What's Hot?Year 10 Science 2012

Section 1: Energy in Chemical reactions

Energy and Chemical Reactions

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions.

Demonstration of exothermic and endothermic reactions

 Record your observations stating whether the reactions are exothermic or endothermic

Exothermic Reactions

Exothermic reactions may occur spontaneously. Exothermic reactions produce heat or may even be explosive.

An exothermic reaction is one which transfers heat energy to the surroundings

Some Exothermic Reactions

• Magnesium + Hydrochloric acid

Gets hot

25o C 45o C

magnesium

Hydrochloricacid

Heatenergygivenout

Exothermic Reactions

Examples of Exothermic reactions

• Skiers and mountain climbers often use hand warmers inside their gloves. These small heat packs rely on exothermic processes.

• Combustion is an example of an exothermic reaction

Bunsen burners burn methane:

Cigarette lighters use butane to produce a flame:butane + oxygen          carbon dioxide + water

Alcohols are also flammable, for example:ethanol + oxygen          carbon dioxide + water

Chemistry is full of patternsWhat pattern do you see here?

Combustion

Examples of exothermic processes

Change of stateWhen steam condenses into water, the water molecules lose energy and this energy is transferred to the surroundings. Similarly, when water freezes, the water molecules lose even more energy to the surroundings.

Exothermic Examples• Chemical Reactionso A candle flameo Rusting irono Burning sugaro Glow stickso Fire crackers

• Physical processeso Some hand warmerso Some dissolvingo Condensingo Freezing

What do you think is the difference between a reaction and process? 

Experiment

Complete page 69 Questions 1-8

Endothermic Reactions

Endothermic reactions cannot occur spontaneously. When endothermic reactions absorb energy, a temperature drop is measured during the reaction.

An endothermic reaction is one which takes heat energy from the surroundings

Endothermic Reaction

 

Cools

HeatenergyIs used up during the reaction so the contents in tube feel cold

Starts 25°C Cools to 5°C

Ammoniumnitrate

Water

Endothermic reactions cause a decrease in temperature.

Endothermic Reactions

Example of Endothermic Reactions

• When you squeeze the plastic bag the water reacts with the ammonium nitrate crystals, and the reaction dissolves the ionic bonds in the ammonium nitrate.

Another example of Endothermic reactions

• An important endothermic reaction is photosynthesis.

• Plants need energy from sunlight to make glucose and oxygen from carbon dioxide and water.

Endothermic Processes

Changes of stateFor ice to melt, the water molecules must gain energy.For water to boil and turn into steam, the water molecules must gain even more energy. These are both endothermic processes. Energy is needed to make these changes happen.

Endothermic Examples

• Chemical Reactionso Baking breado Cooking an egg

• Physical Processeso Sometimes dissolvingo Cold packso Melting and boiling  

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questions 1-5