Chemical Reactions This is the fourth text which will be used for practicing dictation - PowerPoint Presentation

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Chemical Reactions

This is the fourth text which will be used for practicing dictation and translation in the written part of the exam.

The key words are emphasized in bold

and there is a list of them at the end of the presentation.

Chemical Reactions

When a chemical reaction occurs, new substances (called

products

) form from the substances taking part in the reaction (called reactants).Burning is an exothermic reaction – more heat is given out during the reaction than is taken in. Oxidation occurs when a substance combines with oxygen. Reduction occurs when a substance loses oxygen.

Chemical Reactions

Reversible

chemical reactions can go forwards and backwards. When heated, nitrogen dioxide (NO

2) breaks down into nitrogen monoxide (NO) and oxygen (O2), when cooled the change is reversed.Some reactions take in more heat energy than they give out. These reactions are called endothermic.Electron transfer – During the oxidation

process, atoms lose electrons and are

oxidized

. During reduction, atoms gain electrons and are reduced.

Chemical Reactions

Each element has a

chemical symbol

to identify it, and each compound a chemical formula. The formula indicates how the elements in the compound are combined. A chemical equation shows which substances react and the products that result. An atom’s valency shows the number of chemical bonds it can form. It is the number of electrons the atom gains, loses or shares when it makes bonds.

Chemical Reactions

Controlling reactions

– Chemists speed up reactions by making the reacting particles

collide with each other more often or with greater energy. Substances called catalysts speed up reactions by helping substances react together. They remain unchanged by the chemical reaction.Increasing the concentration of a reactant speeds up a reaction. Dyeing a material is faster with a concentrated dye – there are more dye molecules to collide with the material.

Chemical Reactions

The

surface area

of a solid object is the size of its outer surface. Increasing the surface area of a reacting substance speeds up the chemical reaction.Yeast is a fungus containing enzymes, which are biological catalysts. The enzymes in yeast make starches and sugars break down more rapidly into carbon dioxide and ethanol.

Key words:

Product –

produkt

, proizvod – Reactant - reaktantBurning – sagorevanjeReversible/reversed – reverzibilna, povratna

Oxidation/reduction –

oksidacija

/redukcija

Chemical symbol –

hemijski

simbol

Chemical formula –

hemijska

formula

Chemical equation –

hemijska

jednacina

Chemical bonds –

hemijske

veze

Key words

Catalyst –

katalizator

Catalyzation - katalizacijaDye – bojaDyeing – bojenjeSurface area – ukupna

spoljna

povrsinaYeast – kvasac

Starch –

skrob

To break down –

razgraditi