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Understanding Chemical Reactions
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Understanding Chemical Reactions

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    Understanding Chemical Reactions Understanding Chemical Reactions Presentation Transcript

    • Understanding Chemical Reactions
    • In both cases a liquid turned into a solid. Are these changes the same?
    • Physical Change Chemical Changevs
    • Physical Change Chemical Change • Physical changes do not produce new substances. vs • The substances that exist before and after the change are the same, although they might have different physical properties. • This is what happens when liquid water changes to ice. Its physical properties change from a liquid to a solid. But the water, H2O, does not change into a different substance.
    • Physical Change vs Chemical Change • The starting substances and the substances produced have different physical and chemical properties. • A chemical change also is called a chemical reaction. • A chemical reaction is a process in which atoms of one or more substances rearrange to form one or more new substances. • In a chemical change, one or more substances change into new substances.
    • Signs of a Chemical Reaction Change in Properties Bright copper changes to green when the copper reacts with certain gases in the air. Change in color
    • Signs of a Chemical Reaction Change in Properties When food burns or rots, a change in odor is a sign of a chemical change. Change in odor Change in color
    • Signs of a Chemical Reaction Change in Properties Bubbles of carbon dioxide form when baking soda is added to vinegar. Change in odor Change in color Formation of bubbles
    • Signs of a Chemical Reaction Change in Properties Bubbles of carbon dioxide form when baking soda is added to vinegar. Change in odor Change in color Formation of bubbles Bubbles appear when water boils, Is this a chemical reaction?
    • Signs of a Chemical Reaction Change in Properties Formation of a precipitate A precipitate is a solid formed when two liquids react. Change in odor Change in color Formation of bubbles
    • Signs of a Chemical Reaction Change in Energy Thermal energy is either given off or absorbed during a chemical change. Warming or Cooling
    • Signs of a Chemical Reaction Change in Energy A firefly gives off light as the result of a chemical change. Release of Light Warming or Cooling
    • Signs of a Chemical Reaction Change in Energy Release of Light Warming or Cooling Change in Properties Formation of a precipitate Change in odor Change in color Formation of bubbles
    • Chemical Reaction?
    • What happens in a chemical reaction? Atoms rearrange and form new substances • A single drop of water has trillions of oxygen and hydrogen atoms. • All of these atoms are arranged in the same way — two atoms of hydrogen are bonded to one atom of oxygen. • If this arrangement changes, the substance is no longer water. • Instead, a different substance forms with different physical and chemical properties.
    • What happens in a chemical reaction? • Atoms rearrange when chemical bonds between atoms break and other chemical bonds form. Bonds Break and Bonds Form • All substances, including solids, are made of particles that move constantly. As particles move, they collide. • If the particles collide with enough energy, the bonds between atoms can break. • The atoms separate and rearrange, and new bonds can form.
    • Atoms rearrange and form new substances Bonds Break and Bonds Form &
    • Atoms rearrange and form new substances Bonds Break and Bonds Form &
    • Atoms rearrange and form new substances Bonds Break and Bonds Form &
    • Atoms rearrange and form new substances Bonds Break and Bonds Form &
    • Let’s BREAK some bonds H2 + O2 2H2O CH4 + 2O2 CO2 + 2H2O C3H8 + 5O2 3CO2 + 4H2O
    • Chemical Equations C3H8 + 5O2 3CO2 + 4H2O • A chemical equation is a description of a reaction using element symbols and chemical formulas. • The substances produced by the chemical reaction are products. • The starting substances in a chemical reaction are reactants. reactants products say “produces”
    • Conservation of Mass Solution A&B Solution A Solution B and
    • Conservation of Mass • Antoine Lavoisier (1743–1794), a French chemist, measured the masses of substances before and after a chemical reaction inside a closed container. • He found that the total mass of the reactants always equaled the total mass of the products. • Lavoisier’s results led to the law of conservation of mass. • The law of conservation of mass states that the total mass of the reactants before a chemical reaction is the same as the total mass of the products after the chemical reaction.
    • Conservation of Mass • The discovery of atoms helped explain Lavoisier’s observations. • All atoms at the start of a chemical reaction are present at the end of the reaction. • Mass is conserved in a reaction because atoms are conserved. • During a chemical reaction, bonds break and new bonds form. However, a reaction does not destroy atoms, and it does not form new atoms.
    • Conservation of Mass
    • Is the equation balanced? • Because atoms are conserved, the number of atoms of each element must be the same, or balanced, on each side of the arrow.
    • Is the equation balanced? • Because atoms are conserved, the number of atoms of each element must be the same, or balanced, on each side of the arrow.
    • Is the equation balanced? • A balanced equation often does not happen automatically when the formulas for reactants and products are written.
    • Is the equation balanced? • A balanced equation often does not happen automatically when the formulas for reactants and products are written.
    • Is the equation balanced?
    • Balancing Equations • Balancing a chemical equation is the process of counting the atoms in the reactants and the products and then adding coefficients to balance the atoms. • A coefficient is a number placed in front of an element symbol or chemical formula in an equation. • Only coefficients can be changed when balancing an equation. • Changing subscripts changes the identities of the substances that are in the reaction. You CANNOT change the subscript.
    • 1. Write the unbalanced equation. Make sure that all chemical formulas are correct. 2. Count atoms of each element in the reactants and in the products. a. Note which, if any, elements have a balanced number of atoms on each side of the equation. Which atoms are not balanced? b. If all of the atoms are balanced, the equation is balanced. Balancing Equations
    • 3. Add coefficients to balance the atoms. a. Pick an element in the equation that is not balanced, such as oxygen. Write a coefficient in front of a reactant or a product that will balance the atoms of that element. b. Recount the atoms of each element in the reactants and the products. Note which atoms are not balanced. Some atoms that were balanced before might no longer be balanced. c. Repeat step 3 until the atoms of each element are balanced. Balancing Equations
    • 4. Write the balanced chemical equation including the coefficients. Balancing Equations
    • Let’s Balance