On the Profile: Activation Energy is represented by the height of the energy hump (peak) measured from the initial energy level of the reactants up to the peak of the curve.

For the key steps in drawing a reaction profile, look at your book

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

The reactants are at a higher energy level while the products are at a lower energy level.

The overall energy change is negative (energy is released).

The arrow from Reactants to Products points downwards.

What does the reaction profile of an endothermic reaction look like?

The reactants are at a lower energy level while the products are at a higher energy level.

The overall energy change is positive (energy is released).

The arrow from Reactants to Products points upwards.

In a reaction, what happens to the bonds of the reactants and products (not talking about energy)

The atoms in the reactants are held together by chemical bonds. The bonds are broken, splitting the atoms apart. New bonds are formed as atoms join to make products.

Energy must be supplied to….

Energy is released when…

(making and breaking bonds, fill in the gaps)

Energy must be supplied to break bonds in the reactants

Energy is released when bonds in the products are formed

Is breaking bonds exothermic or endothermic? Why?

Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds.

Is making bonds exothermic or endothermic? Why?

Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed

What is the formula for calculating the overall energy change in a reaction?

The difference between

• the sum of the energy needed to break bonds in the reactants
• the sum of the energy released when bonds in the products are formed
  is the overall energy change of the reaction.

Energy absorbed to break bonds - Energy released to make bonds

Explain an exothermic reaction in terms of bonds broken and formed

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

Explain an endothermic reaction in terms of bonds broken and formed

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

What is bond energy?

Bond energy is

• the amount of energy required to break one mole of a specific bond

• the amount of energy released when one mole of that same bond is formed

How would you use bond energies to idneitfy if a reaction is exothermic or endothermic?

1) Add together all the bond energies for all the bonds in the reactants - this is the 'energy in'
2) Add together the bond energies for all the bonds in the products - this is the 'energy out'
3) Energy change = Energy taken in - Energy given out

If change in energy is negative, the reaction is exothermic

If change in energy is positive, the reaction is endothermic

(visualise a reaction profile if that helps)