"It always feels good to hit the ball hard and drive the ball," said Arenado, who also doubled off Felix Doubront. "But there's still a lot of work to be done. I'm not feeling totally comfortable up there, but it was nice to see the ball well today."
Arenado's homer came after Brandon Barnes walked and Roger Bernadina singled, both in lengthy at-bats against Hammel.
"I didn't envision throwing 20 pitches to the first two batters of the year of my 2015," Hammel said. "Hats off to the first two batters for working me deep. A long inning to start the year is always fun.
"Honestly, it was a stupid pitch to Arenado. He didn't take a good hack on my slider before, so I should've gone back to it. It's just [Miguel Montero] and me getting on the same page. First one out of the gate."
Cubs power-hitting third-base prospect Kris Bryant, No. 2 overall in the MLB.com prospect rankings, parked a two-run homer just left of center.
The Cubs selected Bryant second overall out of the University of San Diego in the 2013 First-Year Player Draft, one pick before the Rockies took right-hander Jon Gray out of Oklahoma.
The homer was a blemish on an otherwise positive performance by Rockies non-roster right-hander Brett Marshall, who started because lefty Jorge De La Rosa was skipped with tightness in his left groin. Marshall gave up two hits -- a Jorge Soler single, followed by Bryant's homer -- and struck out two in two innings. Marshall's fastball ran 90-94 mph.
Marshall debuted in the Majors with four relief appearances for the Yankees in 2013. Last year, Marshall was told on the day of the next-to-last Spring Training game he had made the Reds' Opening Day roster, but he suffered a strained tendon in his right middle finger that day and was limited to 16 Triple-A starts. This winter he signed a Minor League deal with the Rockies.
"I just wanted to come in healthy and strong, make sure I can battle for a job," Marshall said.
The Rockies used several veteran relievers. One was non-roster Rafael Betancourt, making his first appearance for the Major League squad since Aug. 22, 2013, when he suffered a right elbow injury that required Tommy John surgery.
Bernadina, a non-roster outfielder, left the game with a bruised left wrist after a collision with first baseman Anthony Rizzo in the second inning. Bernadina said he doesn't expect to miss time. Non-roster infielder Omar Quintanilla also left the game with a left wrist contusion, and was sent for precautionary X-rays.
The Rockies also beat the White Sox, 8-2, in a Saturday morning B game in Glendale, Ariz. First-base prospect Correlle Prime knocked a two-run homer and onetime Athletics prospect Jeremy Barfield also homered.
Up next for the Rockies: Lefty Tyler Matzek will make his first Cactus League start Sunday (2:05 p.m. MT, MLB.TV), against the Padres in Peoria, Ariz. Gray, who threw a 97 mph fastball in his first appearance, and lefties Chris Rusin and John Lannan are scheduled for two innings, with lefty Yohan Flande throwing one as the Rockies take a good look at their starting depth. Second baseman DJ LeMahieu and outfielder Corey Dickerson and catchers Nick Hundley and Wilin Rosario are among the scheduled position players.