Chemical Reactions Table of Contents Observing Chemical Change Describing Chemical Reactions Controlling Chemical Reactions

How Can Changes in Matter Be Described? Matter is anything that has mass and takes up space.

Properties of Matter Matter is often described by its characteristics or properties, and how it changes. Two kinds of properties Chemical- a characteristic of a substance that describes its ability to change into other substances Ex: the ability to burn in the presence of oxygen (flammability) Physical- a characteristic of a substance that can be observed without changing the substance into another substance Ex: Temperature, color, texture, density, and conductivity

Observing Chemical Change Properties of Copper Pennies are coated with copper. Which are physical properties of copper and which are the chemical properties? What are two additional properties?

Changes in Matter Changes in matter can be described in terms of physical changes and chemical changes. Physical change- is any change that alters the form or appearance of a substance but does not change it into another substance. Chemical compositions remain the same. Bending, cutting, crushing, and changes of state of matter Chemical change- a change in matter that produces one or more new substances. Atoms rearrange to form new substances Burning and rusting Reactants- substances that undergo the chemical changes Products- new substances that form

Bonding and Chemical Change Chemical changes occur when existing bonds break and new bonds form. As a result a new substance is produced.

Observing Chemical Change Breaking and Making Bonds Describe what happens to the bonds in each of the steps as oxygen reacts with magnesium.

How Do You Identify a Chemical Reaction? Chemical reactions involve changes in properties and changes in energy that you can often observe.

Changes in Properties One way to detect chemical reactions is to observe changes in the physical properties of the materials. Ex: formation of a precipitate, gas production, and a color change are all possible evidence of a chemical reaction During a chemical reaction, reactants interact to form products with different chemical properties. Although you may observe a change in matter, the change does not always indicate a chemical reaction has taken place. The only sure evidence of a chemical reaction is that one or more new substances are produced.

Formation of a Precipitate The mixing of two liquids may form a precipitate. A precipitate is a solid that forms from liquids during a chemical reaction.

Gas Production Another observable change is the formation of a gas from solid or liquid reactants. Often, the gas formed can be seen as bubbles.

Color Change A color change can signal that a new substance has formed.

Changes in Energy Breaking bonds between atoms or ions requires energy, while forming bonds releases energy. Exothermic reaction- the energy released as the products form is greater than the energy required to break the bonds of the reactants. The energy is usually released as heat. Endothermic reaction- more energy is required to break the bonds of the reactants than is released by the formation of the products. The energy can be absorbed by nearby matter. When energy is absorbed it causes the surroundings to become cooler. Not all endothermic reactions result in a temperature decrease. Many endothermic reactions occur only when heat is constantly added.

Observing Chemical Change Plot the data from the table onto the graph. What would be a good name for the graph?

What Information Does a Chemical Equation Contain? A chemical equation is a way to show a chemical reaction, using symbols instead of words. In chemical equations, chemical formulas and other symbols are used to summarize a reaction.

Formulas in an Equation A chemical formula is a combination of symbols that represents the elements in a compound. The formula tells you the ratio between the atoms in the compound.

Describing Chemical Reactions Chemical Formulas The formula of a compound identifies the elements in the compound and the ratio in which their atoms or ions are present. What are the missing chemical formulas in the table?

Structure of an Equation A chemical equation summarizes everything needed to carry out a chemical reaction. A chemical equation tells you the substances you start with in a reaction and the substances that are formed at the end. The substances you have at the beginning are reactants. The new substances after the reaction are the products. The arrow in a chemical equation is read as yields or reacts to form When there are two or more reactants or products, they are separated by plus signs. The number of reactants and products can vary.

Describing Chemical Reactions Modeling a Chemical Equation Like a skateboard, a chemical equation has a basic structure. How many complete skateboards can be made from the parts pictured?

Describing Chemical Reactions Molecules of nitrogen (N2) and hydrogen (H2) react to form ammonia (NH3). How many H2 and N2 molecules are needed to yield two molecules of NH3?

