EDEXCEL C2

Exothermic reactions

These are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer.

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Examples of exothermic reactions

Burning (combustion). Neutralisation reactions between acids and alkalis. The reaction between water and calcium oxide. Explosions.

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Endothermic reactions

These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer.

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Examples of endothermic reactions

Electrolysis. The reaction between ethanoic acid and sodium carbonate. Photosynthesis. The reaction between ammonium nitrate and water. The thermal decomposition of calcium carbonate in a blast furnace.

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During a chemical reaction...

Bonds in the reactants are broken. New bonds are made in the products. Energy is absorbed to break bonds. Bond-breaking is an endothermic process. Energy is released when new bonds form. Bond-making is an exothermic process.

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Differences between endothermic and exothermic reactions

Endothermic: Energy absorbed is more than the energy released. Exothermic: Energy released is more than the energy absorbed.

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Calculating energy transferred

Energy transferred = mass of water heated × specific heat capacity of water × temperature rise

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How to increase the rate of a reaction

The temperature is increased The concentration of a dissolved reactant is increased The pressure of a reacting gas is increased Solid reactants are broken into smaller pieces A catalyst is used

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If the temperature is increased...

The reactant particles move more quickly More particles have the activation energy or greater The particles collide more often, and more of the collisions result in a reaction The rate of reaction increases

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If a solid reactant is broken into small pieces or ground into a powder...

Its surface area is increased More particles are exposed to the other reactant There is a greater chance of the particles colliding The rate of reaction increases

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If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased...

There are more reactant particles in the same volume There is a greater chance of the particles colliding The rate of reaction increases

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Covalent bonds

Non metals combine together by sharing electrons. The shared pair of electrons holds the two atoms together. It's called a covalent bond. The group of atoms bonded together in this way is called a molecule.

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The allotropes of carbon

Three forms, or 'allotropes', of pure carbon are diamond, graphite and buckminsterfullerene. In all three allotropes, the carbon atoms are joined by strong covalent bonds but in such different arrangements that the properties are v. different.

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Miscible liquids

Miscible liquids are harder to separate as they dissolve in each other. Miscible liquids are often separated using fractional distillation. This is possible as miscible liquids have different boiling points.

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Properties of the groups

Alkali metals are reactive, soft metals with low densities. Transition metals are unreactive and many have everyday uses. Halogens are reactive non-metals that form coloured vapours. Noble gases are unreactive non-metals. All gases fall in to this.

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Malleability

All metals are malleable, meaning they can be hammered into shape. The advantage of this property is that metals have many uses, eg for making jewellery

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EDEXCEL C2

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