chemical reactions and energy

surface area of reactants

When a solid substance is involved in a chemical reaction, only the matter at the surface of the solid is exposed to other reactants. If a solid has more surface area, more of it is exposed and able to react. Therefore, increasing the surface area of solid reactants increases the reaction rate. For example, crushing a solid into a powder exposes more of the substance to other reactants. This may greatly speed up the reaction. You can see another example in Figure 8.18. Iron rusts when it combines with oxygen in the air. The iron hammer head and iron nails will both rust eventually. Which will rust faster?

textbook_image

presence of a catalyst

Some reactions need extra help to occur quickly. They need another substance, called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction but is not changed or used up in the reaction. The catalyst can go on to catalyze many more reactions. Catalysts are not reactants, but they help reactants come together so they can react. You can see one way this happens in the animation at the URL below. By helping reactants come together, a catalyst decreases the activation energy needed to start a chemical reaction. This speeds up the reaction. Living things depend on catalysts to speed up many chemical reactions inside their cells. Catalysts in living things are called enzymes. Enzymes may be extremely effective. A reaction that takes a split second to occur with an enzyme might take billions of years without it!

endothermic reactions

In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products. The word "endothermic" literally means "taking in heat." A constant input of energy, often in the form of heat, is needed in an endothermic reaction. Not enough energy is released when products form to break more bonds in the reactants. Additional energy is needed to keep the reaction going. The general equation for an endothermic reaction is: Reactants + Energy ! Products In many endothermic reactions, heat is absorbed from the surroundings. As a result, the temperature drops. The drop in temperature may be great enough to cause liquid products to freeze. Thats what happens in the endothermic reaction at this URL: One of the most important endothermic reactions is photosynthesis. In this reaction, plants synthesize glucose (C6 H12 O6 ) from carbon dioxide (CO2 ) and water (H2 O). They also release oxygen (O2 ). The energy for photo- synthesis comes from light (see Figure 8.12). Without light energy, photosynthesis cannot occur. The chemical equation for photosynthesis is: 6CO2 + 6H2 O ! C6 H12 O6 + 6O2

textbook_image

concentration of reactants

Concentration is the number of particles of a substance in a given volume. When the concentration of reactants is higher, the reaction rate is faster. At higher concentrations, particles of reactants are crowded closer together, so they are more likely to collide and react. Did you ever see a sign like the one in Figure 8.17? You might see it where someone is using a tank of pure oxygen for a breathing problem. The greater concentration of oxygen in the air makes combustion rapid if a fire starts burning.

textbook_image

temperature of reactants

When the temperature of reactants is higher, the rate of the reaction is faster. At higher temperatures, particles of reactants have more energy, so they move faster. They are more likely to bump into one another and to collide with greater force. For example, when you fry an egg, turning up the heat causes the egg to cook faster. The same principle explains why storing food in a cold refrigerator reduces the rate at which food spoils (see Figure 8.16). Both food frying and food spoiling are chemical reactions that happen faster at higher temperatures.

textbook_image

reaction rate

Any factor that helps reactants come together so they can react lowers the amount of activation energy needed to start the reaction. If the activation energy is lowered, more reactant particles can react, and the reaction occurs more quickly. How fast a reaction occurs is called the reaction rate. Factors that affect the reaction rate include: temperature of reactants concentration of reactants surface area of reactants presence of catalysts

activation energy

All chemical reactions, even exothermic reactions, need a certain amount of energy to get started. This energy is called activation energy. For example, activation energy is needed to start a car. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas wont occur without the spark of energy to begin the reaction. Why is activation energy needed? A reaction wont occur unless atoms or molecules of reactants come together. This happens only if the particles are moving, and movement takes energy. Often, reactants have to overcome forces that push them apart. This takes energy as well. Still more energy is needed to start breaking bonds in reactants. The graphs in Figure 8.15 show the changes in energy in endothermic and exothermic reactions. Both reactions need the same amount of activation energy in order to begin. You have probably used activation energy to start a chemical reaction. For example, if youve ever used a match to light a campfire, then you provided the activation energy needed to start a combustion reaction. Combustion is exothermic. Once a fire starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, wood will not burst into flames on its own.

textbook_image

conservation of energy

Whether a reaction absorbs energy or releases energy, there is no overall change in the amount of energy. Energy cannot be created or destroyed. This is the law of conservation of energy. Energy can change form for example, from electricity to light but the same amount of energy always remains. If energy cannot be destroyed, what happens to the energy that is absorbed in an endothermic reaction? The energy is stored in the chemical bonds of the products. This form of energy is called chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. The excess energy in the reactants is released to the surroundings when the reaction occurs. The graphs in Figure 8.14 show the chemical energy of reactants and products in each type of reaction.

textbook_image

textbook_image

exothermic reactions

In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. The word "exothermic" literally means "turning out heat." Energy, often in the form of heat, is released as an exothermic reaction occurs. The general equation for an exothermic reaction is: Reactants ! Products + Energy If the energy is released as heat, an exothermic reaction results in a rise in temperature. Thats what happens in the exothermic reaction at the URL below. Combustion reactions are examples of exothermic reactions. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in Figure 8.13. You can see the light energy it is giving off. If you were standing near the fire, you would also feel its heat.

instructional diagrams

No diagram descriptions associated with this lesson

questions

Which statement describes a role of energy in chemical reactions?

a. Energy is created in exothermic reactions.

b. Energy is always released in chemical reactions.