How Is Mass Conserved During a Chemical Reaction? Law of conservation of mass- during a chemical reaction, matter is not created or destroyed. The idea of atoms explains the conservation of mass. In a chemical reaction, all of the atoms present at the start of the reaction are present at the end of the reaction. Atoms are not created or destroyed. They may be rearranged to form new substances.

Describing Chemical Reactions Conservation of Mass In a chemical reaction, matter is not created or destroyed. What is the mass of iron sulfide produced by this reaction?

Open and Closed Systems Open system- matter can enter from or escape to the surroundings. If you want to measure all the matter before and after a reaction, you have to be able to contain it. Closed system- matter does not enter or leave. A chemical reaction that occurs inside a sealed, airtight container is a closed system.

Balancing Chemical Equations To be accurate, a chemical equation must show the same number of atoms of each element on both sides of the equation. Chemists say an equation is balanced when conservation of mass is correctly shown. Write the equation- reactants go on the left side of the arrow, products on the right side of the arrow. Count the atoms Use coefficients to balance atoms- coefficient- a number placed in front of a chemical formula in an equation. It tells you the amount of a reactant or product that takes part in a reaction. Look Back and Check

Describing Chemical Reactions Balancing Chemical Equations What are the four steps in balancing chemical equations? Step 1 Step 2 Step 3 Step 4

What are Three Types of Chemical Reactions? Three types of chemical reactions are synthesis, decomposition, and replacement.

Synthesis Synthesize means to put things together. In chemistry, when two or more elements or compounds combine to make a more complex substance, the reaction is classified as synthesis.

Decomposition A decomposition reaction occurs when compounds break down into simpler products.

Replacement When one element replaces another element in a compound, or if two elements in different compounds trade places, the reaction is called a replacement.

Describing Chemical Reactions Types of Reactions What is each type of reaction represented?

How Do Reactions Get Started? Activation Energy A reaction won’t begin until the reactants have enough energy to push them to the “top of the mountain” The energy is used to break the chemical bonds of the reactants. Activation energy- the minimum amount of energy needed to start a chemical reaction. All chemical reactions need a certain amount of activation energy to get started. Adding heat or an electric spark is enough energy to start a reaction

Controlling Chemical Reactions Activation Energy A chemical reaction needs a push to the “top of the mountain” to get started. Where has enough activation energy been added to start the reaction?

Graphing Changes in Energy Every chemical reaction needs activation energy to start. Whether or not a reaction still needs more energy from the environment to keep going depends on whether it is exothermic or endothermic.

Controlling Chemical Reactions Graphs of Exothermic and Endothermic Reactions Each of the graphs shows the amount of energy before and after the reaction. On each graph, determine whether energy is absorbed or released.

What Affects the Rate of a Chemical Reaction? A particular reaction can occur at different rates depending on the conditions. Factors that can affect rates of reactions include surface area, temperature, concentration, and the presence of catalysts and inhibitors.

Surface Area When a piece of solid substance reacts with a liquid or gas, only the particles on the surface of the solid come into contact with the other reactant. If you break the solid into smaller pieces, more particles are exposed to the surface and the reaction happens faster.

Temperature When you heat a substance, its particles move faster. Faster-moving particles have more energy, which helps reactants get over the activation energy barrier more quickly. Reducing temperature slows down reaction rates.

Concentration If you increase the concentration of the reactants, it will increase the rate of a chemical reaction. Concentration- the amount of a substance in a given volume, Increasing the concentration of reactants supplies more particles to react.

Catalysts and Inhibitors A catalyst- increases the reaction rate by lowering the activation energy needed. Cells in our bodies contain thousands of biological catalysts called enzymes, which help these reactions occur at body temperature. Chemical reactions using enzymes require less activation energy and can happen at lower temperatures. A material used to decrease the rate of a chemical reaction is an inhibitor. Inhibitors called preservatives are added to food to prevent spoiling.

Controlling Chemical Reactions Catalysts Adding a catalyst speeds up a chemical reaction. How would the energy graph change for the same chemical reaction if a catalyst were used?