Exothermic and endothermic reactions .1

energy is always…

• conserved in chemical reactions,

• so the total amount of energy in the universe at the end of a reaction is the same as it was before the reaction.

what are the two types of chemical reactions

• exothermic reactions

• endothermic reactions

what happens during a chemical reactions (to do with the surrounding)

energy is transferred to or from the surroundings

what is a exothermic reaction

• energy is transferred to the surroundings

• and the temperature of the surroundings increases

what are examples of exothermic chemical reactions

• combustion reactions

• many oxidation reactions

• most neutralisation reactions

what are some every day uses where exothermic reactions occur

• self-heating cans

• hand warmers

what is an endothermic reaction

• When energy is taken in from the surroundings

• the temperature of the surroundings decreases.

what are some examples of endothermic reactions

• thermal decomposition reactions

• the reaction of citric acid and sodium hydrogencarbonate

what are some every day uses where endothermic reactions occur

• instant ice packs which can be used to treat sports injuries.

example of a exothermic reaction between dilute sodium hydroxide and hydrochloric acid

1. Sodium hydroxide solution is poured into a beaker of hydrochloric acid which contains a thermometer showing room temperature

2. The beaker now contains sodium chloride and water, and the thermometer is showing a rise in temperature, so the neutralisation reaction is exothermic

example of a endothermic reaction between sodium carbonate and ethanoic acid

1. Sodium carbonate powder is tipped into a beaker of ethanoic acid which contains a thermometer showing room temperature

2. The beaker now contains sodium ethanoate, water and carbon dioxide, and the thermometer is showing a fall in temperature, so this was an endothermic reaction

In endothermic reactions….

energy enters

In exothermic reactions…

energy exits.

Required practical - temperature changes in reacting solutions

Reacting two solutions, eg acid and alkali

Reacting two solutions, eg acid and alkali

1. Place the polystyrene cup inside the glass beaker to make it more stable.

2. Measure an appropriate volume of each liquid, eg 25 cm³.

3. Place one of the liquids in a polystyrene cup.

4. Record the temperature of the solution.

5. Add the second solution and record the highest or lowest temperature obtained.

6. Change your independent variable and repeat the experiment. Your independent variable could be the concentration of one of the reactants, or the type of acid/alkali being used, or the type of metal/metal carbonate being used.

Required practical - temperature changes in reacting solutions

Reacting a solid with a solution, eg metal and acid

Reacting a solid with a solution, eg metal and acid

1. Place the polystyrene cup inside the glass beaker to make it more stable.

2. Measure an appropriate volume of the solution, eg 25 cm³.

3. Measure an appropriate mass of the solid, or select a suitable sized piece of metal.

4. Place the solution in a polystyrene cup.

5. Record the temperature of the solution.

6. Add the solid and record the highest or lowest temperature obtained.

7. Change your independent variable and repeat the experiment. Your independent variable could be the surface area of the solid, or the type of acid being used, or the type of metal being used.

Required practical - temperature changes in reacting solutions

analysis

The bigger the temperature change in the reaction, the more energy is absorbed or released

Required practical - temperature changes in reacting solutions

evaluation

The biggest source of error in this experiment is unwanted heat transfer. Using a lid can help to reduce this.

Required practical - temperature changes in reacting solutions

2 hazards

• Dilute acids and alkalis

• Solutions of metal salts (used in displacement reactions)

Required practical - temperature changes in reacting solutions

possible harm + precaution of Dilute acids and alkalis

• May irritate the skin or eyes

• Avoid contact with skin, rinse off skin if necessary, wear eye protection

Required practical - temperature changes in reacting solutions

possible harm + precaution of Solutions of metal salts (used in displacement reactions)

• Dangerous to the environment

• Dispose of metal salt solutions as advised by teacher, some metal salts are collected for safe disposal rather than being poured down the normal drain

what is a energy level diagram

• shows whether a reaction is exothermic or endothermic

• It shows the energy in the reactants and products

• and the difference in energy between them.

what happens to the energy level in an exothermic reaction + why

• decreases

• because energy is given out to the surroundings.

what happens to the energy level in an endothermic reaction + why

• increases

• because energy is taken in from the surroundings.

what is the activation energy in a reaction profile

the minimum energy needed by particles when they collide for a reaction to occur

what is the overall change in energy in a reaction

the difference between the energy of the reactants and products.

what is the reaction profile for an exothermic reaction.

what is the reaction profile for an endothermic reaction

how is energy transferred

when bonds are broken or are formed

what does an endothermic reaction do to bonds

• breaking bonds

• takes energy in

what does an exothermic reaction do to bonds

• building bonds

• gives energy out

what happens during a chemical reaction to the reactants and products

During a chemical reaction:

• bonds in the reactants are broken

• new bonds are made in the products

in an exothermic reaction where is more heat energy needed (making or breaking bonds)

• in a exothermic more heat energy is released in making bonds in the products

• than is taken in when breaking bonds in the reactants

in an endothermic reaction where is more heat energy needed (making or breaking bonds)

• in a endothermic reaction more heat energy is taken in when breaking bonds in the reactants

• than is released in making bonds in the products

how can the energy change in a reaction be calculated

using bond energies

what is a bond energy

A bond energy is the amount of energy needed to break one mole of a particular covalent bond.

how do you calculate the energy taken in (bond energy)

add together the bond energies for all the bonds in the reactants - this is the 'energy in'

hwo do you calculate the energy given out (bond energy)

add together the bond energies for all the bonds in the products - this is the 'energy out'

how do you calculate the energy change

energy change = energy in - energy out

if the energy change is negative what is the reaction

exothermic

if the energy change is positive what is the reaction

endothermic