-->  c. Energy is needed for chemical reactions to start.

d. Energy is destroyed in endothermic reactions.

All chemical reactions need energy to

a. form bonds in products.

b. break bonds in reactants.

c. get started.

-->  d. two of the above

The energy needed for photosynthesis is in the form of

a. glucose.

b. oxygen.

-->  c. light.

d. heat.

Which statement about exothermic reactions is false?

a. They need activation energy.

b. They include combustion.

c. They give off energy.

-->  d. They take in heat.

When products have less chemical energy than reactants, a chemical reaction

a. is endothermic.

-->  b. is exothermic.

c. absorbs energy.

d. two of the above

A constant input of energy is needed in

a. exothermic reactions.

-->  b. endothermic reactions.

c. all chemical reactions.

d. none of the above

You can increase the rate of a chemical reaction by

-->  a. increasing the temperature of reactants.

b. decreasing the surface area of reactants.

c. decreasing the concentration of reactants.

d. all of the above

According to the law of conservation of energy, energy

a. cannot be created.

b. cannot be destroyed.

c. cannot change form.

-->  d. two of the above

Factors that affect reaction rates include

a. temperature.

b. concentration.

c. surface area.

-->  d. all of the above

A catalyst

a. can catalyze just one chemical reaction.

b. is always changed in a chemical reaction.

c. becomes a product in a chemical reaction.

-->  d. helps reactants come together in a chemical reaction.

Crushing a solid reactant into a powder will

a. decrease the reactants surface are

-->  b. increase the rate of the reaction.

c. decrease the concentration of products.

d. increase the temperature of reactants.

Which statement about catalysts is true?

-->  a. They change the rate of chemical reactions.

b. They are reactants in chemical reactions.

c. They are used up in chemical reactions.

d. two of the above

energy stored in chemical bonds

a. activation energy

b. catalyst

c. concentration

d. endothermic

e. exothermic

f. reaction rate

-->  g. chemical energy

substance that speeds up chemical reactions

a. activation energy

-->  b. catalyst

c. concentration

d. endothermic

e. exothermic

f. reaction rate

g. chemical energy

turning out heat

a. activation energy

b. catalyst

c. concentration

d. endothermic

-->  e. exothermic

f. reaction rate

g. chemical energy

An endothermic chemical reaction is a reaction that releases energy.

a. true

-->  b. false

how fast a reaction occurs

a. activation energy

b. catalyst

c. concentration

d. endothermic

e. exothermic

-->  f. reaction rate

g. chemical energy

The general equation for an exothermic reaction is Reactants + Energy Products.

a. true

-->  b. false

energy needed to start a reaction

-->  a. activation energy

b. catalyst

c. concentration

d. endothermic

e. exothermic

f. reaction rate

g. chemical energy

taking in heat

a. activation energy

b. catalyst

c. concentration

-->  d. endothermic

e. exothermic

f. reaction rate

g. chemical energy

Plants synthesize glucose in an endothermic chemical reaction.

-->  a. true

b. false

number of particles of a substance in a given volume

a. activation energy

b. catalyst

-->  c. concentration

d. endothermic

e. exothermic

f. reaction rate

g. chemical energy

The law of conservation of energy does not apply to chemical reactions.

a. true

-->  b. false

Any factor that helps reactants come together lowers the activation energy.

-->  a. true

b. false

All chemical reactions involve energy.

-->  a. true

b. false

One of the most important endothermic reactions is photosynthesis.

-->  a. true

b. false

In an exothermic reaction, it takes more energy to break bonds in reactants than is released when bonds

a. true

-->  b. false

Combustion is an example of an endothermic reaction.

a. true

-->  b. false

There is no overall change in the amount of energy in chemical reactions.

-->  a. true

b. false

Only endothermic reactions need energy to get started.

a. true

-->  b. false

Energy is absorbed in exothermic reactions.

a. true

-->  b. false

An increase in temperature is a sign of an exothermic reaction.

-->  a. true

b. false

Products have less stored chemical energy than reactants in an endothermic reaction.

a. true

-->  b. false

Catalysts in living things are called enzymes.

-->  a. true

b. false

diagram questions

No diagram questions associated with this lesson