#IndicatorsOfAChemicalReactions

#Chemistry

#ChemicalReactions

Introduction

• A chemical reaction is when one or more substances change and produce one or more new chemical substances.

• The substances that are involved in a chemical reaction and that are changed by it are called reactants.

• The substances that are produced by a chemical reaction are called products.

• Chemical reactions happen all around us (and inside us!) all the time. We might not always notice them but there are four indicators that show a chemical reaction has taken place:

  • colour change

  • effervescence

  • precipitation

  • energy (temperature) change

Colour change

• A colour change might take place when two substances react. It can also happen when a compound is broken down by heating it. This is called thermal decomposition.

• Copper carbonate is green and copper oxide is black. You can see a colour change from green to black during the reaction. The carbon dioxide produced can be detected using limewater, which turns cloudy.

Effervescence

• Some chemical reactions result in a product that is in a different state to the reactants.

• example of this is effervescence in which a reaction in a liquid produces bubbles of gas. Effervescence is an indicator of a chemical reaction taking place.

Precipitation

• Some liquids react together to produce an insoluble solid. This is called a precipitation reaction and the solid formed is called a precipitate.

Energy change

• Every chemical reaction involves a change in energy because different substances hold different amounts of energy.

• During the reaction, bonds inside the substances that are reacting together must be broken and new chemical bonds must be formed in the products that are being made.

• Chemical reactions can take in energy or release energy, often in the form of heat. This causes a change in temperature.

• There are two different types of energy change that can take place.

Exothermic reactions

• In an exothermic reaction the reactants hold more energy than the products, so exothermic reactions release energy.

• This energy is most commonly a release of heat energy which would be indicated by a temperature rise.

• Energy could also be released in a chemical reaction in the form of a sound or light being produced but the most exciting chemical reactions will probably have all three going on!

• Burning fuels like coal is an exothermic process

• Exothermic reactions happen around us in everyday life. Fuels burning in combustion reactions involve energy being released. It doesn’t matter if it’s a small object like a match or a whole bonfire that is burning, both heat energy and light energy are given out.

• Not every exothermic reaction is as exciting as a combustion reaction. When acids and alkalis react together, the energy released is not as obvious. Mixing the two solutions and stirring results in a small increase in the temperature of the reaction mixture.

Endothermic reactions

• In an endothermic reaction, the reactants have less energy than the products, so more energy is needed for the reaction to take place.

• Normally, endothermic reactions take in heat. So endothermic reactions can be identified by a decrease in temperature.

• An endothermic reaction reduces the temperature of a cool pack to prevent swelling and aid healing from an injury.

• There are fewer examples of endothermic reactions in everyday life. Chemical cold packs that are used to treat bumps and sprains use an endothermic reaction to cool down.

• Squeezing the cold pack bursts a small inner bag allowing two chemicals to mix. The result is an endothermic reaction that cools the pack down and it can be used to stop or reduce swelling.